The Spinal Cord from Gestational Week 4 to the 4th Postnatal Month (Atlas of Human Central Nervous System Development)

The

SPINAL CORD from GESTATIONAL WEEK 4 to the 4TH POSTNATAL MONTH

ATLAS OF HUMAN CENTRAL NERVOUS SYSTEM DEVELOPMENT SERIES Shirley A. Bayer and Joseph Altman

VOLUME 1 The Spinal Cord from Gestational Week 4 to the 4th Postnatal Month

VOLUME 2 The Brain During the Third Trimester

VOLUME 3 The Brain During the Second Trimester

VOLUME 4 The Brain During the Late First Trimester

VOLUME 5 The Brain During the Early First Trimester

The

SPINAL CORD from GESTATIONAL WEEK 4 to the 4TH POSTNATAL MONTH Shirley A. Bayer and Joseph Altman

CRC PR E S S Boca Raton London New York Washington, D.C.

Library of Congress Cataloging-in-Publication Data Bayer, Shirley A. (Shirley Ann), 1940Atlas of human central nervous system development / Shirley A. Bayer, Joseph Altman. p. ; cm. Includes bibliographical references. Contents: v. 1. The spinal cord from gestational week 4 to the fourth postnatal month -v. 2. The brain during the third trimester -- v. 3. The brain during the second trimester-v. 4.The brain during the late first trimester -- v. 5. The brain during the early first trimester. ISBN 0-8493-1420-8 (alk. paper) 1. Central nervous system--Growth--Atlases. 2. Developmental neurophysiology--Atlases. I. Altman, Joseph, 1925-. II. Title. [DNLM: 1. Brain--growth & development--Atlases. 2. Spinal Cord--growth & development--Atlases. 3. Brain--embryology--Atlases. 4. Spinal Cord--embryology--Atlases. WL 17 B357a 2002] QM451 .B394 2002 2002025938

This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, microfilming, and recording, or by any information storage or retrieval system, without prior permission in writing from the publisher. The consent of CRC Press LLC does not extend to copying for general distribution, for promotion, for creating new works, or for resale. Specific permission must be obtained in writing from CRC Press LLC for such copying. Direct all inquiries to CRC Press LLC, 2000 N.W. Corporate Blvd., Boca Raton, Florida 33431. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.

Visit the CRC Press Web site at www.crcpress.com © 2002 by CRC Press LLC No claim to original U.S. Government works International Standard Book Number 0-8493-1420-8 Library of Congress Card Number 2002025938 Printed in the United States of America 1 2 3 4 5 6 7 8 9 0 Printed on acid-free paper

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PREFACE This book begins an Atlas Series on the Development of the Human Central Nervous System. We start with a volume devoted exclusively to the spinal cord for two reasons: (1) The spinal cord has similar structures throughout its length so a complete picture of its development at any specific age requires only a few different levels–and in early stages, only a single level is needed. (2) A transverse cutting plane is all that is necessary to show spinal cord structures. Thus, a single volume can feature all the stages of spinal cord development. In contrast, many different levels of the brain must be shown in a variety of cutting planes to get an overview of its structure at each stage of development. Four additional volumes (in preparation) in the Atlas Series will present a comprehensive picture of the developing human brain cut in frontal, sagittal, and horizontal planes. The second volume will feature the third trimester; the third volume will feature the second trimester; and finally, volumes 4 and 5 will feature the late and early parts of the first trimester, respectively. Our recent work on the developing spinal cord (Altman, J., and S. A. Bayer, Development of the Human Spinal Cord. An Interpretation Based on Experimental Studies in Animals, Oxford University Press, 2001) and this Atlas are our first publications linking developmental studies of the central nervous system in animals to humans. From the early 1960s to the present, our work has featured the development of the rat nervous system at the cell and tissue levels. We use 3H–thymidine autoradiography to gather a large database that maps the stem cell mosaic in the neuroepithelium, the times of origin of neuronal populations within the nervous system, and the migratory pathways of neuronal populations as they settle in the maturing nervous system. Our initial foray into human central nervous system development began in the late 1980s with a request from Serge Duckett to contribute a chapter on human central nervous system development (Bayer, S.A., J. Altman, R. J. Russo, and X. Zhang, Embryology, In: Pediatric Neuropathology, S. Duckett, Ed., Lea & Febiger, Philadelphia, PA, 1995, pp. 54-107). It was while we were working on that chapter that we realized how little is known about the development of the human central nervous system. An up-to-date interpretation simply does not exist.

The best work was published in German (Hochstetter, F.: Beiträge zur Entwicklungsgeschichte des menschlichen Gehirns. I. Teil. Vienna and Leipzig, Franz Deuticke, 1919) and has never been translated into English. There is a striking similarity between the developing human and rat central nervous systems. But Hochstetter did his pioneering work on development when even the mature anatomy of the nervous system was not well understood. A re-examination of human central nervous system development is needed to link what has been learned from animal studies to normal development in humans. Medical textbooks of human embryology are in need of an update on central nervous system development. In addition, the many disorders resulting from abnormal neural development can be better understood if normal development is better known. We decided to apply our knowledge of rat nervous system development to humans by directly examining histological sections of normal human specimens. The National Museum of Health and Medicine in the Armed Forces Institute of Pathology (AFIP), Walter Reed Hospital, Washington D.C. has the Carnegie Collection of human embryos and the Yakovlev Collection of human fetuses; while we were there we accidentally found the excellent Minot Collection of human embryos. Unfortunately, attempts to obtain funding for research at the AFIP were unsuccessful, so we decided to finance the research ourselves. During several trips to Washington D.C. in 1996 and mostly during an extended stay in the summer of 1997, we examined and took over 10,000 photographs of the best preserved normal specimens in the Yakovlev, Carnegie, and Minot collections. All of the film developing was done by us. Technicians were employed to scan each negative and create high resolution digitized files. These files are the sources for a comprehensive database on the development of the human central nervous system at the cell and tissue levels. This material is being analyzed in our Laboratory of Developmental Neurobiology that is equipped with photographic instruments, high resolution scanners, printers, and Apple computers capable of handling large files and running 3-dimensional reconstruction software. This Atlas Series is the result of our efforts.

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It is our hope that this first volume will update knowledge of the developing human spinal cord and provide new insights into the steps involved in the morphogenesis of the mature spinal cord. All Companion Plates in this Atlas are presented in a “user friendly” format so that the reader can view unlabeled and labeled photographs on facing pages. The first Companion Plate (2A-2B) shows the human spinal cord at the 4th gestational week at its most primitive stage consisting mainly of neuroepithelial stem cells (NEP cells). The last set of Companion

Plates (104A-104B to 117A-117B) shows a spinal cord nearing maturity in the 4th postnatal month. What lies in between these Companion Plates are “freeze frames” of the developmental record. The large sections on 3-dimensional reconstructions provide holistic views of the developing spinal cord at critical stages when maturing structures first appear during the first and early second trimesters (Figures 2 through 32). A Glossary at the end of the book gives a brief definition of each structure that is labeled in the Plates and Figures. Shirley A. Bayer Joseph Altman

DEDICATION We dedicate this book to the memory of Dr. Raymond J. Russo, an enthusiastic Professor of Biology, Indiana University–Purdue University in Indianapolis. He introduced us to the marvel-

ous graphic capabilities of Macintosh Computers and generously helped us with our first attempts at 3D reconstructions of the developing nervous system.

ACKNOWLEDGMENTS We thank our friend, Dr. William DeMyer, a pediatric neurologist at Riley Hospital for Children, Indiana University Medical Center, Indianapolis, Indiana for access to his personal library on human central nervous system development, which included a fragile copy of Hochstetter’s 1919 publication. We also thank the staff of the National Museum of Health and Medicine at the Armed Forces Institute of Pathology, Walter Reed Hospital, Washington, D.C.: Dr. Adrianne Noe, Director; Archibald J. Fobbs, Curator of

the Yakovlev Collection; Elizabeth C. Lockett, and William Discher. We are most grateful to Dr. James M. Petras at the Walter Reed Institute of Research who made his dark room facilities available so that we could develop all the photomicrographs on location rather than in our laboratory in Indiana. Finally, we thank Barbara Norwitz, Billi W. van der Putten, Tanya Li, and Susan Fox at CRC Press for their personal attention to us and for expert help during production of the manuscript.

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CONTENTS PREFACE ------------------------------------------------------------------------------- ix Figure 1: Dorsal View of the Brain and Spinal Cord in a Newborn Infant ------------------------------------------------------xvi I. INTRODUCTION ----------------------------------------------------------------------- 1 A. Overview of Atlas Organization -------------------------------------------------- 1 B. Specimen Collections ----------------------------------------------------------------- 1 C. Methods:Plate Preparation -------------------------------------------------------- 1 D. Methods: 3-D Reconstructions ---------------------------------------------------- 2 E. Terminology --------------------------------------------------------------------------- 2 F. References ----------------------------------------------------------------------------- 5 II. THE FIRST TRIMESTER ------------------------------------------------------------ 6 A. Cervical levels only: Embryos at GW4.0 to GW8.5 -------------------------- 6 Plate 1: Overview --------------------------------------------------------------------- 7 Plates 2A, 2B: GW4.0 --------------------------------------------------------------8-9 Plates 3A, 3B: GW4.5 -----------------------------------------------------------10-11 Plates 4A, 4B: GW4.75 ---------------------------------------------------------12-13 Plates 5A, 5B: GW5.25 (M2065) ----------------------------------------------14-15 Plates 6A, 6B: GW5.25 (C8998) ----------------------------------------------16-17 Plates 7A, 7B: GW5.5 -----------------------------------------------------------18-19 Plates 8A, 8B: GW6.5 -----------------------------------------------------------20-21 Plates 9A, 9B: GW6.7 -----------------------------------------------------------22-23 Plates 10A, 10B: GW6.8 --------------------------------------------------------24-25 Plates 11A, 11B: GW7.25 ------------------------------------------------------26-27 Plates 12A, 12B: GW8.5 (C609) ----------------------------------------------28-29 B. Spinal cord of a GW8.5 embryo --------------------------------------------------30 Plate 13: Overview ------------------------------------------------------------------31 Plates 14A, 14B: Upper Cervical -----------------------------------------------32-33 Plates 15A, 15B: Cervical Enlargement----------------------------------------34-35 Plates 16A, 16B: Lower Cervical -----------------------------------------------36-37 Plates 17A, 17B: Middle Thoracic----------------------------------------------38-39 Plates 18A, 18B: Lower Thoracic-----------------------------------------------40-41 Plates 19A, 19B: Lumbar Enlargement ----------------------------------------42-43 Plates 20A, 20B: Sacral and Coccygeal ----------------------------------------44-45 C. Spinal cord of a GW10.5 embryo ------------------------------------------------46 Plate 21: Overview ------------------------------------------------------------------47 Plates 22A, 22B: Lower Medulla/Upper Cervical ----------------------------48-49 Plates 23A, 23B: Cervical Enlargement----------------------------------------50-51 Plates 24A, 24B: Upper Thoracic -----------------------------------------------52-53 Plates 25A, 25B: Lower Thoracic-----------------------------------------------54-55 Plates 26A, 26B: Upper Lumbar ------------------------------------------------56-57 Plates 27A, 27B: Lumbar Enlargement ----------------------------------------58-59 Plates 28A, 28B: Sacral ----------------------------------------------------------60-61

xii III. THE SECOND TRIMESTER ---------------------------------------------------------62 A. Spinal cord of a GW14 fetus -----------------------------------------------------62 Plate 29: Overview ------------------------------------------------------------------63 Plates 30A, 30B: Upper Cervical -----------------------------------------------64-65 Plates 31A, 31B: Cervical Enlargement----------------------------------------66-67 Plates 32A, 32B: Upper Thoracic -----------------------------------------------68-69 Plates 33A, 33B: Middle Thoracic----------------------------------------------70-71 Plates 34A, 34B: Lower Thoracic-----------------------------------------------72-73 Plates 35A, 35B: Upper Lumbar ------------------------------------------------74-75 Plates 36A, 36B: Lumbar Enlargement ----------------------------------------76-77 Plates 37A, 37B: Sacral/Coccygeal ---------------------------------------------78-79 B. Spinal cord of a GW19 fetus -----------------------------------------------------80 Table III B–1: Density of proliferating glia in the white matter at GW 19 ------------------------------------------------------------80 Plate 38: Overview ------------------------------------------------------------------81 Plates 39A, 39B: Upper Cervical ----------------------------------------------82-83 Plates 40A, 40B: Cervical Enlargement --------------------------------------84-85 Plates 41A, 41B: Middle Thoracic --------------------------------------------86-87 Plates 42A, 42B: Lower Thoracic ---------------------------------------------88-89 Plates 43A, 43B: Upper Lumbar -----------------------------------------------90-91 Plates 44A, 44B: Lumbar Enlargement ---------------------------------------92-93 Plates 45A, 45B: Lumbosacral -------------------------------------------------94-95 C. Matched myelin and cell body stained sections in the spinal cord of a GW26 fetus ----------------------------------------------96 Table III C–1: Glia types and concentration in the white matter at GW 26 ------------------------------------------------------------96 Plate 46: Overview ------------------------------------------------------------------97 Plates 47A, 47B: Upper Cervical Enlargement (myelin) -------------------98-99 Plates 48A, 48B: Upper Cervical Enlargement (cell body) -------------100-101 Plates 49A, 49B: Lower Cervical Enlargement (myelin) ---------------102-103 Plates 50A, 50B: Lower Cervical Enlargement (cell body) -------------104-105 Plates 51A, 51B: Upper Thoracic (myelin) -------------------------------106-107 Plates 52A, 52B: Upper Thoracic (cell body) -----------------------------108-109 IV. THE THIRD TRIMESTER ---------------------------------------------------------- 110 A. Matched myelin and cell body stained sections in the spinal cord of a GW31 fetus -------------------------------------------- 110 Table IV A–1: Glia types and concentration in the white matter at GW 31 ---------------------------------------------------------- 110 Plate 53: Overview ---------------------------------------------------------------- 111 Plates 54A, 54B: Lower Thoracic (myelin) -------------------------------112-113 Plates 55A, 55B: Lower Thoracic (cell body) ----------------------------114-115 Plates 56A, 56B: Upper Lumbar (myelin) --------------------------------116-117 Plates 57A, 57B: Upper Lumbar (cell body) ------------------------------118-119 Plates 58A, 58B: Lumbar Enlargement (myelin) -------------------------120-121 Plates 59A, 59B: Lumbar Enlargement (cell body) ----------------------122-123

xiii Plates 60A, 60B: Sacral/Coccygeal (myelin) -----------------------------124-125 Plates 61A, 61B: Sacral/Coccygeal (cell body) --------------------------126-127 B. Matched myelin and cell body stained sections in the spinal cord of a GW37 fetus -------------------------------------------- 128 Table IV B–1: Glia types and concentration in the white matter at GW 37 ---------------------------------------------------------- 128 Plate 62: Overview ---------------------------------------------------------------- 129 Plates 63A, 63B: Upper Cervical (myelin) --------------------------------130-131 Plates 64A, 64B: Upper Cervical (cell body) -----------------------------132-133 Plates 65A, 65B: Cervical Enlargement (myelin) ------------------------134-135 Plates 66A, 66B: Cervical Enlargement (cell body) ---------------------136-137 Plates 67A, 67B: Upper Thoracic (myelin) -------------------------------138-139 Plates 68A, 68B: Upper Thoracic (cell body) -----------------------------140-141 Plates 69A, 69B: Middle Thoracic (myelin) ------------------------------142-143 Plates 70A, 70B: Middle Thoracic (cell body) ---------------------------144-145 Plates 71A, 71B: Lower Thoracic (myelin) -------------------------------146-147 Plates 72A, 72B: Lower Thoracic (cell body) ----------------------------148-149 Plates 73A, 73B: Upper Lumbar (myelin) --------------------------------150-151 Plates 74A, 74B: Upper Lumbar (cell body) ------------------------------152-153 Plates 75A, 75B: Lumbar Enlargement (myelin) -------------------------154-155 Plates 76A, 76B: Lumbar Enlargement (cell body) -----------------------156-157 Plates 77A, 77B: Sacral/Coccygeal (myelin) -----------------------------158-159 Plates 78A, 78B: Sacral/Coccygeal (cell body) --------------------------160-161 V. THE EARLY POSTNATAL PERIOD: Infants at 4 days, 4 weeks, and 4 months ------------------------------------------ 162 A. Matched myelin and cell body stained sections in the spinal cord of a 4–day infant ------------------------------------------- 162 Table V A–1: Glia types and concentration in the white matter in a 4-Day Infant -------------------------------------------------- 162 Plate 79: Overview------------------------------------------------------------------ 163 Plates 80A, 80B: Cervical Enlargement (myelin) ------------------------164-165 Plates 81A, 81B: Cervical Enlargement (cell body) ---------------------166-167 Plates 82A, 82B: Upper Thoracic (myelin) -------------------------------168-169 Plates 83A, 83B: Upper Thoracic (cell body) -----------------------------170-171 Plates 84A, 84B: Lower Thoracic (myelin) -------------------------------172-173 Plates 85A, 85B: Lower Thoracic (cell body) ----------------------------174-175 Plates 86A, 86B: Upper Lumbar (myelin) --------------------------------176-177 Plates 87A, 87B: Upper Lumbar (cell body) ------------------------------178-179 Plates 88A, 88B: Lumbar Enlargement (myelin) -------------------------180-181 Plates 89A, 89B: Lumbar Enlargement (cell body) ----------------------182-183 Plates 90A, 90B: Sacral/Coccygeal (myelin) -----------------------------184-185 Plates 91A, 91B: Sacral/Coccygeal (cell body) --------------------------186-187

xiv B. Matched myelin and cell body stained sections in the spinal cord of a 4–week infant ----------------------------------------- 188 Table V B–1: Glia types and concentration in the white matter in a 4-Week Infant ------------------------------------------------ 188 Plate 92: Overview ---------------------------------------------------------------- 189 Plates 93A, 93B: Upper Cervical (myelin) --------------------------------190-191 Plates 94A, 94B: Upper Cervical (cell body) -----------------------------192-193 Plates 95A, 95B: Cervical Enlargement (myelin) ------------------------194-195 Plates 96A, 96B: Upper Thoracic (myelin) -------------------------------196-197 Plates 97A, 97B: Upper Thoracic (cell body) -----------------------------198-199 Plates 98A, 98B: Lower Thoracic (myelin) -------------------------------200-201 Plates 99A, 99B: Lower Thoracic (cell body) ----------------------------202-203 Plates 100A, 100B: Upper Lumbar (myelin) -----------------------------204-205 Plates 101A, 101B: Lumbar Enlargement (myelin) ----------------------206-207 Plates 102A, 102B: Lumbar Enlargement (cell body) -------------------208-209 C. Matched myelin and cell body stained sections in the spinal cord of a 4–month infant --------------------------------------- 210 Table V C–1: Glia types and concentration in the white matter in a 4-Month Infant ----------------------------------------------- 210 Plate 103: Overview --------------------------------------------------------------- 211 Plates 104A, 104B: Upper Cervical (myelin) -----------------------------212-213 Plates 105A, 105B: Upper Cervical (cell body) --------------------------214-215 Plates 106A, 106B: Cervical Enlargement (myelin) ---------------------216-217 Plates 107A, 107B: Cervical Enlargement (cell body) ------------------218-219 Plates 108A, 108B: Upper Thoracic (myelin) ----------------------------220-221 Plates 109A, 109B: Upper Thoracic (cell body) --------------------------222-223 Plates 110A, 110B: Middle Thoracic (myelin) ---------------------------224-225 Plates 111A, 111B: Middle Thoracic (cell body) ------------------------226-227 Plates 112A, 112B: Lower Thoracic (myelin) ----------------------------228-229 Plates 113A, 113B: Lower Thoracic (cell body) -------------------------230-231 Plates 114A, 114B: Upper Lumbar (myelin) ------------------------------232-233 Plates 115A, 115B: Upper Lumbar (cell body) ---------------------------234-235 Plates 116A, 116B: Lumbar Enlargement (myelin) ----------------------236-237 Plates 117A, 117B: Lumbar Enlargement (cell body) -------------------238-239 VI. 3-D RECONSTRUCTIONS ---------------------------------------------------------- 240 A. The cervical level of eight first–trimester specimens ---------------------- 240 Figure 2: Overview of Spinal Cord Development (GW3.5-GW14) -------- 241 Figure 3: Neuroepithelium (GW3.5-GW4.0) ---------------------------------- 242 Figure 4: Neuroepithelium and Gray Matter (GW5.25-GW5.5) ------------ 243 Figure 5: Neuroepithelium and Gray Matter (GW7.0-GW7.2) -------------- 244 Figure 6: Neuroepithelium and Gray Matter (GW8.5-GW7.2) -------------- 245 Figure 7: Neuroepithelium, Roof and Floor Plates (GW3.5-GW5.5) ------ 246 Figure 8: Neuroepithelium, Roof and Floor Plates (GW7.0-GW14) ------- 247 Figure 9: Growth of the White Matter (GW5.25-GW14) -------------------- 248

xv B. The progressive segregation of ventral horn motoneurons into columns ----------------------------------------------------------------------- 249 Figure 10: Cervical Sections in the GW8.5 Model ---------------------------- 250 Figure 11: Thoracic, Lumbar, and Sacral Sections in the GW8.5 Model - 251 Figure 12: The Entire GW8.5 Model --------------------------------------252-253 Figure 13: Cervical Part of GW8.5 Model ------------------------------------- 254 Figure 14: Thoracic Part of GW8.5 Model ------------------------------------- 255 Figure 15: Lumbosacral Part of GW8.5 Model -------------------------------- 256 Figure 16: Cervical Sections in the GW10.5 Model -------------------------- 257 Figure 17: Thoracic Sections in the GW10.5 Model -------------------------- 258 Figure 18: Lumbar and Sacral Sections in the GW10.5 Model ------------- 259 Figure 19: The Entire GW10.5 Model -------------------------------------260-261 Figure 20: Cervical Part of GW10.5 Model ------------------------------------ 262 Figure 21: Thoracic Part of GW10.5 Model ----------------------------------- 263 Figure 22: Lumbar Part GW10.5 Model ---------------------------------------- 264 Figure 23: Cervical Sections in the GW14 Model ---------------------------- 265 Figure 24: Thoracic Sections in the GW14 Model ---------------------------- 266 Figure 25: Lumbar and Sacral Sections in the GW14 Model ---------------- 267 Figure 26: The Entire GW14 Model ---------------------------------------268-269 Figure 27: Cervical Part of GW14 Model (top view) ------------------------- 270 Figure 28: Cervical Part of GW14 Model (side view) ------------------------ 271 Figure 29: Lumbosacral Part of GW14 Model (top view) ------------------- 272 Figure 30: Lumbosacral Part of GW14 Model (side view) ------------------ 273 Figure 32: Thoracic Part of GW14 Model -------------------------------------- 274 VII. SUMMARY AND CONCLUSIONS ------------------------------------------------ 275 Figure 32: Changes in Area During the First Trimester ---------------------- 276 Table VII-1: Developmental Events in Neuronal Populations during the First Trimester ----------------------------------------- 277 Figure 33: Changes in Area from GW14 to the 4th Postnatal Month ------ 277 Figure 34: Proportional Areas at Cervical Enlargement (1) ------------------- 278 Figure 35: Proportional Areas at Cervical Enlargement (2) ------------------- 279 Figure 36: Level Differences in Proportional Areas: GW8.5-GW19 -------------------------------------------------------- 280 Figure 37: Level Differences in Proportional Areas: GW37-4th Month ---------------------------------------------------- 281 Table VII-2: Myelination Sequences in Major Fiber Tracts (Cervical Enlargement) ------------------------------------------- 282 Glossary

--------------------------------------------------------------------------------------------- 283

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FIGURE 1

Dorsal View of the Brain and Spinal Cord in a Newborn Infant

Cerebrum

Cerebellum Medulla C1 spinal nerve Cervical region (C) C2-C8 dorsal root ganglia

Thoracic region (T) Figure 1. Dissection exposing the dorsal aspect of the brain and spinal cord in a neonate, drawn by Johannes Sobotta and published in the first German edition (1906) of his Atlas of Human Anatomy (reproduced from the 8th English edition, 1963). The entire dorsal aspect of the spinal cord is seen connected to the dorsal roots of the spinal nerves. All spinal nerves have dorsal root ganglia except C1. Top thick line separates the spinal cord from the medulla; lower thick lines successively segregate the cervical, thoracic, and lumbosacral regions. The conus medullaris is located just below the coccygeal terminus of the spinal cord, and the cauda equina contains the elongated roots of the spinal nerves that exit from the bony spine far below their point of attachment to the spinal cord. The illustrations that follow throughout this Atlas trace the development of the spinal cord from just after closure of the embryonic neural tube to the 4th postnatal month.

T1-T12 dorsal root ganglia Thoracic spinal nerve Lumbar region (L) Sacral region Conus medullaris L1-L5 dorsal root ganglia Lumbar spinal nerve Cauda equina

Sacral dorsal root ganglia and spinal nerves

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I. Introduction A. Overview of Atlas Organization This Atlas focuses on the development of the human spinal cord, and is the first volume in the series, Atlas of Human Central Nervous System Development. The largest sections of this Atlas feature photographs of transversely cut spinal cords from normal specimens ranging in age from gestational week (GW) 4.0 up to and including the 4th postnatal month. The first trimester is described in Part II, the second in Part III, the third in Part IV, and the postnatal period in Part V. Each specimen or set of specimens is introduced by an overview plate that shows thumbnail photographs of all sections in that part of the Atlas at the same scale. The overview plate is followed by companion plates designated as A and B on facing pages. The A part of each plate on the left page shows the full contrast high magnification photograph of the specimen without any labels; the B part of each plate on the right page shows a low contrast copy of the same photograph with superimposed outlines and unabbreviated labels. Part VI presents 3-dimensional reconstructions of the cervical spinal cord during the first trimester (VIA) and of motoneuron columns in the ventral horn at cervical, thoracic, and lumbar levels in three specimens at the end of the first trimester and the beginning of the second (VIB). In Part VII, quantitative summaries of spinal cord development are presented in several graphs along with estimates of timetables for neurogenesis, dates of cell migration and settling, and sequences of myelination in major fiber tracts. A Glossary gives brief definitions for each label in the Plates, and defines other terms that are used in notes and figure captions. The concepts presented here are based on Altman and Bayer (2001), a research work that links human spinal cord development to the large body of developmental experiments in animals.

B. Specimen Collections The 117 Plates in this Atlas are from 20 specimens in three histological collections of human embryos and fetuses housed in the National Museum of Health and Medicine, Armed Forces Institute of Pathology, Washington D.C. Five of the specimens (designated by a C prefix) are from the Carnegie Collection, which originated in the Department of Embryology of the Carnegie Institution of Washington, under the leadership of Franklin P. Mall (1862-1917), George L. Streeter (1873-1948), and George W. Corner (1889-1981). These specimens were collected over a span of 40 to 50 years and were histologically prepared with a variety of different fixatives, embedding media, cutting planes and histological stains. Initial descriptions and analyses of this material in relation to spinal cord development (most of them in the context of other facets of embryonic development) were published in the early 1900s in Contributions to Embryology, The Carnegie Institution of Washington.

Seven of the specimens (designated by an M prefix) are from the Minot Collection, which is the work of Dr. Charles S. Minot (1852-1914), an embryologist at Harvard University. Throughout his career, Minot collected about 1900 embryos from a variety of species. The 100 human embryos in the collection were probably acquired between 1900 and 1910. From our examination of these specimens and their similar appearance, we presume that they are preserved in the same way although there are no existing records describing fixation procedures. The slides contain information regarding the section number, the section thickness (10 µm), and the stain (aluminum cochineal). Most embryos are cut in the same transverse plane, probably by Minot himself. Because of the constancy in their preparation, the Minot specimens are the source of quantitative data that are used to determine developmental trends during the early first trimester. To our knowledge, the nervous system has never been examined in the Minot specimens, other than by Altman and Bayer (2001). Eight of the specimens (designated by a Y prefix) are from the Yakovlev Collection. Over a 40-year span, Paul I. Yakovlev (1894-1983) collected about 1500 normal and abnormal human specimens, ranging in age from the early second trimester of fetal development through old age (Haleem, 1990). All of the specimens are fixed in formalin, embedded in celloidin, and are cut in 35 µm sections. The sections are preserved on two sets of slides, one set is stained for cell bodies using cresylviolet. The other set is stained for myelin using the Loyez modification of Weigert’s hematoxylin. Although sections of the brains are consecutively numbered on the slides, only two normal specimens (Y380–62, GW10.5 and Y68–65, GW14), have consecutively numbered sections of the spinal cord; 3D reconstructions of those specimens are presented in Part VIB. To our knowledge, this Atlas and the reference work on human spinal cord development (Altman and Bayer, 2001) are the only studies to analyze the spinal cord in the normal specimens of this collection.

C. Methods: Plate Preparation All sections are photographed using either an Olympus photomicroscope (the Carnegie and Minot specimens) or a Wild photomakroskop (the Yakovlev specimens) using Kodak technical pan black and white negative film #TP442. The film is developed for 6 to 7 minutes in dilution F of Kodak HC–110 developer, followed by stop bath for 30 seconds, Kodak fixer for 5 minutes, Kodak hypo clearing agent for 1 minute, running water rinse for 10 minutes, and a brief rinse in Kodak photo–flo before drying. Each specimen is photographed at the magnification that filled the microscopic field with the largest cross-section of the spinal cord.

2

Introduction The negatives are scanned at 2700 dots per inch (dpi) using a Nikon Coolscan–1000 35 mm film scanner interfaced to a Macintosh PowerMac G3 computer. To bring out all of the subtle shades of gray, the exposures are scanned as color positives rather than as black and white negatives. The software driving the scanner is a plug–in for Adobe Photoshop and generates raw files that are approximately 27 megabytes (mb) each. The raw files are reversed to positives and are changed to grayscale (reduces file size to 8 to 9 mb). The image resolution is set to 300 dpi. Using the enhancement features built into Adobe Photoshop and the additional features of Extensis Intellihance, adjustments are made to bring out contrast and increase sharpness. A normal-contrast image is in part A of each companion plate along with a low-contrast copy in part B. Labels are placed onto the low-contrast image using Adobe Illustrator. Small copies of each normal-contrast image are shown in the overview plates. A small image showing the spinal cord with the surrounding structures of the vertebral column is included in the plates of the first 11 specimens since the spinal cords have not been dissected from the surrounding tissues. For specimens in the late first, second, and third trimesters, the area surrounding the section is removed from the images.

D. Methods: 3D Reconstructions Cervical Cord in the First Trimester (Part VIA): Eight specimens were used to reconstruct the cervical part of the spinal cord from GW 4.0 to GW10.5, following a 5-step procedure. First, photographs of sections are made at regularly spaced intervals through the cervical spinal cord; the negatives are scanned and converted to computer files (as described in Section C above). Second, all the files from a single specimen are placed into one large Photoshop file that contains a separate photograph in each layer. By altering the visibility and transparency of these layers the sections are aligned to each other using the middle dorsoventral part of the spinal canal as the reference for alignment. In the living state, there is a curve in the cervical region of the developing cord; that is difficult to reconstruct accurately and often makes interpretation of the rendered images confusing. Therefore to simplify the reconstructions, the centers of the sections are aligned straight. Third, Adobe Illustrator is used to outline the following 14 structures in each section: (1) the outside edge of the entire section, (2) the entire neuroepithelium on the left side, (3–5) the dorsal, intermediate, and ventral parts of the neuroepithelium on the right side, (6–7) the roof and floor plates, (8) the undivided gray matter on the left side, (9–11) the dorsal horn, intermediate gray, and ventral horn on the right side, (12) the dorsal funiculus on both sides, (13) the lateral funiculus on both sides, and (14) the ventral funiculus on both sides and the ventral commissure. Since structures are usually symmetrical in most specimens, the outlines from one side are copied to the other side to simplify the images. The outlines from each section are saved in sepa-

rate Adobe Illustrator eps (encapsulated postscript) files. Fourth, the eps files are imported into 3D space (x, y, and z coordinates) using Cinema4DXL (C4D, Maxon Computer, Inc.), a modeling and animation software package. For each section, points on the outlines have unique x–y coordinates and share the same z coordinate. By calculating the distance between sections, the entire array of outlines is stretched out in the z axis. The outlines are segregated into 14 different goups, one for each structure. The C4D loft tool builds a “skin” for each structure by creating a spline mesh of polygons. The polygons start from the x–y points on the first outline with the most anterior z coordinate, to the x–y points on the next outline behind it, and finish with the x–y points on the last outline at the most posterior z coordinate. The polygons are rendered either as completely opaque or as partly transparent surfaces using the C4D ray-tracing engine. Selected parts of the model can be made either invisible or visible during rendering using the various options in C4D. Fifth, the rendered images are converted to Adobe Photoshop files, and Adobe Illustrator is used to draw thin lines on some of the surfaces to make the images more easy to understand. Motor Columns in the Ventral Horn (Part VIB): Using the same steps as described in the preceding paragraph, the entire spinal cord from upper cervical to sacral levels is reconstructed in three specimens at the end of the first trimester and at the beginning of the second trimester (GW8.5, GW10.5, and GW14). The following structural outlines are drawn: (1) the outside edge of the entire section, (2) the gray matter on both sides, (3) the central canal, (4) motoneuron columns in the right ventral horn, (5) motoneuron columns in the left ventral horn. The images are rendered so that the white and gray matter are shown as transparent, glass-like surfaces surrounding the opaque surfaces of the central canal and the motoneuron columns. Because the spinal cord is many times longer than it is wide, the thin motoneuron columns are difficult to see when all axes are rendered at the same scale, especially in the GW14 model. Consequently, the x–y axis is three times larger than the z axis in all models.

E. Terminology The labels in this Atlas come from a review of the literature (Altman and Bayer, 2001) on experimental studies of anatomical connections in the spinal cord of various mammals. In addition, several atlases of the adult human spinal cord are used (Fix,1987; Roberts et al., 1987; DeArmond et al., 1989; Haines, 2000) and some classical neuroanatomy books are consulted (Ranson and Clark, 1959; Crosby et al., 1962; Truex and Carpenter, 1969; Brodal, 1981). Some of these references use the directional terms anterior and posterior. Since our orientation is to link the work in humans to experimental work in animals, dorsal and ventral are the directional terms used throughout this Atlas. For the most part, the labels follow the terminology in these atlases and texts, especially in the gray matter.

Introduction However, some parts of the white matter are labeled differently from the standard adult atlases and there are departures from some developmental terminology. New Subdivisions in the Dorsal Funiculus. The largest components of the dorsal funiculus contain the principal ascending axons of dorsal root ganglion cells in the fasciculus gracilis and the fasciculus cuneatus. However, experimental evidence indicates that the entry zone of the dorsal root fibers and the fibers bordering the medial side of the dorsal horn have a more complex organization. As the dorsal root axons enter, they first bifurcate into ascending and descending branches (see Figures 12-15 and 12-16 in Truex and Carpenter, 1969; see Chapter 4, Section 4.1.2 in Altman and Bayer, 2001). That part of the dorsal funiculus is labeled the dorsal root bifurcation zone. Indeed, that part of the dorsal funiculus is the first to develop, appearing as an indistinct structure around GW4.75 (Plate 4) and as a more definite swelling in the white matter by GW5.25 (Plate 5). In the developmental literature, that structure is called the oval bundle of His, and labels in the youngest specimens give both names to the structure. After the dorsal root fibers bifurcate, there is developmental evidence in rats (Kudo and Yamada, 1987) that both the ascending and descending branches give off short, local collaterals (see Figure 143 in Ranson and Clark, 1959; see Figure 12-16 in Truex and Carpenter, 1969; see Figure 2-3 in Brodal, 1981; see Chapter 4, Section 4.2.2 in Altman and Bayer, 2001). These collaterals hug the medial edge of the dorsal horn before they enter the gray matter. For that reason, that part of the dorsal funiculus is called the dorsal root collateralization zone. Some of the collaterals invade the gray matter from the top of the dorsal horn in a dorsal bundle that first grows down, then curves upwards to form elaborate terminal arbors in various laminae of the dorsal horn. Other collaterals enter the dorsal horn in a dorsomedial bundle that grows ventrally and forms arbors within the motoneuron columns in the ventral horn (Kudo and Yamada, 1987). Just like the dorsal root bifurcation zone, the dorsal root collateralization zone is an early developing part of the dorsal funiculus. It first appears around GW6.7 (Plate 9) and GW6.8 (Plate 10). The latest developing and eventually the largest component of the dorsal funiculus contains the principal ascending axons of the dorsal root ganglia in the traditionally named fasciculus gracilis and fasciculus cuneatus. Most probably, the ascending branch that initially bifurcates and gives off local collaterals in the lateral part of the dorsal funiculus near its point of entry shifts medially as it grows toward the brain to enter these fasciculi (see comments on p. 189 in Ranson and Clark, 1959; see Chapter 4, Section 4.2.3 in Altman and Bayer, 2001). The first evidence that these fasciculi appear is on GW8.5 (Plates 12, 14-20).

3 The Spinocephalic (Spinothalamic) Tract. This tract occupies a crescent-shaped region in the superficial ventral and lateral funiculi. The reason this tract is called the spinocephalic tract is because a higher proportion of its fibers terminate in the brainstem rather than the thalamus (Altman and Bayer, 2001). The suffix cephalic refers to the brain rather than the thalamus. This tract has been divided into two parts, a ventral and a lateral, but Brodal (1981) maintains that there is no distinction between the two. Haines (2000) calls these tracts the anterolateral system that includes the spino–olivary, spinoreticular, and spinotectal tracts. In addition, some axons terminate in the midbrain central gray and the hypothalamus. The Intraspinal (Propriospinal) Tract. This tract, which carries local connections between neurons in the spinal cord that do not ascend to the brain, is traditionally called the propriospinal tract or fasciculus. The prefix proprio- means “within itself.” Unfortunately, that prefix can be confused with a set of proprioceptive fibers that transmit sensory information from muscles and tendons. By that criterion, the spinocerebellar tracts along the margin of the lateral funiculus are also propriospinal tracts because they send proprioceptive information to the cerebellum. To avoid that confusion, we name the propriospinal tract the intraspinal tract. The Overlapping of Tracts. Other than the spinocerebellar tracts, the fiber tracts in the ventral and lateral funiculi of the spinal cord do not have distinct borders (Ranson and Clark, 1959; Crosby et al., 1962; Truex and Carpenter, 1969; Brodal, 1981). The lines drawn in the white matter of the sections in this Atlas designate different densities of the myelin stain or different densities of glia in the cell body stains. They are not the actual borders of fiber tracts, and comments on the B parts of several plates indicate that. The ventral fibers of the lateral corticospinal tract overlap with the rubrospinal tract (labeled in only a few Plates). The spinocephalic tracts overlap with the vestibulospinal, spino–olivary (not labeled in any Plate), and spinotectal tracts (not labeled in any Plate). The part of the white matter that is designated as the intraspinal tract in this Atlas is a circumferential zone around the ventral horn, lateral intermediate gray, and lateral dorsal horn that overlaps with the medial longitudinal fasciculus (labeled at cervical levels), the tectospinal tract (labeled at cervical levels), and the lateral reticulospinal tract (only labeled in some Plates). In actuality, the intraspinal tract also extends along the medial surface of the dorsal horn and overlaps with the dorsal root collateralization zone (Fix,1987; Roberts et al., 1987; DeArmond et al., 1989; Haines, 2000). To simplify the plates, the dorsal root collateralization zone is not double-labeled as part of the intraspinal tract. Developmental Terminology. His (1889) identified three different layers of the embryonic brain and spinal cord: (1) an inner layer surrounding the brain ventricles and spinal canal (Innerplatte), (2) a cell-rich mantle layer,

4

Introduction and (3) a cell-sparse marginal layer. His further subdivided the inner layer into four plates: a roof plate, a floor plate, and two lateral plates (alar and basal). The sulcus limitans forms the dividing line between the two lateral plates. That terminology is modified as described below. This Atlas keeps His’ designation of the roof and floor plates because these are different cytologically and developmentally from the thick lateral plate that contains the neuroepithelium, a term used by Langman et al. (1966). The neuroepithelium contains the stem cells that give rise to neurons and glia. This Atlas modifies His’ segregation of the lateral plate by partitioning it into three parts rather than two, the dorsal neuroepithelium (the source of secondary sensory neurons in the dorsal horn), the intermediate neuroepithelium (the source of neurons in the intermediate gray), and ventral neuroepithelium (the source of motoneurons and interneurons in the ventral horn). The sulcus limitans marks that part of the spinal canal that will persist as the central canal and is surrounded by the intermediate neuroepithelium. His’ terms mantle and marginal layers are not useful because developing structures should be named according to what they will become at maturity, not be given a different set of names during development. That is a policy followed throughout the Atlas of Prenatal Rat Brain Development (Altman and Bayer, 1995). In the spinal cord, the cells that leave the neuroepithelium are specific types of neurons that migrate in a variety of dif-

ferent directions, settle in specific locations, and begin to differentiate in the primordial gray matter, not the mantle layer. Cell groups in the gray matter are named as soon as they are recognizable; for example, cells in the early ventral horn, intermediate gray, and dorsal horn appear as early as GW4.75 (Plate 4). The cell-sparse zone beneath the pial membrane in the developing spinal cord is the primordial white matter, not the marginal layer. Definite axons appear in the primordial white matter around GW5.25 (Plate 5) simultaneously in the ventral and dorsal funiculi; axons appear later in the lateral funiculus. Two other terms are used throughout this Atlas that refer to developing features not widely known. Sojourn zone describes an accumulation of premigratory neurons in the neuroepithelium; the evidence that these are aggregates of postmitotic neurons rather than neuroepithelial cells comes from short survival 3H–thymidine autoradiography (see Figure 3-10 in Altman and Bayer, 2001). The dorsal root boundary cap is an aggregate of specialized glial cells adjacent to the external wall of the spinal cord (Altman and Bayer, 1984; see Chapter 4, Section 4.1.3 in Altman and Bayer, 2001). Boundary caps mark spots on the pia where dorsal root fibers penetrate the spinal cord. Indeed, specific boundary caps mark nerve entry or exit points in the developing brain, indicating that they are important developmental structures (see Figure 62 in Altman and Bayer, 1981).

Introduction

5

F. References Altman, J., and S. A. Bayer (1981) Development of the Cranial Nerve Ganglia and Related Nuclei in the Rat. (Advances in Anatomy Embryology and Cell Biology, Vol. 74.) Berlin: Springer-Verlag.

His, W. (1889) Die Neuroblasten und deren Entstehung im embryonalen Mark. Abhandlungen der königlisches Gesellschaft der Wissenschaften, mathematisch-physikalische Klasse, 15:313–372.

Altman, J., and S. A. Bayer (1984) Development of the Rat Spinal Cord. (Advances in Anatomy Embryology and Cell Biology, Vol. 85.) Berlin: Springer-Verlag.

Kudo, N., and T. Yamada (1987) Morphological and physiological studies of the development of the monosynaptic reflex pathway in the rat lumbar spinal cord. Journal of Physiology, 389:441-459.

Altman, J., and S. A. Bayer (1995) Atlas of Prenatal Rat Brain Development. Boca Raton, FL: CRC Press. Altman, J., and S. A. Bayer (2001) Development of the Human Spinal Cord. An Interpretation Based on Experimental Studies in Animals. New York: Oxford University Press. Brodal, A. (1981) Neurological Anatomy in Relation to Clinical Medicine, (3rd edition). New York: Oxford University Press. Corner, G. W. (1929) A well-preserved human embryo of 10 somites. Carnegie Institution of Washington, Contributions to Embryology, 20:81-102. Crosby, E. C., T. Humphrey, and E. W. Lauer (1962) Correlative Anatomy of the Nervous System. New York: The Macmillan Company. DeArmond, S. J., M. M. Fusco, and M. M. Dewey (1989) Structure of the Human Brain, A Photographic Atlas, (3rd edition). New York: Oxford University Press. Fix, J. D. (1987) Atlas of the Human Brain and Spinal Cord. Gaithersburg, MD: Aspen Publishers. Haines, D. (2000) Neuroanatomy: An Atlas of Structures, Sections, and Systems, (5th edition). Philadelphia, PA: Lippincott Williams & Wilkins. Haleem, M. (1990) Diagnostic Categories of the Yakovlev Collection of Normal and Pathological Anatomy and Development of the Brain. Washington, D.C.: Armed Forces Institute of Pathology.

Langman, J., R. L. Guerrant, and B. G. Freeman (1966) Behavior of neuroepithelial cells during closure of the neural tube. Journal of Comparative Neurology, 131:15-26. Minot, C. S. (1903) A Laboratory Text-Book of Embryology. Philadelphia, PA: Blakiston. Ranson, S. W., and S. L. Clark (1959) The Anatomy of the Nervous System, its Development and Function, (10th edition). Philadelphia, PA: W. B. Saunders Company. Roberts, M., J. Hanaway, and D. K. Morest (1987) Atlas of the Human Brain in Section, (2nd edition). Philadelphia, PA: Lea & Febiger. Sobotta, J. (1963) Atlas of Human Anatomy, (8th English Edition). (Volume III, Part II: Atlas of Neuroanatomy. Central Nervous System, Autonomic Nervous System Eye, Ear, and Skin, F. H. J. Figge, Ed.) New York: Hafner. Truex, R. C., and M. B. Carpenter (1969) Human Neuroanatomy, (6th edition). Baltimore, MD: Williams and Wilkins Company. Streeter, G. L., C. H. Heuser, and G. W. Corner (1951) Developmental horizons in human embryos: Description of age groups XIX, XX, XXI, XXII, and XXIII, being the fifth issue of a survey of the Carnegie Collection. Carnegie Institution of Washington, Contributions to Embryology, 34:165-196.

6

II. The First Trimester A. Cervical levels only: Embryos at GW4.0 to GW8.5 Plate 1 is a survey of sections from the cervical levels of the spinal cord in the youngest 11 specimens. All sections are shown at the same scale. The boxes enclosing each section list: the gestational age in weeks (GW); the crown rump length (CR) in millimeters (mm); the specimen number preceded by M (Minot Collection) or C (Carnegie Collection); the slide number and section number from the set of slides containing all the sections of that specimen; and the total area of the section in square millimeters (mm2). Since all Minot specimens have consecutive section numbers, no slide number is given. Fullpage normal contrast photographs of each specimen are in Plates 2A-12A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 2B-12B. All A-B plates are on facing pages to allow the user to glance back and forth at the unlabeled structure on the left and the labeled one on the right. Plates 2-12 also feature low magnification views of the spinal cord surrounded by other parts of the embryo. Gross changes in morphology. It is during early stages of embryonic development (GW4.0-GW8.5) that the most dramatic structural changes take place in the spinal cord. On GW4.0 (M714) the most prominent component is the neuroepithelium, the germinal matrix of proliferating stem cells (NEP cells), that surround a slit-shaped spinal canal. The roof and floor plates are cell bridges that close off the spinal canal in the dorsal midline and ventral midline, respectively. Only a few cells are in the differentiating field of future gray matter, and the entire spinal cord is surrounded by a thin layer of primordial white matter. By GW8.5 (C609), the gray matter is the most prominent component of the spinal cord; the ventral horn and the intermediate gray already have complete populations of neurons, and nearly all neuronal populations are complete in the dorsal horn and the central autonomic area. Most major fiber tracts are present in the white matter. The spinal canal remains in a central location (central canal). A remnant of neuroepithelium lines the dorsal part of the central canal, and an ependymal/glial matrix lines the intermediate and ventral part. Roof and floor plates are still visible in the midline. The sequence of neurogenesis between various components of the gray matter. Between GW4.0 and GW4.75, the ventral part of the neuroepithelium is the largest component. The first neurons outside the ventral neuroepithelium at GW4 are differentiating motoneurons in the ventral horn. Between GW4.75 and GW5.5, ventral horn motoneurons increase as the ventral neuroepithelium recedes. By GW5.5, the center of the ventral neuroepithelium is

thinner than its dorsal or ventral borders (see Plate 6B). The thinned area may mark the exhaustion of the NEP (stem) cell population that produces motoneurons. Other parts of the ventral neuroepithelium are presumably producing ventral horn interneurons. By GW6.8, the ventral neuroepithelium is becoming an ependymal/glial proliferative matrix that no longer produces neurons. The intermediate neuroepithelium expands rapidly between GW5.25 and GW5.5 prior to growth of the intermediate gray. The intermediate neuroepithelium gradually declines by GW7.25 as the intermediate gray expands. By GW8.5, the intermediate neuroepithelium becomes an ependymal/ glial matrix. Between GW5.5 and GW6.5, the dorsal neuroepithelium dramatically increases its size, remains large on GW7.25, and is much smaller on GW8.5. The most rapid growth of the dorsal horn occurs between GW7.25 and GW8.5, coinciding with the shrinkage of the dorsal neuroepithelium (see Chapter 5, Section 5.3, Altman and Bayer, 2001). The dorsal neuroepithelium is characterized by “undulations” on its outer edges. These may be groups of premigratory postmitotic neurons, the postulated “sojourn zones” that have been well documented with 3H–thymidine autoradiography in the spinal cord of rats on embryonic day 15 (see Chapter 3, Figure 3-10 in Altman and Bayer, 2001). These undulations are still present on GW8.5 (Plate 12) indicating that a few neurons are still being generated (or have just been generated) by the dorsal neuroepithelium. The sequence of appearance of various components of the white matter. The oval bundle of His, first definite at GW5.25 (Plate 5), is the bifurcation zone of ingrowing axons of the dorsal root ganglion cells, the earliest component of the dorsal funiculus. From that lateral position, the dorsal funiculus grows medially to approach the dorsal midline by GW7.25 (Plate 11). By GW8.5, the dorsal funiculus grows downward along the midline, occupying the space vacated by the retreating roof plate and the receding dorsal neuroepithelium (Plate 12). The ventral funiculus also appears by GW5.25 in the wedge-like space between the floor plate, the ventral neuroepithelium, and the ventral horn. By GW5.5, axons of the early generated commissural neurons are crossing the midline in the ventral commissure beneath the floor plate. The lateral part of the white matter is still small at GW5.5 in C8998 but is larger at GW5.5 in M2161, indicating the beginning growth of the lateral funiculus. The dorsal, ventral, and lateral funiculi continue to expand throughout the rest of the first trimester (see Chapter 5, Section 5.6, Altman and Bayer, 2001).

PLATE 1 Plates 2A, 2B GW4.0 CR 4.0 mm

Plates 3A, 3B GW4.5 CR 6.3 mm

Plates 4A, 4B GW4.75 CR 6.7 mm

M714 Section 70 Total area: 0.065 mm2

Plates 9A, 9B GW6.7 CR 17.1 mm

M2300 Section 150 Total area: 0.1795 mm2

M2285 Section 350 Total area: 0.2324 mm2

Plates 10A, 10B GW6.8 CR 17.5 mm

Plates 5A, 5B GW5.25 CR 8.0 mm

Plates 6A, 6B GW5.5 CR 11.0 mm

M2155 Section 555 Total area: 1.2977 mm2

M2065 Section 350 Total area: 0.2478 mm2

C8998 Slide 19 Section 2 Total area: 0.6447 mm2

Plates 11A, 11B GW7.25 CR 22.5 mm

Plates 7A, 7B GW5.5

C8965 Slide 29 Section 10 Total area: 1.0092 mm2

C7254 Slide 34 Section 5 Total area: 1.4555 mm2

M2161 Section 165 Total area: 0.7530 mm2

CR 11.1 mm

Plates 12A, 12B GW8.5 Plates 8A, 8B GW6.5 CR 14.5 mm

C7707 Slide 34 Section 15 Total area: 0.8541 mm2

CR 32 mm

C609 Slide 32 Section 7 Total area: 1.6824 mm2

7

8

PLATE 2A GW4.0 CR 4.0 mm M714 Cervical Cell body stain

Areas (mm2) Spinal canal

.0046

Neuroepithelium

.0459

Roof plate

.0021

Floor plate

.0023

Primordial white matter

.0101

9

PLATE 2B Roof plate Lateral plate (neuroepithelium, NEP cells) Dorsal root ganglion

Primordial white matter

Somite

Slit-shaped spinal canal expanding ventrally

Floor plate Notochord

LATERAL PLATE Roof plate Dorsal neuroepithelium (alar plate)

Mitotic figures in neuroepithelium

Primordial white matter

Intermediate neuroepithelium

Slit-shaped spinal canal

Earliest differentiating motoneurons? Ventral neuroepithelium (basal plate)

Primordial white matter Floor plate

Large arrow extending from the neuroepithelium indicates migrating neurons. Small arrow in the spinal canal indicates the direction of expansion.

10

PLATE 3A GW4.5 CR 6.3 mm M2300 Cervical Cell body stain

Areas (mm2) Spinal canal

.0130

Neuroepithelium

.1163

Roof plate

.0036

Floor plate

.0043

Gray matter

.0098

White matter

.0325

11 Roof plate

PLATE 3B

Lateral plate (neuroepithelium, NEP cells) Primordial white matter

Dorsal root ganglion

Somite

Primordial gray matter Slit-shaped spinal canal expanding ventrally

Floor plate Notochord Dorsal canal

LATERAL PLATE Roof plate

Earliest neurons in dorsal horn (lamina I?)

Dorsal neuroepithelium (alar plate)

Primordial white matter

Approximate location of the future sulcus limitans (see Plate 4)

Intermediate neuroepithelium

Earliest commissural neurons? Central canal Primordial white matter Ventral neuroepithelium (basal plate)

Earliest motoneurons

Expanding ventral canal Large arrows extending from the neuroepithelium indicate migrating neurons into the primordial gray matter. Progressively smaller arrows extending from the neuroepithelium indicate progressively fewer migrating neurons. Small arrow in the spinal canal indicates the direction of expansion.

Floor plate

12

PLATE 4A GW4.75 CR 6.7 mm M2285 Cervical Cell body stain

Areas (mm2) Spinal canal

.0400

Neuroepithelium

.1080

Roof plate

.0037

Floor plate

.0048

Gray matter

.0476

White matter

.2324

13

PLATE 4B

Roof plate Lateral plate (neuroepithelium, NEP cells)

Expanding gray matter

Expanding dorsal root ganglion

Diamond-shaped spinal canal expanding in dorsal, lateral, and ventral directions

Expanding white matter

Floor plate Earliest dorsal horn neurons (laminae I and IV)

LATERAL PLATE Roof plate

Incipient dorsal funiculus Dorsal neuroepithelium

Oval bundle of His (dorsal root bifurcation zone) Dorsal canal Sulcus limitans Edge of gray matter Intermediate interneurons Central canal

Primordial white matter Migrating ventral horn motoneurons and interneurons

Intermediate neuroepithelium

Ventral root Incipient ventral funiculus

ling Sett l horn a s r vent neuron o t mo

Ventral canal Ventral neuroepithelium

Floor plate

Large arrows extending from the neuroepithelium indicate many migrating neurons. Progressively smaller arrows extending from the neuroepithelium indicate progressively fewer migrating neurons. Arrows in the spinal canal indicate the direction of expansion.

14

PLATE 5A GW5.25 CR 8.0 mm M2065 Cervical Cell body stain

Areas (mm2) Spinal canal

.0255

Neuroepithelium

.1103

Roof plate

.0012

Floor plate

.0067

Gray matter

.0752

White matter

.0289

15

PLATE 5B

Roof plate

Expanding white matter

Dorsal root boundary cap

Expanding gray matter

Dorsal root ganglion

Diamond-shaped spinal canal expanding dorsally and ventrally

Lateral plate (neuroepithelium, NEP cells)

Ventral horn

Ventral root Spinal nerve Floor plate

Expanding white matter Roof plate Dorsal canal Earliest dorsal horn neurons (laminae I and IV)

Dorsal neuroepithelium

Dorsal funiculus Oval bundle of His (dorsal root bifurcation zone)

Sulcus limitans Central canal

Earliest intermediate interneurons

Intermediate neuroepithelium

Primordial white matter Migrating ventral horn motoneurons and interneurons

First evidence of vascularization in the ventral neuroepithelium

Ventral neuroepithelium

Settling ventral horn motoneurons Large arrows extending from the neuroepithelium indicate many migrating neurons. Progressively smaller arrows extending from the neuroepithelium indicate progressively fewer migrating neurons. Arrows in the spinal canal indicate the direction of expansion.

Ventral canal

Floor plate

Ventral funiculus

16

PLATE 6A GW5.5 CR 11 mm C8998 Cervical Cell body stain

Areas (mm2) Spinal canal

.0523

Neuroepithelium

.2376

Roof plate

.0029

Floor plate

.0079

Gray matter

.2522

White matter

.0919

17

PLATE 6B

Roof plate Lateral plate (neuroepithelium, NEP cells)

Expanding gray matter Expanding white matter

Dorsal root (nerve + boundary cap) Diamond-shaped spinal canal expanding dorsally and ventrally

Dorsal root ganglion Floor plate Ventral rootlets Spinal nerve

Ventral horn Expanding white matter Roof plate

Dorsal canal (expanding rapidly)

Migrating and settling early dorsal horn neurons (laminae I and IV)

Dorsal neuroepithelium

Dorsal funiculus: Expanding tip Oval bundle of His (dorsal root bifurcation zone) Dorsal root

Sulcus limitans

Intermediate interneurons Central canal Primordial white matter

Dorsally advancing vascularization

Migrating ventral horn motoneurons and interneurons

Intermediate neuroepithelium Ventral canal (expansion ending, see Plate 7B) Ventral neuroepithelium compartments: Active ventral interneuron source? Depleted motoneuron source? Sojourn zone of late motoneurons and interneurons?

Settled ventral horn motoneurons

Active source of glial precursors?

Floor plate Ventral funiculus Ventral white commissure (decussation of commissural axons) Large arrows extending from the neuroepithelium indicate migrating neurons. Arrows in the spinal canal indicate the direction of expansion.

18

PLATE 7A GW5.5 CR 11.1 mm M2161 Cervical Cell body stain

Areas (mm2) Spinal canal

.0974

Neuroepithelium

.2160

Roof plate

.0066

Floor plate

.0070

Gray matter

.2842

White matter

.1418

19 Expanding white matter

Roof plate

PLATE 7B

Lateral plate (neuroepithelium, NEP cells) Dorsal root

Diamond-shaped spinal canal with expanding dorsal and receding ventral components Dorsal root ganglion

Expanding gray matter

Floor plate Postulated sojourn zone of premigratory neurons destined for the dorsal horn n ic Mi u lu do grat rsa ing s l h an orn d ne sett uro lin ns g

Expanding tip

Dorsal root bifurcation zone

fu

Do rs al

Dorsal funiculus

Roof plate

Expanding dorsal canal

Dorsal neuroepithelium

Vascularization reaches the dorsal neuroepithelium

Sulcus limitans Central canal Intermediate neuroepithelium

ulus funic

Intermediate interneurons

Ventral neuroepithelium

r a l Late

Migrating ventral horn motoneurons and interneurons

Active ventral interneuron source? Depleted motoneuron source? Active source of glial precursors?

Settled ventral horn motoneurons

Floor plate

Ventral white commissure

V

e n

Large arrows extending from the neuroepithelium indicate many migrating neurons. Arrows in the spinal canal indicate the direction of expansion or contraction.

Receding ventral canal

t r a l

u u l c f u n i

s

Incipient ventral median fissure

Crisscrossing streams of migrating neurons growing axons ipsilaterally and contralaterally

20

PLATE 8A GW6.5 CR 14.5 mm C7707 Cervical Cell body stain

Areas (mm2) Spinal canal

.0559

Neuroepithelium

.1810

Roof plate

.0136

Floor plate

.0057

Gray matter

.3932

White matter

.2047

21

PLATE 8B

Expanding white matter

Roof plate Lateral plate (neuroepithelium, NEP cells) Dorsal root Diamond-shaped spinal canal with expanding dorsal and receding ventral components Expanding gray matter

Dorsal root ganglion Floor plate

Ventral root Spinal nerve Cellular roof plate Fibrous roof plate

Migrating and settling dorsal horn neurons

Dorsal funiculus Expanding tip

lu

s

Dorsal root bifurcation zone

D

or

sa

l

fu

n

ic

u

Sojourn zone of premigratory neurons destined for the dorsal horn

Expanding dorsal Dorsal canal neuroepithelium

Dorsal root

Sulcus limitans

Intermediate neuroepithelium

Central canal

f u n

i

c

u

l u s

Intermediate interneurons

Criscrossing Crisscrossing streams of migrating neurons growing axons ipsilaterally and contralaterally

L a t e r a l

Active ventral interneuron source?

Ventral horn motoneurons are beginning to segregate into motor columns

V

e

Migrating ventral horn interneurons

Depleted motoneuron source? Receding ventral canal Floor plate Ventral white commissure

n

t r a l

l u f u n i c u

s

Incipient ventral median fissure

Large arrows extending from the neuroepithelium indicate many migrating neurons. Arrows in the spinal canal indicate the direction of expansion or contraction.

Active source of glial precursors?

Ventral neuroepithelium

22

PLATE 9A GW6.7 CR 17.1 mm C8965 Cervical Cell body stain

Areas (mm2) Spinal canal

.0166

Neuroepithelium

.1549

Roof plate

.0069

Floor plate

.0051

Gray matter

.5557

White matter

.2701

23

PLATE 9B Roof plate Lateral plate (neuroepithelium, NEP cells) Expanding white matter

Dorsal root Spinal canal with expanding dorsal and receding ventral components

Expanding gray matter

Dorsal root ganglion Floor plate Cellular roof plate

Fibrous roof plate

Migrating and settling dorsal horn neurons

Dorsal root bifurcation zone

Dorsal neuroepithelium

Lateral cervical nucleus?

Dorsal root

Sojourn zone of premigratory neurons destined for the dorsal horn

Sulcus limitans Expanding dorsal canal

Intermediate interneurons

at er

Depleting ventral interneuron source? Depleted motoneuron source? Active source of glial precursors?

Ventral neuroepithelium

Floor plate

s

Ventral white commissure

V Large arrows extending from the neuroepithelium indicate migrating neurons.

Incipient ventral median fissure

u

lu

axial motoneurons

ic en n tral fu

funiculus

Settling ventral horn interneurons

al

Receding ventral canal

proximal upper limb motoneurons Segregating ventral horn motoneurons

L

Central canal

Intermediate neuroepithelium

distal upper limb motoneurons

Dorsal funiculus

Incipient dorsal root collateralization zone

24

PLATE 10A GW6.8 CR 17.5 mm M2155 Cervical Cell body stain

Areas (mm2) Spinal canal

.0510

Neuroepithelium

.2350

Roof plate

.0140

Floor plate

.0126

Gray matter

.6475

White matter

.3376

25

PLATE 10B

Roof plate Lateral plate (neuroepithelium, NEP cells)

Expanding white matter

Dorsal root Spinal canal with expanding dorsal and receding ventral components Expanding gray matter

Dorsal root ganglion Floor plate

Cellular roof plate

Dorsal funiculus

Fibrous roof plate

Dorsal root collateralization zone

Dorsal neuroepithelium Migrating and settling dorsal horn neurons

Dorsal root bifurcation zone

Lateral cervical nucleus? Sulcus limitans Intermediate interneurons

Dorsal canal

Intermediate neuroepithelium

Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray

L

at

Central canal

era

Receding ventral neuroepithelium

Settling ventral horn interneurons

Segregating ventral horn motoneurons

Floor plate Ventral white commissure

s

proximal upper limb motoneurons

axial motoneurons

l funiculus

Receding ventral canal distal upper limb motoneurons

Dorsal root

Large arrows extending from the neuroepithelium indicate migrating neurons. Small arrows in the spinal canal indicate the direction of expansion or contraction.

V

Incipient ventral median fissure

e

i ntr l fun a

l cu

u

26

PLATE 11A GW7.25 CR 22.5 mm C7254 Cervical Cell body stain

Areas (mm2) Spinal canal

.0887

Neuroepithelium

.2207

Roof plate

.0149

Floor plate

.0153

Gray matter

.6798

White matter

.4362

27

PLATE 11B Roof plate Dorsal root Lateral plate (neuroepithelium with NEP cells)

Expanding white matter

Spinal canal with stable dorsal part and receding ventral part Floor plate

Expanding gray matter

Dorsal root ganglion Ventral root Spinal nerve

Dorsal funiculus Dorsal root collateralization zone

Cellular roof plate Fibrous roof plate

Dorsal root bifurcation zone

Migrating and settling dorsal horn neurons

Dorsal neuroepithelium Dorsal canal

Sojourn zone of premigratory neurons destined Dorsal root for the dorsal horn and intermediate gray

Intermediate interneurons

Intermediate neuroepithelium

Central canal

nic

Receding ventral neuroepithelium (source of glia?)

Ventral canal

ulus

Segregating ventral horn motoneuron columns

Settling ventral horn interneurons

Sulcus limitans

fu

distal upper limb motoneurons

l Latera

Lateral cervical nucleus?

Floor plate

proximal upper limb motoneurons

Ventral white commissure axial motoneurons

V Incipient ventral median fissure Large arrows extending from the neuroepithelium indicate migrating neurons. Small arrow in the spinal canal indicates the direction of contraction.

ul en tral funic

us

28

PLATE 12A GW8.5 CR 32 mm C609 Cervical Cell body stain

Areas (mm2) Central canal

.0439

Neuroepithelium

.0470

Roof plate

.0092

Floor plate

.0056

Gray matter

.8925

White matter

.6842

29

PLATE 12B Expanding white matter Retreating roof plate Receding dorsal neuroepithelium Dorsal root Ependyma Floor plate Expanding gray matter Persisting central canal Dorsal root ganglion

Dorsal funiculus Fasciculus gracilis? Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal root

Retreating roof plate

Lissauer's tract?

Dorsal horn

Surrounding meningeal membranes

Lamina I Substantia gelatinosa Laminae IV-V

Receding dorsal neuroepithelium

Intermediate interneurons

Floor plate Ventral white commissure

axial motoneurons

en tra l fun iculus

Ventral median fissure Large arrows extending from the neuroepithelium indicate migrating neurons. Small arrow in the spinal canal indicates the direction of contraction.

us

V Segregating ventral horn motoneuron columns

icu l

proximal upper limb motoneurons

Ventral horn interneurons

l fun

distal upper limb motoneurons

Persisting central canal

tera

Ependyma

Central autonomic area

La

Lateral cervical nucleus

30

Part II: The First Trimester (continued)

B. Spinal cord of a GW8.5 embryo Plate 13 is a survey of sections from seven levels of the spinal cord in M2050, a specimen in the Minot Collection with a crown rump length of 36 mm (see Chapter 5 in Altman and Bayer, 2001). All sections are shown at the same scale. The boxes enclosing each section list: the level, ranging from upper cervical to coccygeal; the section number; and the total area of the section in square millimeters (mm2). Note that the areal measurements are determined after fixation, while the crown rump length is measured before fixation. The unfixed area of each section could be 40 to 60% larger. Thus, the areal measurements are given only for comparison purposes between levels. Full-page normal contrast photographs of each specimen are in Plates 14A-20A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 14B-20B. Prior to GW8.5, the spinal cord progressively decreases in size from cervical to coccygeal levels. By GW8.5, this pattern begins to change. First, the cervical enlargement is about the same size as the upper cervical level. Second, regional size differences are also beginning to show. For example, the middle thoracic level is 66% smaller than the cervical enlargement and 30% smaller than the lumbar enlargement. However, the lumbar enlargement is still 50% smaller than the cervical enlargement, and the smallest cross-sectional areas are at sacral and coccygeal levels. These size differences reflect a rostral to caudal gradient in spinal cord maturation. The proliferating neuroepithelium is declining in this specimen but at all levels an active dorsal neuroepithelium is still present. The basal edges of that neuroepithelium are indistinct as a multitude of cells, presumably young neurons destined for the dorsal horn and the central autonomic area, migrate away to settle. Basal “undulations” are prominent and may be clumps of premigratory young neurons that sojourn in the neuroepithelium just after their generation. The upper part of the intermediate neuroepithelium still appears to be active at low thoracic and lumbar levels, in contrast to cervical levels. At the coccygeal level, even the ventral neuroepithelium appears to be active. Thus, neurons are still being generated in the spinal cord on GW8.5.

The ventral part of the spinal canal is absent except at sacral/coccygeal levels, and the dorsal part of the canal is receding. The central part of the canal will persist into adulthood. Concomitant with the decline of the neuroepithelium, an ependymal layer is appearing as low as lumbar levels. The gray matter displays maturational changes indicative of the adult spinal cord. In the ventral horn, motoneurons are segregating into columns around the ventral and lateral borders. That is clearly seen in the cervical and lumbar enlargements. The thoracic ventral horn does not have lateral accumulations of motoneurons, but only medial ones. We postulate that the medial columns of motoneurons represent those that supply the axial (skull, neck, and trunk) muscles, while the lateral columns represent those that supply limb muscles. At cervical levels, the intermediate gray has a distinguishable migratory stream of large neurons that may be destined to settle in the lateral cervical nucleus, and an accumulation of large neurons near the central canal that may represent the central cervical nucleus. At thoracic levels, there is only a hint of some large neurons accumulating in Clarke’s column (on the right side of the section in Plates 17A and 17B). Throughout all levels of the dorsal horn, there are distinctive clumps of densely packed small cells, presumably settling neurons of laminae II and III (substantia gelatinosa). In addition, migratory streams of cells leaving the dorsal neuroepithelium appear to be heading for the substantia gelatinosa. The central autonomic area can be distinguished as a more dense accumulation of small cells surrounding the central and dorsal parts of the spinal canal. All components of the white matter are expanding. The most notable changes are occuring in the ventral commissure, ventral funiculus, and dorsal funiculus. The ventral commissure thickens beneath the floor plate, and moves upward as the ventral neuroepithelium and ventral spinal canal disappear. The dorsal funiculus is accumulating axons on either side of the dorsal midline along vertically aligned cells that may connect the retreating roof plate to the pia.

31

PLATE 13

GW8.5, CR 36 mm, M2050 Plates 14A, 14B Upper Cervical Section 730 Total area: 2.8238 mm2

Plates 17A, 17B Middle Thoracic Section 1593 Total area: 0.9715 mm2

Plates 18A, 18B Lower Thoracic Section 1884 Total area: 1.1852 mm2 Plates 15A, 15B Cervical Enlargement Section 921 Total area: 2.8308 mm2

Plates 16A, 16B Lower Cervical Section 1113 Total area: 1.7018 mm2

Plates 19A, 19B Lumbar Enlargement Section 2134 Total area: 1.3964 mm2

Plates 20A, 20B (top sections) Sacral Section 2607 Total area: 0.6220 mm2 Plates 20A, 20B (bottom sections) Sacral/Coccygeal Section 2674 Total area: 0.3906 mm2

32

PLATE 14A GW8.5 CR 36 mm M2050 Upper Cervical Cell body stain

Areas (mm2) Spinal canal

.0622

Neuroepithelium

.1072

Roof plate

.0190

Floor plate

.0209

Gray matter

1.7430

White matter

.8715

33

PLATE 14B Dorsal median septum Incipient dorsal intermediate septum

Dorsal funiculus

Retreating roof plate Fasciculus gracilis?

Dorsal neuroepithelium Postulated sojourn zone of premigratory neurons in the dorsal neuroepithelium

Fasciculus cuneatus Dorsal root collateralization zone

Lamina I

Dorsal root bifurcation zone

Dorsal horn Substantia gelatinosa

Dorsal root

Laminae IV-V e au ss Li

Spinal nucleus of the trigeminal

r 's

Dorsal canal

trac

Migrating lateral cervical nucleus neurons?

Central cervical nucleus?

axial motoneurons

Floor plate Ventral gray commissure

Ventral white commissure

s tr u l al f u nicu

n

Incipient ventral median fissure

Ve

Segregating ventral horn motoneuron columns

Ependyma

f u n i c u l u s

imb er l rons upptoneu mo

Ventral horn interneurons

Central canal

Intermediate interneurons

l L a t e r a

Lateral cervical nucleus

t?

Central autonomic area

34

PLATE 15A GW8.5 CR 36 mm M2050 Cervical Enlargement Cell body stain

Areas (mm2) Spinal canal

.0834

Neuroepithelium

.0966

Roof plate

.0221

Floor plate

.0161

Gray matter

1.5254

White matter

1.0872

35

PLATE 15B

Dorsal funiculus Dorsal median septum Dorsal intermediate septum Retreating roof plate Dorsal neuroepithelium Postulated sojourn zone of premigratory neurons in the dorsal neuroepithelium

Fasciculus gracilis? Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal horn

Lissauer's tract?

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root

Dorsal canal Central autonomic area

Migrating lateral cervical nucleus neurons?

Ventral horn interneurons forearm, arm, and shoulder motoneurons

Floor plate

Ventral gray commissure Ventral white commissure

Ventral median fissure

r

t

Segregating ventral horn motoneuron columns

Ependyma

n Ve

axial motoneurons

Central canal

f u n i c u l u s

digit and hand motoneurons

Intermediate interneurons

l

Central cervical nucleus?

r a L a t e

Lateral cervical nucleus

a

l

u f u n i c u l

s

36

PLATE 16A GW8.5 CR 36 mm M2050 Lower Cervical Cell body stain

Areas (mm2) Spinal canal

.0557

Neuroepithelium

.0874

Roof plate

.0124

Floor plate

.0156

Gray matter

.8918

White matter

.6389

37

PLATE 16B

Dorsal median septum Dorsal intermediate septum

Dorsal funiculus

Retreating roof plate

Fasciculus gracilis?

Dorsal neuroepithelium

Fasciculus cuneatus (beginning)

Postulated sojourn zone of premigratory neurons in the dorsal neuroepithelium

Dorsal horn

Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract?

Dorsal canal Lateral cervical nucleus Intermediate interneurons

Dorsal root

l ra La t e

Lateral horn motoneurons

Central autonomic area

Central canal Ependyma

digit motoneurons

axial motoneurons

Floor plate

Ventral gray commissure Ventral white commissure

Indistinct clusters of ventral horn motoneurons?

Ve

tr

n

Segregating ventral horn motoneuron columns

funiculu s

Ventral horn interneurons

Ventral median fissure

al

funiculu

s

38

PLATE 17A GW8.5 CR 36 mm M2050 Middle Thoracic Cell body stain

Areas (mm2) Spinal canal

.0238

Neuroepithelium

.0625

Roof plate

.0094

Floor plate

.0110

Gray matter

.4796

White matter

.3852

39

PLATE 17B

Dorsal median septum

Dorsal funiculus

Retreating roof plate Dorsal neuroepithelium

Fasciculus gracilis?

Lamina I Dorsal horn Substantia

Dorsal root collateralization zone Dorsal root bifurcation zone

gelatinosa Laminae IV-V Intermediate neuroepithelium

Dorsal root Central autonomic area Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray Neurons settling in Clarke's column?

Central canal

funi

Ependyma Ventral horn interneurons Ventral gray Floor plate

cul

axial motoneurons

commissure Ventral white commissure

Ve

us

nt Ventral median fissure

ra

us

Lateral

Lateral horn motoneurons (autonomic) Intermediate interneurons

Lissauer's tract?

Dorsal canal

l funicu

l

40

PLATE 18A GW8.5 CR 36 mm M2050 Lower Thoracic Cell body stain

Areas (mm2) Spinal canal

.0317

Neuroepithelium

.0857

Roof plate

.0097

Floor plate

.0132

Gray matter

.6133

White matter

.4317

41

PLATE18B

Dorsal median septum

Dorsal funiculus

Retreating roof plate

Fasciculus gracilis?

Dorsal neuroepithelium

Dorsal horn

Dorsal root collateralization zone

Lamina I Substantia gelatinosa

Dorsal root bifurcation zone

Laminae IV-V Dorsal root

Lissauer's tract?

Dorsal canal

Intermediate neuroepithelium

Intermediate interneurons

L atera

Lateral horn motoneurons? (autonomic)

Central canal

s

Ve

axial motoneurons

Floor plate Ventral white commissure

l funiculu

Ependyma Ventral horn interneurons

Central autonomic area Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray

nt

r

al s Ventral f uniculu median fissure

Migratory stream of lateral horn motoneurons?

42

PLATE 19A GW8.5 CR 36 mm M2050 Lumbar Enlargement Cell body stain

Areas (mm2) Spinal canal

.0353

Neuroepithelium

.0957

Roof plate

.0238

Floor plate

.0113

Gray matter

.7637

White matter

.4666

43

PLATE 19B

Dorsal median septum Retreating roof plate Dorsal neuroepithelium Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray

Dorsal funiculus Fasciculus gracilis? Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal Lamina I horn Substantia gelatinosa Laminae IV-V

Lissauer's tract?

Dorsal canal

Dorsal root

Central canal

Intermediate interneurons Ependyma

axial motoneurons

n

h moip an ton d k eu nee ron s

Floor plate Ventral white commissure

Ve

ankle and foot motoneurons

Ventral horn interneurons

ra

t

Segregating ventral horn motoneuron columns

Ventral median fissure

l f uni culu s

s

Intermediate neuroepithelium

uniculu La t e r a l f

Central autonomic area

44

PLATE 20A GW8.5 CR 36 mm M2050 Sacral Cell body stain

Sacral/Coccygeal Cell body stain

Areas (mm2) Spinal canal

.0184

Neuroepithelium

.0647

Roof plate

.0135

Floor plate

.0066

Gray matter

.3275

White matter

.1913

Areas (mm2) Spinal canal

.0089

Neuroepithelium

.0386

Roof plate

.0052

Floor plate

.0029

Gray matter

.1962

White matter

.1389

45

PLATE 20B

Roof plate Dorsal neuroepithelium Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray

Dorsal root collateralization zone

Lamina I

Dorsal horn

Dorsal funiculus

Dorsal root bifurcation zone

Substantia gelatinosa

Lissauer's tract?

Laminae IV-V Central autonomic area

Dorsal canal

Intermediate neuroepithelium Intermediate interneurons

Central canal

Ventral horn interneurons

Ventral canal

Ventral neuroepithelium Ventral horn motoneurons

Lateral funiculus Floor plate plate Floor Ventral white commissure

Roof plate Sojourn zone of premigratory neurons destined for the dorsal horn and intermediate gray

Dorsal root collateralization zone

Lamina I

Dorsal horn

Dorsal root bifurcation zone

Substantia gelatinosa Laminae IV-V

Dorsal funiculus

Dorsal and intermediate neuroepithelia

Central autonomic area

Spinal canal with persisting central part and receding dorsal and ventral parts Lateral funiculus

Intermediate interneurons Ventral horn interneurons Ventral horn motoneurons

Ventral funiculus

Ventral neuroepithelium Floor plate Ventral white commissure

Ventral funiculus

46

Part II: The First Trimester (concluded)

C. Spinal cord of a GW10.5 embryo Plate 21 is a survey of sections from seven levels of the spinal cord in Y380-62, a specimen in the Yakovlev Collection with a crown rump length of 56 mm (see Chapter 5 in Altman and Bayer, 2001). All sections are shown at the same scale. The boxes enclosing each section list: the level, ranging from upper cervical to coccygeal; the section number; and the total area of the section in square millimeters (mm2). Note that the areal measurements are determined after fixation, while the crown-rump length is measured before fixation. The unfixed area of each section could be 40 to 60% larger. Thus, the areal measurements are given only for comparison purposes between levels. Full-page normal contrast photographs of each specimen are in Plates 22A-28A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 22B-28B. The size differences between levels seen at GW8.5, reflecting a rostral-to-caudal gradient in spinal cord maturation, persist in this specimen. The lumbar enlargement is 11% smaller than the cervical enlargement, and the smallest cross-sectional area is at the sacral level. However, the lower thoracic level is 37% smaller than the cervical enlargement and 29% smaller than the lumbar enlargement, reflecting regional size differences characteristic of the adult spinal cord.

The dorsal neuroepithelium is no longer present in this specimen. At all levels, the central canal is shaped like a triangle, and is lined by a dense layer of cells, presumably the ependymal layer. However, at the most caudal level (Plate 28), a small ventral neuroepithelium may be the source of glia. The lining of the persisting central canal is considered to be composed of ependymal cells. The gray matter is continuing to show maturational changes indicative of the adult spinal cord. In the ventral horn, the motoneurons are segregating into more discrete motor columns than in the GW8.5 specimen. The thoracic ventral horn has a large accumulation of motoneurons in the medial motor column. At cervical levels, the intermediate gray has a distinguishable lateral cervical nucleus, and an accumulation of large neurons near the central canal that may represent the central cervical nucleus. However, the migratory stream of large neurons that appeared to be moving into the lateral cervical nucleus (seen in the GW8.5 specimen) is no longer evident. The dense groups of cells that appear to be migrating to and settling in the substantia gelatinosa, prominent at GW8.5, are less distinct in this more mature specimen. In the white matter, all components are expanding. The dorsal funiculus continues to fill in with fibers as it descends from the dorsal midline, following the retreating roof plate.

47

PLATE 21

GW10.5, CR 56mm, Y380-62 Plates 22A, 22B Lower Medulla/ Upper Cervical Section 361 Total Area: 3.2032 mm2

Plates 25A, 25B Lower Thoracic Section 881 Total Area: 1.2365 mm2

Plates 26A, 26B Upper Lumbar Section 911 Total Area: 1.4099 mm2 Plates 23A, 23B Cervical Enlargement Section 521 Total Area: 1.9640 mm2 Plates 27A, 27B Lumbar Enlargement Section 1066 Total Area: 1.7510 mm2 Plates 24A, 24B Upper Thoracic Section 671 Total Area: 1.4213 mm2

Plates 28A, 28B Sacral Section 1211 Total Area: 0.6813 mm2

48

PLATE 22A GW10.5 CR 56 mm Y380-62 Lower Medulla/Upper Cervical Cell body stain

Areas (mm2) Central canal

.0728

Neuroepithelium

.0395

Roof plate

.0199

Floor plate

.0139

Gray matter

1.7140

White matter

1.3431

49

PLATE 22B

Dorsal funiculus

Dorsal median septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone

Dorsal intermediate septum? Retreating roof plate

Dorsal horn

Dorsal root bifurcation zone

Lamina I

r's ue ssa t? Li trac

Substantia gelatinosa

Laminae IV-V

Central cervical nucleus?

Ependyma

Central canal Intermediate interneurons

di

Ventral horn motoneuron columns

Floor plate

Ventral white commissure

r a Ve n t

ap hr ag ax m ? (sk ial m ul o l a to nd ne ne uro ck ns ?)

Ventral horn interneurons

l

Ventral median fissure

funi cu l us

Lateral cervical nucleus

La t eral

Central autonomic area

l u u c f u n i

s

50

PLATE 23A GW10.5 CR 56 mm Y380-62 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0105

Neuroepithelium

.0135

Roof plate

.0133

Floor plate

.0086

Gray matter

1.0320

White matter

.8861

51

PLATE 23B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal intermediate septum? Retreating roof plate

Dorsal horn

Lamina I

Liss a trac uer's t?

Laminae IV-V

Central autonomic area

wrist and hand?

Ventral horn interneurons neck?

Floor plate

a l funicu l u s

Ventral horn motoneuron columns

Ventral median fissure

s

arm and shoulder?

Ventral white commissure

tr Ve n

arm and forearm?

Ependyma Central canal

fu n i c u lu

Intermediate interneurons Central cervical nucleus?

Lateral cervical nucleus

t l roo ra sal te L a

Dor

Substantia gelatinosa

52

PLATE 24A GW10.5 CR 56 mm Y380-62 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0206

Neuroepithelium

.0198

Roof plate

.0143

Floor plate

.0083

Gray matter

.6812

White matter

.6771

53

PLATE 24B

Dorsal median septum Fasciculus gracilis Dorsal Fasciculus cuneatus funiculus Dorsal root collateralization zone

Dorsal intermediate septum? Retreating roof plate

Lamina I

er's sau Lis tract?

Dorsal horn

Dorsal root bifurcation zone

Substantia gelatinosa Laminae IV-V

Ependyma Ventral horn interneurons

s

s

Ventral median fissure

lu

Ventral white commissure

Floor plate

Ve n t r a l

trunk

Ventral horn motoneuron column

eral fu

Central canal

nic u

Intermediate interneurons Lateral horn motoneurons (autonomic)

Lat

Central autonomic area

Clarke's column?

f unicu

lu

Migratory stream of lateral horn (autonomic) motoneurons?

54

PLATE 25A GW10.5 CR 56 mm Y380-62 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0114

Neuroepithelium

.0207

Roof plate

.0100

Floor plate

.0082

Gray matter

.6527

White matter

.5336

55

PLATE 25B

Dorsal Fasciculus gracilis funiculus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Retreating roof plate

Lamina I

Lissauer's tract?

Substantia gelatinosa

Clarke's column? Intermediate interneurons

Central canal

Ependyma Floor plate

s

l Ve n t r a

Ventral white commissure Ventral median fissure

lu

Ventral horn interneurons trunk

Ventral horn motoneuron column

Central autonomic area

nic u

Lateral horn motoneurons (autonomic)

Lateral fu

Laminae IV-V

s

Dorsal horn

funicu

lu

56

PLATE 26A

GW10.5 CR 56 mm Y380-62 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0129

Neuroepithelium

.0232

Roof plate

.0143

Floor plate

.0091

Gray matter

.7558

White matter

.5992

57

PLATE 26B

Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone

Retreating roof plate

Dorsal horn

Dorsal root bifurcation zone

Lamina I

Lissauer's tract?

Substantia gelatinosa

La Central autonomic area

ankle? leg?

us

Ventral white commissure Ventral median fissure

Floor plate

cu l

trunk

Ependyma

Ve n t r a l

Ventral horn interneurons leg? thigh?

Ventral horn motoneuron columns

Central canal

Intermediate interneurons

trunk hip?

funiculus

teral funi

Laminae IV-V

Lateral horn motoneurons? (autonomic)

Dorsal funiculus

58

PLATE 27A GW10.5 CR 56 mm Y380-62 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0128

Neuroepithelium

.0183

Roof plate

.0128

Floor plate

.0081

Gray matter

.9752

White matter

.7221

59

PLATE 27B

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Retreating roof plate Lamina I

Dorsal horn Substantia

Lissauer's tract?

gelatinosa Laminae IV-V

Central canal

Ependyma Floor plate

trunk

leg? hip and thigh?

Ventral horn motoneuron columns Ventral white commissure

ankle and foot?

trunk

l Ve n t r a

ankle and foot?

Ventral horn interneurons

eral funiculus

Intermediate interneurons

Lat

Central autonomic area

leg? thigh? hip?

fu niculus Ventral median fissure

Dorsal funiculus

60

PLATE 28A GW10.5 CR 56 mm Y380-62 Sacral Cell body stain

Areas (mm2) Central canal

.0144

Neuroepithelium

.0311

Roof plate

.0201

Floor plate

.0058

Gray matter

.3385

White matter

.2754

61

PLATE 28B

Dorsal median septum Retreating roof plate Lamina I

Dorsal horn Substantia gelatinosa

Laminae IV-V Dorsal neuroepithelium Intermediate interneurons Ventral horn interneurons Ventral horn motoVentral horn neuron column motoneurons (lower trunk) Ventral median fissure

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal funiculus

Lissauer's tract?

Central autonomic area Central canal Lateral funiculus Ependyma Ventral neuroepithelium (glioepithelium?) Floor plate Ventral white commissure Ventral funiculus

62

III. The Second Trimester

A. Spinal cord of a GW14 fetus Plate 29 is a survey of sections from Y68-65, a specimen in the Yakovlev Collection with a crown-rump length of 108 mm (see Chapters 5 and 6 of Altman and Bayer, 2001). All sections are shown at the same scale. The boxes enclosing each section list: the level from upper cervical to sacral/coccygeal; the section number from the set of slides containing all the sections of that specimen; and the total area (post-fixation) of the section in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 30A-37A. Low contrast photographs with superimposed labels and outlines of section details are on facing pages in Plates 30B-37B. The outer surface of this specimen has sharp spikelike projections, and the white matter is thin around the gray matter, interpreted to be a shrinkage artifact of histological processing. Cells in the gray matter are not distinct, except for the prominent clumps of large motoneurons in the ventral horn, possibly due to inadequate penetration of

fixative. In spite of these histological artifacts, this specimen is illustrated becaue it is the only one in the Yakovlev Collection where sections of the spinal cord are consecutively numbered from cervical to coccygeal levels. Most of the size differences between levels at GW14 are similar to those in the adult spinal cord. The smallest cross-sectional area is in the middle thoracic level, which is 27% smaller than the sacral level. That the cervical enlargement level is smaller than both the upper cervical and lumbar enlargement levels reflects variability unique to this specimen. Within the gray matter, the most obvious sign of ongoing maturation is the prominent columnar arrangement of motoneurons in the ventral horn. Not only are these columns larger than at GW10.5, but there are more of them. Within the white matter, the dorsal funiculus is deepening further in the dorsal midline, accompanying the retreating roof plate.

63

PLATE 29

GW14, CR 108 mm, Y68-65 Plates 30A, 30B Upper Cervical Section 741 Total area: 4.1972 mm2

Plates 31A, 31B Cervical Enlargement Section 981 Total area: 3.0426 mm2

Plates 32A, 32B Upper Thoracic Section 1141 Total area: 1.4745 mm2

Plates 33A, 33B Middle Thoracic Section 1581 Total area: 1.1659 mm2

Plates 34A, 34B Lower Thoracic Section 1741 Total area: 1.6474 mm2

Plates 35A, 35B Upper Lumbar Section 1821 Total area: 2.7717 mm2

Plates 36A, 36B Lumbar Enlargement Section 1901 Total area: 3.8584 mm2

Plates 37A, 37B Sacral/ Coccygeal Section 2061 Total area: 1.6071 mm2

64

PLATE 30A GW14 CR 108 mm Y68-65 Upper Cervical Cell body stain

Areas (mm2) Central canal

.0053

Neuroepithelium

.0182

Roof plate

.0290

Floor plate

.0029

Gray matter

2.5381

White matter

1.6037

65

PLATE 30B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Fasciculus cuneatus Dorsal root collateralization zone

Dorsal intermediate septum

Dorsal horn

Lamina I Substantia gelatinosa

Dorsal root bifurcation zone

Laminae IV-V Lissauer's tract? tract

L

Central autonomic area

r a l t e

Intermediate interneurons Central cervical nucleus?

a

Lateral cervical nucleus

Roof plate

Central canal

f u n i c u l u s

Ependyma Ventral horn interneurons shoulder? diaphragm?

neck?

skull and neck?

ra nt

Ventral gray commissure

Ventral white commissure

Ve

Ventral horn motoneuron columns

Floor plate

l

Ventral median fissure

cu i f u n

lu

s

Ventral root

66

PLATE 31A

GW14 CR 108 mm Y68-65 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0056

Neuroepithelium

.0127

Roof plate

.0192

Floor plate

.0035

Gray matter

1.9505

White matter

1.0511

67

PLATE 31B

Dorsal funiculus Dorsal median septum Dorsal intermediate septum Dorsal root

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I

Dorsal horn

Substantia gelatinosa Laminae IV-V

Lissauer's tract

L

a t e

r

Lateral cervical nucleus

arm?

forearm?

shoulder?

e

Ventral horn motoneuron columns

neck and upper trunk?

Ventral white commissure

V

arm and shoulder?

Ventral horn interneurons

n

Ventral median fissure

t r a l

f

n i c u l u s

Central canal Ependyma Floor plate

forearm and wrist?

l

u

Intermediate interneurons Central cervical nucleus?

wrist and hand?

a

Roof plate Central autonomic area

u s f u n i c u l

68

PLATE 32A

GW14 CR 108 mm Y68-65 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0021

Neuroepithelium

.0063

Roof plate

.0135

Floor plate

.0040

Gray matter

.8242

White matter

.6243

69

PLATE 32B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum Dorsal intermediate septum

Dorsal horn

Fasciculus cuneatus Dorsal root collateralization zone Dorsal root Dorsal root bifurcation zone bifurcation zone

Lamina I Substantia gelatinosa

Lissauer's Lissauer's tract tract?

Laminae IV-V

La

Clarke's column?

Ventral gray commissure

s

trunk

Ve n t r a l

Ventral horn motoneuron column

Ventral white commissure

unicu al f lu

Central autonomic area Ependyma Central canal Floor plate

Intermediate interneurons Ventral horn interneurons

ter

Lateral horn motoneurons (autonomic)

Roof plate

cu f uni

Ventral median fissure

lu

s

70

PLATE 33A

GW14 CR 108 mm Y68-65 Middle Thoracic Cell body stain

Areas (mm2) Central canal

.0018

Neuroepithelium

.0084

Roof plate

.0152

Floor plate

.0035

Gray matter

.6913

White matter

.4458

71

PLATE 33B

Dorsal median septum

Dorsal horn

Fasciculus gracilis Dorsal root collateralization zone

Lamina I

Dorsal funiculus

Dorsal root bifurcation zone

Substantia gelatinosa

Lissauer's tract? tract

Laminae IV-V

Clarke's column?

Central autonomic area

Lateral funiculus

Lateral horn motoneurons (autonomic)

Roof plate

Intermediate interneurons

Ependyma Ventral horn interneurons

Central canal trunk

Ventral white commissure

Ventral horn motoneuron columns

Floor plate Ventral gray commissure

Ventral funiculus Ventral median fissure

72

PLATE 34A

GW14 CR 108 mm Y68-65 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0039

Neuroepithelium

.0128

Roof plate

.0171

Floor plate

.0019

Gray matter

1.0249

White matter

.5868

73

PLATE 34B

Fasciculus gracilis

Dorsal median septum

Dorsal horn

Lamina I

Dorsal funiculus

Dorsal root collateralization zone

Substantia gelatinosa

Dorsal root bifurcation zone

Laminae IV-V

Lissauer's tract?

Clarke's column

Central autonomic area

Lateral funiculus

Lateral horn motoneurons (autonomic)

Roof plate

Intermediate interneurons

Ependyma Ventral gray commissure

Ventral horn interneurons

Central canal Floor plate

trunk

Ventral white commissure

Ventral horn motoneuron columns

Ventral funiculus Ventral median fissure

74

PLATE 35A

GW14 CR 108 mm Y68-65 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0056

Neuroepithelium

.0140

Roof plate

.0315

Floor plate

.0076

Gray matter

1.6817

White matter

1.0312

75

PLATE 35B

Fasciculus gracilis

Dorsal median septum

Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root

Dorsal horn

Dorsal funiculus

Lamina I Lissauer's tract? tract

Substantia gelatinosa

Laminae IV-V

La

Roof plate

teral f

Intermediate interneurons

Lateral horn motoneurons (autonomic)

Central autonomic area

unic u

Clarke's column?

Central canal Ependyma

Ventral horn interneurons

Ventral white commissure

s

hip?

s

thigh?

lu

Floor plate Ventral gray commissure

lower trunk?

V

Ventral horn motoneuron columns

en

tra

Ventral median fissure

l fu

u nic

lu

76

PLATE 36A GW14 CR 108 mm Y68-65 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0059

Neuroepithelium

.0118

Roof plate

.0271

Floor plate

.0056

Gray matter

2.5458

White matter

1.2621

77

PLATE 36B

Dorsal horn

Dorsal funiculus

Fasciculus gracilis

Dorsal median septum

Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I

Substantia gelatinosa

Lissauer's tract? tract

Laminae IV-V

Dorsal root

Intermediate interneurons

t e r a l L a

Central autonomic area Roof plate

f

u

u s u l

thigh?

c

Ventral gray commissure

Ventral white commissure

hip? leg?

i

ankle and foot?

n

Ependyma Floor plate

Central canal

Ventral horn interneurons

V

t

Ventral root

n

Ventral horn motoneuron columns

e

lower trunk?

r

Ventral median fissure

a

l

l u s f u n i c u

78

PLATE 37A

GW14 CR 108 mm Y68-65 Sacral/Coccygeal Cell body stain

Areas (mm2) Central canal

.0045

Neuroepithelium

.0134

Roof plate

.0254

Floor plate

.0057

Gray matter

1.1322

White matter

.4258

79

PLATE 37B

Dorsal median septum

Fasciculus gracilis

Dorsal Dorsal root collateralization zone funiculus Dorsal root bifurcation zone

Lamina I Dorsal horn Substantia gelatinosa Laminae IV-V

Lissauer's tract? tract

Intermediate interneurons

Roof plate Central autonomic area Central canal

lus

Ependyma Floor plate Ventral gray commissure Ventral white commissure

ic

Ventral horn motoneuron?

ulu

s

Ventral horn interneurons

Lateral funicu

Lateral horn motoneurons?

Dorsal root

V

e n a l fun tr

Ventral median fissure

Ventral root

80

Part III: The Second Trimester (continued) B. Spinal cord of a GW19 fetus Plate 38 is a survey of sections from Y52-61, a specimen in the Yakovlev Collection with a crown-rump length of 130 mm. All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post-fixation) in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 39A-45A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 39B-45B. In this specimen, the section numbers are not given because they are placed on large glass plates without any numbers. Twenty sections were photographed ranging from upper cervical to sacral levels. Low sacral and coccygeal levels were not preserved. To determine the approximate level, the 20 photographs were intuitively arranged using features such as the appearance and progressively smaller rostral-to-caudal size of the ventral and lateral corticospinal tracts. The most notable characteristic of this specimen is the dense accumulation of glial cells in specific regions of the white matter. These are assumed to be glial cells proliferating prior to myelination (myelination gliosis). The myelination of fiber tracts in the spinal cord follows a progressive sequence (see Chapter 6, Section 6.3 in Altman and Bayer, 2001). Differences in the concentration of glia allow several major fiber tracts to be tentatively identified by this age (Table III B-1). The lateral and ventral corticospinal tracts (the last to myelinate) stand out as very sparse regions. Other regions in the ventral and lateral funiculi have different densities of proliferative glia in bands and clumps. A densely populated outer band at the cervical level (only in the lateral funiculus) is postulated to be the spinocerebellar tracts. A sparsely populated band just inside that (the outermost band in the ventral funiculus) is postulated to be the vestibulospinal tract and the spinocephalic tracts. A very dense to dense inner band adjacent to the ventral gray matter is postulated to contain the medial longitudinal fasciculus, the tectospinal tract (only at the cervical levels), and the intraspinal (propriospinal) tracts. Both of these fiber tracts contain density gradients of proliferating glia (see notes beneath Table III B-1). In the dorsal funiculus, the dorsal root collateralization and bifurcation zones contain very dense to dense proliferative glia all the way down to the lumbar enlargement. At cervical and middle thoracic levels, the cuneate fasciculus can be distinguished from the gracile fasciculus by the greater concentration of proliferating glia. However, the gracile fasciculus, especially its deep part, contains proliferating glia that is most dense in the lumbar enlargement and gradually declines through thoracic levels. It is least dense at cervical levels, except for the deep wedge. Generally, the

lower concentration of proliferating glia in the white matter at lumbosacral levels (column 4, Table III B-1) reflects the gradient of maturation from upper cervical to coccygeal levels. Within the gray matter, columns of motoneurons continue to show progressive segregation in the ventral horn. The accumulation of lateral horn (autonomic) motoneurons is obvious at thoracic levels, and possibly at the sacral level. Large neurons in Clarke’s column are evident at the low thoracic level, and may also be present at the middle thoracic level.

Table III B-1: Density of proliferating glia in the white matter at GW19 Lumbosacral Thoracic Name

Cervical

DORSAL ROOT VENTRAL ROOT DORSAL FUNICULUS: dorsal root bif. zone dorsal root col. zone deep fas. gracilis

Sparse

---

Sparse

Very dense

---

Very dense

Dense

Dense

Very dense Very dense

Sparse Dense*

Dense

Very dense

---

superficial fas. gracilis

Very sparse

Sparse

Sparse

deep fas. cuneatus

Very dense

Dense

---

Sparse

Very sparse

---

superficial fas. cuneatus Lissauer's tract

Very sparse Very sparse Very sparse

LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract ven. cortricospinal tract rubrospinal tract spinocerebellar tracts ven. commissure intraspinal tracts** spinocephalic tracts*** med. long. fasiculus tectospinal tract vestibulospinal tract

Very sparse very sparse Dense Dense Very dense Gradient† Gradient†† Very dense Very dense Sparse

* **

Very sparse Very sparse very sparse ------Sparse --Dense Sparse Gradient† -Gradient†† ---------Sparse ---

Sparse at the most caudal level (Plate 45). Overlaps with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract. † Very dense medial to the ventral horn, dense around the remaining ventral horn, sparse around the intermediate gray and lateral dorsal horn. *** Contains anterior and lateral parts; overlaps with the vestibulospinal tract, the spinotectal tract, and the spino-olivary tract. †† Deep parts (adjacent to intraspinal tracts) are more dense than superficial parts (adjacent to the ventrolateral pial membrane).

81

PLATE 38

GW19, CR 130 mm, Y52-61 Plates 39A, 39B Upper Cervical Total area: 4.1202 mm2

Plates 42A, 42B Lower Thoracic Total area: 1.4929 mm2

Plates 43A, 43B Upper Lumbar Total area: 2.2128 mm2 Plates 40A, 40B Cervical Enlargement Total area: 3.7511 mm2

Plates 44A, 44B Lumbar Enlargement Total area: 2.6480 mm2

Plates 41A, 41B Middle Thoracic Total area: 1.3996 mm2

Plates 45A, 45B Lumbosacral Total area: 2.0164 mm2

82

PLATE 39A GW19 CR 130 mm Y52-61 Upper Cervical Cell body stain

Areas (mm2) Central canal

.0050

Neuroepithelium

.0170

Roof plate

.0152

Floor plate

.0054

Gray matter

1.8505

White matter

2.2271

83 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of this plate.

PLATE 39B

Dorsal funiculus Superficial fasciculus gracilis Deep fasciculus gracilis

Dorsal median septum

Superficial fasciculus cuneatus Dorsal intermediate septum

Dorsal horn

Deep fasciculus cuneatus Dorsal root collateralization zone

Lamina I

Substantia gelatinosa Laminae IV-V

Dorsal root bifurcation zone Lissauer's tract

Do

Rubrospinal tract?

rs a

m?

e r e be llar tract

Floor plate

Ventral horn motoneurons dle

?

for

ear

Ventral gray commissure Ventral white commissure

Examples of proliferating glial densities

Very sparse Very dense

c Ve t s ntr al spi

noc

Dense

tr a l a I n t r a s p i n t lic a h cep o n i Sp

Ventral funiculus

Sparse

skull motoneurons?

s

Ventral corticospinal tract

ct

rn ho ons l ra r nt eu Ve tern in

ra

sho

uld

er

gir

and arm

no co Lat c e re rtic era o l tra spin ct al

ac t

Central canal Ependyma

Intermediate interneurons

pi

tr llar be

Lateral funiculus

Central autonomic area

Central cervical nucleus?

Lateral cervical nucleus

ls

Roof plate

Ventral median fissure

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

The fine lines in the lateral and ventral funiculi segregate regions of differing densities of proliferating glia, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar and the lateral corticospinal tracts. Other lines run across fiber tracts, such as the very dense region adjacent to the ventral corticospinal tract that includes the medial longitudinal fasciculus, the tectospinal tract, and the medial part of the intraspinal tracts. Many of the fiber tracts in the ventral and lateral funiculi overlap and do not have distinct borders.

84

PLATE 40A GW19 CR 130 mm Y52-61 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0069

Neuroepithelium

.0172

Roof plate

.0154

Floor plate

.0037

Gray matter

1.8661

White matter

1.8419

85 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 40B

Dorsal funiculus Superficial fasciculus gracilis Deep fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus

Dorsal median septum

Dorsal root collateralization zone

Dorsal intermediate septum Dorsal root

Dorsal root bifurcation zone

Dorsal horn

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

Do

r

sa

Roof plate Central autonomic area Central canal

Intermediate interneurons

tract?

t

Lateral funiculus Rubrospinal

Ventral white commissure

arm? shoulder? shoulder and arm?

Ventral horn motoneurons

Ventral corticospinal tract

Ventral funiculus Ventral median fissure

ac t V s r en tra a c t s ls pin ocer

Ventral gray commissure

Ventral horn interneurons

forearm?

ebella

Floor plate

wrist?

Ventral root

r tract

Ependyma digits?

c

trac llar er e b e al al ter pin La icos ct rt ra co t

Lateral cervical nucleus

l spi no

l I n t r a s pi n a tr ic l ha ep c o n Spi

t

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

Note the larger lateral and ventral corticospinal tracts on one side of the spinal cord and the smaller ones on the other (see also Plates 39 and 41). Axons in the lateral corticospinal tract cross the midline, while axons in the ventral corticospinal tract remain ipsilateral. The larger lateral corticospinal tract (crossed component) is linked to the smaller ventral corticospinal tract (uncrossed component) and vice versa. It is unknown which is the true right or left side. This specimen is the only one in the Atlas to show such a pronounced asymmetry.

86

PLATE 41A

GW19 CR 130 mm Y52-61 Middle Thoracic Cell body stain

Areas (mm2) Central canal

.0019

Neuroepithelium

.0092

Roof plate

.0156

Floor plate

.0029

Gray matter

.6000

White matter

.7701

87 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 41B

Dorsal funiculus Superficial fasciculus gracilis Deep fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum

Dorsal horn

Lamina I Substantia gelatinosa

Lissauer's tract

Laminae IV-V Clarke's column?

Roof plate Central autonomic area

Intermediate interneurons

Ependyma

Floor plate Ventral gray commissure

Ventral horn interneurons

a

ha

l

Ventral white commissure

trunk?

In t r a s p

Ventral funiculus

oc

ep

in

Ventral corticospinal tract Ventral horn motoneurons

tr ac t s ? lic trac ts?

Central canal

Lateral funiculus

Sp

in

Vestibulospinal tract? Ventral median fissure The fine lines in the lateral and ventral funiculi segregate regions of differing densities of proliferating glia, not the borders of fiber tracts.

racts? bellar t

Lateral horn motoneurons (autonomic)

n

oc e r e

al l ter na La ospi tic ct cor tra

Spi

88

PLATE 42A

GW19 CR 130 mm Y52-61 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0025

Neuroepithelium

.0103

Roof plate

.0170

Floor plate

.0031

Gray matter

.7999

White matter

.6600

89 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 42B

Dorsal funiculus Dorsal median septum

Superficial fasciculus gracilis Deep fasciculus gracilis Dorsal root collateralization zone

Lamina I

Dorsal horn

Dorsal root bifurcation zone

Substantia gelatinosa Laminae IV-V

Clarke's column?

cerebella r tr S p ino a

Lateral funiculus

Lissauer's tract??

Roof plate Central autonomic area Central,autonomic Ependyma

Intermediate interneurons

Central canal Floor plate Ventral gray commissure Ventral white commissure

l

trunk?

Ventral horn motoneurons

Ventral funiculus

a t r a p i n s in Sp

oc

I

n

ep

?

tr ha a c ts lic tra ? cts?

Ventral horn interneurons

cts

Lateral horn motoneurons (autonomic)

Vestibulospinal tract? Ventral median fissure The fine lines in the lateral and ventral funiculi segregate regions of differing densities of proliferating glia, not the borders of fiber tracts.

90

PLATE 43A

GW19 CR 130 mm Y52-61 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0049

Neuroepithelium

.0152

Roof plate

.0175

Floor plate

.0031

Gray matter

1.3115

White matter

.8607

91 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 43B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Dorsal root collateralization zone

Dorsal horn

Dorsal root Dorsal root bifurcation zone Lamina I

Substantia gelatinosa

Lissauer's tract??

Laminae IV-V

Roof plate

?

?

Central autonomic area Central canal Intermediate interneurons

Lateral funiculus

Ependyma Floor plate Ventral gray commissure

ankle?

pelvic girdle?

lower trunk?

Ventral horn motoneurons

ts?

I n t r

thigh?

Ventral funiculus Ventral median fissure

c s ? tr ac

thigh and leg?

t c a a t r h p e oc n i

li

leg?

Ventral white commissure Ventral horn interneurons

a s p i n a l Sp

Ventral root

92

PLATE 44A

GW19 CR 130 mm Y52-61 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0072

Neuroepithelium

.0151

Roof plate

.0170

Floor plate

.0031

Gray matter

1.8235

White matter

.7822

93 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 44B

Dorsal Fasciculus gracilis funiculus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum

Dorsal horn

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root

Lissauer's tract?

Roof plate Central canal Central autonomic area

Lateral funiculus

Intermediate interneurons Ependyma Floor plate

foot?

Ventral gray commissure ankle? leg?

thigh and leg?

Ventral white commissure

Ventral horn interneurons

lower trunk? thigh?

Ventral horn motoneurons

pelvic girdle?

Ventral funiculus Ventral median fissure

94

PLATE 45A

GW19 CR 130 mm Y52-61 Lumbosacral Cell body stain

Areas (mm2) Central canal

.0027

Neuroepithelium

.0187

Roof plate

.0201

Floor plate

.0019

Gray matter

1.4461

White matter

.5269

95 See Table III B-1 for the list of glial densities in each fiber tract. Examples of glial densities in each category are labeled in the lower left corner of Plate 39B.

PLATE 45B

Dorsal funiculus Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum

Dorsal horn

Lamina I Dorsal root Lissauer's tract?

Substantia gelatinosa Laminae IV-V

Autonomic motoneurons?

Roof plate Central autonomic area Intermediate interneurons

foot?

Central canal

foot?

Ependyma

leg?

Lateral funiculus

Floor plate

ankle?

Ventral gray commissure

Ventral horn interneurons

Ventral white commissure

thigh? lower trunk?

Ventral funiculus Ventral horn motoneurons

Ventral median fissure

Ventral root

96

Part III: The Second Trimester (concluded) C. Matched myelin and cell body stained sections in the spinal cord of a GW26 fetus Plate 46 is a survey of matched myelin stained and cell body stained sections from Y60-61, a specimen in the Yakovlev Collection with a crown rump length of 210 mm. All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post-fixation) of the section in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 47A-52A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 47B-52B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without any section numbers. Eighteen myelin stained and 17 cell body stained sections were photographed ranging from upper cervical to thoracic levels. There were no sections preserved at lumbar, sacral, and coccygeal levels. The 35 photographic prints were intuitively arranged in order from upper cervical to thoracic levels, using internal features such as the size of the corticospinal tracts, and the width of the ventral horn. Then, myelin and cell body stained sections were matched. The upper cervical level sections and several middle to lower thoracic level sections were either damaged or had no matches. There were good matches in the region of the cervical enlargement (upper and lower levels are illustrated) and in the upper level of the thoracic cord.

The densities of reactive and proliferating glia within a fiber tract vary independently of each other (compare rows in Table III C-1). Some tracts have dense reactive glia and dense proliferating glia (for example, deep gracile and cuneate fasciculi). Others have dense reactive glia but sparse proliferating glia (for example, intraspinal tracts), sparse reactive glia and dense proliferating glia (for example, superficial gracile and cuneate fasciculi), sparse reactive glia and sparse proliferating glia (for example, the spinocephalic tracts at the upper cervical enlargement level). These variations are probably due to the different rates of myelination in each fiber tract. In general, glial proliferation precedes reactive gliosis (see the differing concentrations of proliferating glia in the GW19 specimen, Plates 38-45), and that precedes myelination in different fiber tracts (see Chapter 6 in Altman and Bayer, 2001).

Table III C-1: Glia types and concentration in the white matter at GW26

Name

The cross- sectional area of a myelin stained section is smaller than the matching cell body stained section in all cases. Evidently, the myelin staining procedure produces greater tissue shrinkage than the cell body staining procedure. The upper cervical enlargement section has a larger ventral corticospinal tract but is nearly the same size as the lower section in the cervical enlargement; the thoracic section is 47% smaller than the cervical sections. Y60-61 is one of the youngest specimens to show any myelin stained areas. Actually, staining for myelin products begins around GW20 (Y27-60, CR 160 mm, Fig. 6-2 in Altman and Bayer, 2001), but that specimen is very incomplete, and is not shown in this Atlas. Three structures contain solid black stain indicative of myelinating axons (see myelination column in Table III C-1). Throughout the rest of the white matter in the myelin stained sections, there is either punctate staining or no staining. Dense punctate stained areas are assumed to have high concentrations of glia that react with the stain prior to production of the myelin sheath, sparse punctate stained areas have low concentrations, and unstained areas have none (reactive glia column, Table III C-1). In the cell body stained sections, there are various densities of what is assumed to be proliferating interfascicular glia in the dorsal, lateral, and ventral funiculi (proliferating glia column, Table III C-1), but generally, the concentration of proliferating glia is less pronounced than in the GW19 specimen.

Proliferating glia Reactive glia Myelination

DORSAL ROOT VENTRAL ROOT DORSAL FUNICULUS: dorsal root bif. zone

---

Dense

Dense

Advanced

---

sparse

Some fibers

---

Dense

dorsal root col. zone

---

Dense

Dense

deep fas. gracilis

---

Dense

Dense

superficial fas. gracilis

---

Sparse

Dense

deep fas. cuneatus

---

Dense

Dense

superficial fas. cuneatus

---

Sparse

Dense

Lissauer's tract

---

None

Very sparse

LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract --None Very sparse ven. cortricospinal tract --None Very sparse spinocerebellar tracts --Dense Dense ven. commissure Some fibers Dense Dense **intraspinal tracts --Gradient* Sparse ††spinocephalic tracts --Gradient† Sparse med. long. fasiculus --Dense Sparse tectospinal tract --Dense Sparse vestibulospinal tract --Sparse Sparse ** * †† †

Overlaps with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract. The central part of the tract (adjacent to the ventral and lateral parts of the ventral horn) stains more intensely than medial parts or dorsolateral parts. Contains ventral and lateral parts. Overlaps with the vestibulospinal, spinotectal, and spino-olivary tracts. Deep parts (adjacent to intraspinal tracts) are more dense than superficial parts (adjacent to the ventrolateral pial membrane).

97

PLATE 46

GW26, CR 210 mm, Y60-61 MYELIN STAIN

CELL BODY STAIN

Plates 47A, 47B Upper Cervical Enlargement Total area: 11.455 mm2

Plates 48A, 48B Upper Cervical Enlargement Total area: 12.070 mm2

Plates 49A, 49B Lower Cervical Enlargement Total area: 11.380 mm2

Plates 50A, 50B Lower Cervical Enlargement Total area: 12.589 mm2

Plates 51A, 51B Upper Thoracic Total area: 6.029 mm2

Plates 52A, 52B Upper Thoracic Total area: 6.982 mm2

98

PLATE 47A

GW26 CR 210 mm Y60-61 Upper Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0069

Ependyma

.0163

Gray matter

4.7293

White matter

6.7021

99

PLATE 47B See the matched cell body stained section in Plates 48A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Examples of reactive glia densities and myelination Sparse reactive glia Dense reactive glia No reactive glia

S

Lissauer's tract

Do

rsa

oo

o

t

c

e r e

l a i n tera al tr p La spin co

rti

co

Myelination

lr

b

ct

e

Dorsal gray

a r l l

Ventral gray

t r a c tr t s ac ? ts

Ependyma

Lateral funiculus

Central canal Ventral gray commissure Ventral white commissure

li

a

Ventral funiculus Ventral rootlets (myelinated)

Ventral median fissure

ha

n i p s a ce o I n t r in Sp

p

Ventral corticospinal tract

c

l

Myelinated proximal axons of ventral horn motoneurons

t

l tra

roo

n Ve

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

The fine lines in the lateral and ventral funiculi segregate regions of differing densities of reactive glia, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar and the lateral corticospinal tracts. Other lines run across fiber tracts, such as the dense region adjacent to the ventral and lateral part of the ventral horn that includes the central part of the intraspinal tracts and the deep parts of the spinocephalic tracts. Many of the fiber tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The spinotectal, spino-olivary, spinoreticular, and ventrolateral reticulospinal tracts (all unlabeled) intermingle with the spinocephalic fibers. The medial longitudinal fasciculus and tectospinal tract are interspersed with the medial fibers of the intraspinal tracts; dorsal and lateral fibers of the intraspinal tract are infiltrated by the lateral reticulospinal tract (unlabeled).

t r a c t s

Intermediate gray

100

PLATE 48A GW26 CR 210 mm Y60-61 Upper Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0055

Ependyma

.0206

Gray matter

5.0505

White matter

6.9934

101

PLATE 48B See the matched myelin stained section in Plates 47A and B

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Example of dense proliferating glia Dorsal root boundary cap

Lissauer's tract Do rsa l ro ot S

Dorsal horn

e er oc in p co rti t co ac al l tr ter na La spi

Lamina I Substantia gelatinosa Laminae IV-V

be

lla acts r tr

Central autonomic area

Lateral cervical nucleus Intermediate interneurons digits? wrist and hand?

wrist?

forearm? arm?

Lateral funiculus

Central canal

Central cervical nucleus?

Ependyma Ventral gray commissure Ventral white commissure

Ventral horn interneurons neck?

shoulder?

Ventral horn motoneurons

Ventral corticospinal tract

Example of sparse proliferating glia

t

Ventral funiculus

n Ve

l tra

Ventral median fissure

Example of very sparse proliferating glia Only the corticospinal and spinocerebellar tracts can be clearly delineated in the ventral and lateral funiculi. The corticospinal tracts stand out as clear areas with very sparse proliferating glia. The spinocerebellar tracts have a more dense concentration of proliferating glia. A sparse population of proliferating glia fills the remaining ventral and lateral funiculi and contains several fiber tracts (unlabeled in this section). Refer to the matching myelin stained section for the approximate locations of the medial longitudinal fasciculus, tectospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

roo

102

PLATE 49A GW26 CR 210 mm Y60-61 Lower Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0090

Ependyma

.0196

Gray matter

5.1514

White matter

6.2003

103

PLATE 49B See the matched cell body stained section in Plates 50A and B

Dorsal funiculus

Dorsal median septum Dorsal intermediate septum

Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Examples of reactive glia densities and myelination Sparse reactive glia Dense reactive glia in collateralization zone Myelination in bifurcation zone

Lissauer's tract

No reactive glia

t oo llar lr be rsa re Do ce o in l l era ina p S Lat osp t c rti ac co tr

Dorsal root boundary cap

Dorsal gray

Lateral funiculus Central canal Ependyma

Ventral gray commissure Ventral white commissure

l

Ventral median fissure

in

n S pi

a

oc

lic

Intrasp

ha

Ventral corticospinal tract

Ventral funiculus

Ventral root and rootlets (myelinated)

tra

tr

ac ts? cts?

Ventral gray Myelinated proximal axons of ventral horn motoneurons

trac ts

Intermediate gray

ep

l tra

n Ve Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

ot

ro

104

PLATE 50A GW26 CR 210 mm Y60-61 Lower Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0086

Ependyma

.0234

Gray matter

5.3014

White matter

7.2555

105

PLATE 50B See the matched myelin stained section in Plates 49A and B

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Example of dense proliferating glia Dorsal root boundary cap

Lissauer's tract e oc in t p oo S co lr rti rsa co ct Do al tra ter nal La spi

Dorsal horn

Lamina I Substantia gelatinosa Laminae IV-V

wrist and hand? wrist? forearm?

Ependyma

Lateral funiculus

Ventral gray commissure Ventral horn interneurons

Ventral white commissure

arm?

oo lr

Ventral funiculus

tra

Ventral corticospinal tract

Ve n

Ventral horn motoneurons

t

neck? shoulder?

Example of sparse proliferating glia Example of very sparse proliferating glia

Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the medial longitudinal fasciculus, tectospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

ellar tracts

Central autonomic area

Central cervical nucleus?

digits?

reb

Central canal

Intermediate interneurons

Lateral cervical nucleus

106

PLATE 51A GW26 CR 210 mm Y60-61 Upper Thoracic Myelin stain

Areas (mm2) Central canal

.0071

Ependyma

.0144

Gray matter

2.3030

White matter

3.7045

107

PLATE 51B See the matched cell body stained section in Plates50A and B

Dorsal median septum Dorsal intermediate septum

Dorsal funiculus

Examples of reactive glia densities and myelination

Superficial fasciculus gracilis Deep fasciculus gracilis Superficial fasciculus cuneatus

Sparse reactive glia Dense reactive glia

Deep fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Myelinating? No reactive glia

Lissauer's tract

Dorsal gray S p i

t r a c t s al ? ic tra cts ? ph

i

p t r a s

ce

In

o

Ventral

Ventral corticospinal funiculus tract

n

a

l

Ventral gray

t r a c t s

Ependyma Central canal Ventral gray commissure Ventral white commissure

l l a r o c e r e b e

Lateral funiculus

n

al al ter pin La icos ct rt ra co t

Intermediate gray

i Sp

Ventral rootlets (myelinated)

Ventral median fissure

n

Ventral root

Vestibulospinal tract?

108

PLATE 52A GW26 CR 210 mm Y60-61 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0083

Ependyma

.0204

Gray matter

2.8518

White matter

4.1012

109

PLATE 52B See the matchedmyelin stained section in Plates 48A and B

Dorsal funiculus

Dorsal median septum Dorsal intermediate septum

Fasciculus gracilis Fasciculus cuneatus

Example of dense proliferating glia

Dorsal root bifurcation zone Dorsal root collateralization zone

Dorsal Dorsal horn horn

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

S

p i n

Lateral horn motoneurons Intermediate interneurons

Lateral funiculus

Central canal

Clarke's column Central autonomic area Ventral gray commissure Ventral white commissure trunk

Ventral horn motoneurons

Ventral corticospinal tract

Ventral funiculus Example of sparse proliferating glia Example of very sparse proliferating glia

Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the vestibulospinal, intraspinal, and spinocephalic tracts.

t r a c t s

Ependyma

Ventral horn interneurons

l a r e l o c e r e b

Lateral corticospinal tract

110

IV. The Third Trimester A. Matched myelin and cell body stained sections in the spinal cord of a GW31 fetus

Plate 53 is a survey of matched myelin stained and cell body stained sections from Y162-61, a specimen in the Yakovlev Collection with a crown rump length of 270 mm (see Chapter 6 in Altman and Bayer, 2001). All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post-fixation) of the section in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 54A-61A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 54B-61B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without any numbers. Thirteen myelin stained and 13 cell body stained sections were photographed ranging from lower thoracic to sacral/coccygeal levels. There were no sections preserved at cervical, upper thoracic, and middle thoracic levels. The 26 photographic prints were intuitively arranged in order from thoracic to sacral/ coccygeal levels, using internal features such as the diminishing size of the corticospinal tracts from rostral to caudal levels, and the width of the ventral horn. Then, the myelin stained and the cell body stained sections were matched. As in the previous specimen, the cross-sectional area of a myelin stained section is smaller than the matching cell body stained section in all cases. Evidently, the myelin staining procedure produces greater tissue shrinkage than the cell body staining procedure. The lumbar enlargement expands relative to regions above and below. Using the myelin-stained section areas for comparison, the lumbar enlargement is 90% larger than the lower thoracic level, 41% larger than the upper lumbar level, and 278% larger than the sacral/coccygeal level. Myelination in this specimen is more advanced than in the previous specimen, even though the two are only 5 weeks apart, and we are dealing with a more immature (caudal) region of the spinal cord. Dense staining indicative of true myelination is seen in the ventral commissure, the ventral rootlets, the dorsal root bifurcation zone, the dorsal root collateralization zone, and deep regions of the fasciculus gracilis (see column 2 in Table IV A-1). Myelinated fibers from the dorsal root collateralization zone penetrate the gray matter, and there is a light dusting of reactive glia in the subgelatinosal plexus in the dorsal gray as finer collateral axons prepare for later myelination (see Fig. 6-39 in Altman and Bayer, 2001). Clumps of myelinated axons are in the lateral part of the intermediate gray and the lateral neck region of the dorsal gray (the reticulated area, labeled

in Figs. 9-25 through 9-31 in Altman and Bayer, 2001). The lateral corticospinal tract and Lissauer’s tract contain very sparse reactive glia. The dorsal root bifurcation zone is a complex area that not only contains heavily myelinated fibers, but also unmyelinated fibers. The heavily myelinated axons are presumably from large sensory neurons in the dorsal root ganglia, while the unmyelinated axons are from small nociceptive ganglion cells. The remaining fiber tracts contain varying densities of reactive glia (see column 3 in Table IV A-1). In the cell body stained sections, there is a different density of proliferating glia in various fiber tracts. It is lowest in the lateral and ventral corticospinal tracts and highest in the dorsal funiculus (except Lissauer’s tract) and spinocerebellar tracts (see column 4 in Table IV A-1). In the cell body stained sections, columns of motoneurons continue to be prominent and show greater degrees of segregation in the ventral horn. The dorsal horn has fairly well-defined clusters of small neurons in the substantia gelatinosa. The accumulation of lateral horn motoneurons is very obvious at the thoracic level. Clarke’s column is also prominent in this specimen.

Table IV A-1: Glia types and concentration in the white matter at GW31

Name

Proliferating glia Reactive glia Myelination

DORSAL FUNICULUS: dorsal root bif. zone

Many fibers*

---

Sparse

dorsal root col. zone

Many fibers

---

Very dense Very dense

deep fas. gracilis

Many fibers

---

superficial fas. gracilis

Some fibers

Dense

Dense

---

None

Very sparse

Lissauer's tract LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract ven. cortricospinal tract rubrospinal tract spinocerebellar tracts lat. reticulospinal tract intraspinal tracts spinocephalic tracts vestibulospinal tract ven. commissure

--Very sparse Very sparse --Sparse Very sparse Some fibers Dense Sparse Some fibers Dense Dense Some fibers Dense Sparse Some fibers Dense Sparse Some fibers Dense Sparse Some fibers Dense Sparse Many fibers --Sparse

* intermingled in a bed of nonreactive glia (associated with Lissauer's tract fibers?)

111

PLATE 53

GW31, CR 270 mm, Y162-61 MYELIN STAIN

CELL BODY STAIN

Plates 54A, 54B Lower Thoracic Total area: 5.1272 mm2

Plates 55A, 55B Lower Thoracic Total area: 5.2380 mm2

Plates 56A, 56B Upper Lumbar Total area: 6.9121 mm2

Plates 57A, 57B Upper Lumbar Total area: 7.1380 mm2

Plates 58A, 58B Lumbar Enlargement Total area: 9.7553 mm2

Plates 60A, 60B Sacral/Coccygeal Total area: 3.5153 mm2

Plates 59A, 59B Lumbar Enlargement Total area: 10.918 mm2

Plates 61A, 61B Sacral/Coccygeal Total area: 3.5751 mm2

112

PLATE 54A GW31 CR 270 mm Y162-61 Lower Thoracic Myelin stain

Areas (mm2) Central canal

.0012

Ependyma

.0057

Gray matter

1.4171

White matter

3.7032

113

PLATE 54B See the matched cell body stained section in Plates 55A and B

Dorsal median septum

Dorsal funiculus

Examples of myelination and concentrations of reactive glia

Deep fasciculus gracilis

Some myelinating fibers

Superficial fasciculus gracilis

Many myelinating fibers

Dorsal root collateralization zone

Dense reactive glia Very sparse reactive glia

Dorsal root bifurcation zone Lissauer's tract

S Lateral corticospinal tract

Rubrospinal tract?

Ventral funiculus

Ventral gray commissure Ventral white commissure Ventral corticospinal tract?

ts? trac

ic al

h

I n

t

Central canal with surrounding ependyma

s?

Ventral gray

tra c

Lateral funiculus

Reticulated area t r a c t s ?

Clarke's column

t

Intermediate gray

no c e r e b e ll a r

Myelinating terminals of dorsal root collaterals

pi

Dorsal gray Lateral reticulospinal tract?

Incipient subgelatinosal plexus

l ep r a s p i n a c o in Sp

Vestibulospinal tract? Ventral median fissure

An example of sparse reactive glia The fine lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar and the lateral corticospinal tracts. Other lines run across fiber tracts, such as the area surrounding the ventral horn that includes the central part of the intraspinal tracts, the vestibulospinal tract, and the ventral and central fibers of the spinocephalic tracts. Most tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The ventral part of the spinocephalic tracts overlaps with the vestibulospinal tract; its central part includes fibers in the ventrolateral reticulospinal tract (not labeled), the spinotectal tract (not labeled), and the spino-olivary tract (not labeled). The intraspinal tracts overlap with the lateral reticulospinal tract. Some fibers of the rubrospinal tract may intermingle with ventral fibers of the lateral corticospinal tract.

114

PLATE 55A

GW31 CR 270 mm Y162-61 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0011

Ependyma

.0087

Gray matter

1.5228

White matter

3.7054

115

PLATE 55B See the matched myelin stained section in Plates 54A and B

Dorsal median septum Examples of concentrations of proliferating glia

Dorsal funiculus Deep fasciculus gracilis

Dense Very dense

Superficial fasciculus gracilis Dorsal root collateralization zone

Sparse Very sparse

Dorsal root bifurcation zone Lissauer's tract

Dorsal Lamina I horn Substantia gelatinosa Laminae IV-V

Ventral horn motoneurons

Ventral funiculus Ventral median fissure

Ventral gray commissure

ts trac cer

ebe

Central canal surrounded by ependyma

Ventral horn interneurons trunk motoneurons

llar

Clarke's column Intermediate interneurons

ino

Lateral funiculus

Central autonomic area

Sp

Lateral horn motoneurons

Lateral corticospinal tract

Ventral white commissure

Ventral corticospinal tract?

Only the corticospinal and spinocerebellar tracts can be clearly delineated in the ventral and lateral funiculi. The corticospinal tracts stand out as clear areas with very sparse proliferating glia. The spinocerebellar tracts have a slightly more dense concentration of proliferating glia. A sparse population of proliferating glia fills the remaining ventral and lateral funiculi and contains several fiber tracts (unlabeled in this section). Refer to the matching myelin stained section for the approximate locations of the rubrospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

116

PLATE 56A GW31 CR 270 mm Y162-61 Upper Lumbar Myelin stain

Areas (mm2) Central canal

.0038

Ependyma

.0110

Gray matter

2.5641

White matter

4.3333

117

PLATE 56B See the matched cell body stained section in Plates 57A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Dorsal median septum

Dorsal root collateralization zone

Myelinating terminals of dorsal root collaterals

Dorsal root bifurcation zone

Lissauer's tract Incipient subgelatinosal plexus

Dorsal gray

Lateral corticospinal tract

r tr acts ?

Clarke's column

Intermediate gray

Reticulated area

be

lla

Ventral gray commissure Ventral white commissure

Ventral gray

Ventral median fissure

o Spin

ce

Vestibulospinal tract

The increased density of the myelin stain that was interpreted to be the rubrospinal and lateral reticulospinal tracts in Plate 54 cannot be detected in this section, so these fiber tracts are not labeled. However, the medial and central fibers in the intraspinal tracts have a greater staining density than the dorsolateral parts. That same gradient is seen in the thoracic cord of the GW26 specimen (Plate 51).

c

tr

li

I

nt l raspina

p

ha

ce no

pi

a

Ventral funiculus

Central canal with surrounding ependyma

tr

c

ac

ts

ts

?

?

re

Lateral funiculus

S

118

PLATE 57A GW31 CR 270 mm Y162-61 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0050

Ependyma

.0135

Gray matter

2.7017

White matter

4.4178

119

PLATE 57B See the matched myelin stained section in Plates 56A and B

Dorsal funiculus Dorsal median septum

Dorsal horn

Lamina I

Substantia gelatinosa Laminae IV-V

Deep fasciculus gracilis Superficial fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Lissauer's tract

Central canal surrounded by ependyma

trunk?

act

leg? trunk?

hip?

Ventral funiculus

ce

reb

hip?

trunk?

Central autonomic area Ventral gray commissure Ventral white commissure ell ar tr

Ventral horn interneurons

Clarke's column

no

Lateral funiculus

Intermediate interneurons

Lateral corticospinal tract?

s

Lateral horn motoneurons

Sp Ventral horn motoneurons

Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the vestibulospinal, intraspinal, and spinocephalic tracts.

i

120

PLATE 58A GW31 CR 270 mm Y162-61 Lumbar Enlargement Myelin stain

Areas (mm2) Central canal

.0030

Ependyma

.0085

Gray matter

4.9128

White matter

4.8309

121

PLATE 58B See the matched cell body stained section in Plates 59A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis

Dorsal median septum Myelinating terminals of dorsal root collaterals

Dorsal root collateralization zone Dorsal root bifurcation zone

Incipient subgelatinosal plexus

Dorsal gray

Intermediate gray

Lateral funiculus

al l ter na La cospi rti ct co tra

Reticulated area Central canal surrounded by ependyma

Lissauer's tract

Ventral gray commissure Ventral white commissure

Ventral gray

I

t s ?

Myelinated proximal axons of ventral horn motoneurons

c

n

Myelinated ventral rootlets

Ventral median fissure

s p ? i n t s i c c a l t r a a l S p i n o c e p h Vestibulospinal tract?

t

a

r

a

t

r

Ventral funiculus

122

PLATE 59A GW31 CR 270 mm Y162-61 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0028

Ependyma

.0154

Gray matter

5.7639

White matter

5.1354

123

PLATE 59B See the matched myelin stained section in Plates 58A and B

Dorsal funiculus Deep fasciculus gracilis Lamina I Dorsal horn Substantia

Dorsal median septum

Superficial fasciculus gracilis Dorsal root collateralization zone

gelatinosa Laminae IV-V

Dorsal root bifurcation zone Lissauer's tract

foot?

Ventral horn interneurons

Central canal surrounded by ependyma Central autonomic area Ventral gray commissure Ventral white commissure

thigh? leg and ankle? hip? leg?

lower trunk?

hip and thigh?

Ventral horn motoneurons

Ventral funiculus Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the vestibulospinal, intraspinal, and spinocephalic tracts.

l era al Lat ospin tic ? cor tract

Intermediate interneurons

Lateral funiculus

124

PLATE 60A

GW31 CR 270 mm Y162-61 Sacral/Coccygeal Myelin stain

Areas (mm2) Central canal

.0053

Ependyma

.0184

Gray matter

2.1433

White matter

1.3482

125

PLATE 60B See the matched cell body stained section in Plates 61A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract

Lateral corticospinal tract?

Dorsal gray

Lateral funiculus Intermediate gray Central canal surrounded by ependyma

ac ts ?

Ventral gray commissure Ventral white commissure ?

tr

Ventral gray

I

Ventral funiculus The border of the lateral corticospinal tract is no longer detectable in this section because the dorsal half of the lateral funiculus has very sparse reactive glia. The fiber tracts at this level of the spinal cord are more immature than the three rostral levels shown in this specimen. Thus, the overall rostral (first) to caudal (last) gradient of maturation is evident in this third trimester specimen.

s c ct li a tr ha nt raspinal cep o S pin

Ventral median fissure

Dark areas that are likely edge artifact staining

126

PLATE 61A

GW31 CR 270 mm Y162-61 Sacral/Coccygeal Cell body stain

Areas (mm2) Central canal

.0046

Ependyma

.0253

Gray matter

2.2183

White matter

1.3268

127

PLATE 61B See the matched myelin stained section in Plates 60A and B

Dorsal funiculus Dorsal median septum

Dorsal horn

Fasciculus gracilis Dorsal root collateralization zone

Lamina I

Dorsal root bifurcation zone

Substantia gelatinosa

Laminae IV-V

Lissauer's tract

Lateral corticospinal tract?

Intermediate interneurons

Central autonomic area

Lateral funiculus

Central canal surrounded by ependyma

Ventral horn interneurons

Ventral gray commissure Ventral white commissure lower trunk?

Ventral funiculus Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the intraspinal and spinocephalic tracts.

Ventral horn motoneurons?

128

Part IV: The Third Trimester (concluded)

B. Matched myelin and cell body stained sections in the spinal cord of a GW37 fetus Plate 62 is a survey of matched myelin stained and cell body stained sections from Y117-61, a specimen in the Yakovlev Collection with a crown rump length of 310 mm (see Chapter 6 in Altman and Bayer, 2001). All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post fixation) of the section in square millimeters (mm2). Fullpage normal contrast photographs of each specimen are in Plates 63A-78A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 63B-78B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without any section numbers. This specimen has the most complete set of spinal cord sections in the Yakovlev Collection with all levels preserved. Twenty myelin stained sections and 19 cell body stained sections were photographed ranging from upper cervical to sacral/ coccygeal levels. The 39 photographic prints were intuitively arranged in order from upper cervical to sacral/ coccygeal levels, using internal features such as the size of the corticospinal tracts and the width of the ventral horn. Then, myelin and cell body stained sections were matched and eight different levels were analyzed. As in the previous specimens, the cross-sectional area of a myelin stained section is smaller than the matching cell body stained section in all cases, except at the upper thoracic level, where the areas are the same. Using the total areas of the myelin stained sections, the overall size differences between levels indicate the following: the cervical enlargement has the largest area, being larger than the lumbar enlargement by 30%. The middle thoracic level has the smallest area and is 6% smaller than the sacral/ coccygeal level. Myelination continues to advance in this specimen (see columns 2 and 3 in Table IV B-1). Dense staining indicative of either advanced or beginning myelination is seen throughout the ventral funiculus and the lateral funiculus except the ventral and lateral corticospinal tracts, the sacral/coccygeal level excepted. All parts of the dorsal funiculus are either myelinated or are myelinating, except Lissauer’s tract and the areas in the dorsal root bifurcation zone which most likely contains incoming axons that will

join Lissauer’s tract. The reticulated area is more prominently myelinated in the lateral intermediate gray and lateral neck region of the dorsal gray. The reactive glia in the subgelatinosal plexus is more concentrated, indicating that terminal parts of the dorsal root collaterals are approaching myelination. It is interesting to note that the white matter in the cell body stained sections (except the sacral/ coccygeal level, Plate 78) show a uniformly reduced density of glia, suggesting the decline or cessation of myelination gliosis (see column 4 in Table IV B-1). But again, the lateral and ventral corticospinal tracts generally stand out as having slightly lower concentrations of glia; in these tracts, myelination gliosis has not yet begun.

Table IV B-1: Glia types and concentration in the white matter at GW37

Name

Proliferating glia Reactive glia Myelination

DORSAL ROOT

Myelinated

---

Dense

VENTRAL ROOT

Myelinated

---

---

Many fibers*

---

Sparse

Myelinated

---

Sparse†

DORSAL FUNICULUS: dorsal root bif. zone dorsal root col. zone deep fas. gracilis

Myelinated

---

Sparse

superficial fas. gracilis

Many fibers

---

Sparse

deep fas. cuneatus

Myelinated

---

Sparse

superficial fas. cuneatus

Many fibers

---

Sparse

---

None

Sparse

Lissauer's tract LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract ven. cortricospinal tract lat. reticulospinal tract spinocerebellar tracts ven. commissure intraspinal tract spinocephalic tract med. long. fasiculus vestibulospinal tract

--Very sparse Very sparse --Very sparse Very sparse Many fibers --Sparse Many fibers Sparse --Myelinated --Sparse Many fibers --Sparse Some fibers Very dense Sparse Many fibers --Sparse --Dense Sparse

* intermingled in a bed of nonreactive glia (associated with Lissauer's tract fibers?) † dense at the sacral/coccygeal level

129

PLATE 62 GW37, CR 310 mm, Y117-61 MYELIN STAIN

CELL BODY STAIN

Plates 63A, 63B Upper Cervical Total area: 15.138 mm2

Plates 64A, 64B Upper Cervical Total area: 15.896 mm2

Plates 65A, 65B Cervical Enlargement Total area: 16.046 mm2

Plates 66A, 66B Cervical Enlargement Total area: 17.036 mm2

Plates 67A, 67B Upper Thoracic Total area: 8.4267 mm2

Plates 68A, 68B Upper Thoracic Total area: 8.4266 mm2

Plates 69A, 69B Middle Thoracic Total area: 6.1539 mm2

Plates 70A, 70B Middle Thoracic Total area: 6.7020 mm2

Plates 71A, 71B Lower Thoracic Total area: 7.2005 mm2

Plates 72A, 72B Lower Thoracic Total area: 7.9554 mm2

Plates 73A, 73B Upper Lumbar Total area: 11.530 mm2

Plates 74A, 74B Upper Lumbar Total area: 12.297 mm2

Plates 75A, 75B Lumbar Enlargement Total area: 12.297 mm2

Plates 76A, 76B Lumbar Enlargement Total area: 12.872 mm2

Plates 77A, 77B Sacral/Coccygeal Total area: 6.5068 mm2

Plates 78A, 78B Sacral/Coccygeal Total area: 6.7829 mm2

130

PLATE 63A GW37 CR 310 mm Y117-61 Upper Cervical Myelin stain

Areas (mm2) Central canal

.0044

Ependyma

.0102

Gray matter

5.2519

White matter

9.8713

131

PLATE 63B See the matched cell body stained section in Plates 64A and B

Dorsal median septum Dorsal intermediate septum

Dorsal funiculus

Myelinating terminals of dorsal root collaterals

Deep fasciculus gracilis Superficial fasciculus gracilis

Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract Sp i

Incipient subgelatinosal plexus

Dorsal gray

ce

re be

Ventral funiculus Very dense reactive glia Some myelinated fibers Examples of myelination and concentrations of reactive glia

Ventral median fissure

r I nt

a

Sp

in

na

o

l

p ce

ac

c

i sp

tr

tr

Ventral corticospinal tract

li

Myelinated ventral rootlets

ac

Myelinated proximal axons of ventral horn motoneurons

ts?

Ventral gray Lateral funiculus

ts?

Reticulated area Spinal canal with surrounding ependyma Ventral gray commissure Ventral white commissure

Intermediate gray

Lateral reticulospinal tract?

s? ract r t

co

lla

rti L co ate sp ra in l al tra

Deep fasciculus cuneatus

no

ct

Examples of myelination and concentrations of reactive glia Dorsal root Myelinated Very sparse reactive glia Many myelinated fibers

ha

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

The fine lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar and the lateral corticospinal tracts. Other lines run across fiber tracts, such as the area surrounding the ventral horn that includes the medial and central parts of the intraspinal tracts, and the internal fibers of the spinocephalic tracts. Most tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The ventral part of the spinocephalic tracts overlaps with the vestibulospinal tract; its central part includes fibers in the ventrolateral reticulospinal tract (not labeled), the spinotectal tract (not labeled), and the spino-olivary tract (not labeled). The intraspinal tracts overlap with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract.

132

PLATE 64A GW37 CR 310 mm Y117-61 Upper Cervical Cell body stain

Areas (mm2) Central canal

.0038

Ependyma

.0175

Gray matter

5.8125

White matter

10.0620

133

PLATE 64B See the matched myelin stained section in Plates 63A and B

Dorsal funiculus Dorsal median septum

Deep fasciculus gracilis

Dorsal intermediate septum

Superficial fasciculus gracilis Deep fasciculus cuneatus

Dorsal Dorsal horn horn

Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

Spi

no

Lateral corticospinal tract

be

Ventral horn interneurons

shoulder?

Ventral horn motoneurons Sparse Very sparse Examples of concentrations of proliferating glia

Central canal with surrounding ependyma

Ventral white commissure

Ventral gray commissure

Lateral funiculus

neck and skull?

Ventral corticospinal tract

Ventral funiculus

Ventral median fissure

Only the corticospinal and spinocerebellar tracts can be clearly delineated in the ventral and lateral funiculi. The corticospinal tracts stand out as clear areas with very sparse proliferating glia. The spinocerebellar tracts have a slightly more dense concentration of proliferating glia. A sparse population of proliferating glia fills the remaining ventral and lateral funiculi and contains several fiber tracts (unlabeled in this section). Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

r tracts?

nd arm ader? l shou

lla

Central cervical nucleus? Intermediate interneurons

forearm and wrist?

re

Lateral cervical nucleus

ce

Central autonomic area

134

PLATE 65A GW37 CR 310 mm Y117-61 Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0073

Ependyma

.0137

Gray matter

6.0566

White matter

9.9683

135

PLATE 65B See the matched cell body stained section in Plates 66A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Myelinating terminals of dorsal root collaterals

Incipient subgelatinosal plexus

Dorsal gray

la r

ct ral l tra e t a La spin o c rti

Lissauer's tract Sp in oc er eb el

Myelinated ventral rootlets Ventral median fissure

ic

tr

ac

?

al

t r a s p i cep o in Sp

n

In

al

t rac

ract corti Ventral cosp inal t

Ventral funiculus

ts

ts?

Reticulated Central canal area surrounded by ependyma Ventral gray commissure Ventral white commissure

Intermediate gray Ventral gray Myelinated proximal axons of ventral horn motoneurons

ts?

Lateral funiculus

ac

Lateral reticulospinal tract?

tr

co

h

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus?

136

PLATE 66A GW37 CR 310 mm Y117-61 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0079

Ependyma

.0230

Gray matter

6.6710

White matter

10.3340

137

PLATE 66B See the matched myelin stained section in Plates 65A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Lamina I Dorsal horn Substantia gelatinosa Laminae IV-V

Lissauer's tract Sp

in

er

eb

el

l

digits?

arm and forearm?

Ventral horn interneurons Ventral white commissure

Central canal surrounded by ependyma Ventral gray commissure

neck? shoulder and arm?

Ventral corticospinal tract

shoulder?

Ventral horn motoneurons

Ventral funiculus Ventral Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

cts?

Intermediate interneurons wrist and hand?

Central autonomic area

Central cervical nucleus?

tra

Lateral cervical nucleus

oc

ar

Lateral corticospinal tract

138

PLATE 67A

GW37 CR 310 mm Y117-61 Upper Thoracic Myelin stain

Areas (mm2) Central canal

.0037

Ependyma

.0086

Gray matter

2.1599

White matter

6.2545

139

PLATE 67B See the matched cell body stained section in Plates 68A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Dorsal median septum

Deep fasciculus cuneatus

Dorsal intermediate septum Myelinating terminals of dorsal root collaterals

Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root Lissauer's tract

S

er

Intermediate gray

Reticulated area Central canal surrounded by ependyma

Lateral funiculus

Ventral funiculus

Int

ra

na

tra

ph

i sp

ce

Ventral corticospinal tract

al

tr

ic

ac

ts

?

Ventral gray

cts?

Ventral gray commissure Ventral white commissure

l

Lateral reticulospinal tract?

llar tracts?

cor

ebe

Dorsal gray

oc

ticoLater spi al nal tra

in

ct

p

Incipient subgelatinosal plexus

i Sp

no

Vestibulospinal tract? Ventral median fissure The medial and central fibers in the intraspinal tracts have a greater staining density than the dorsolateral parts. That same gradient is seen at the thoracic level of the GW26 (Plate 51) and GW 31 (Plate 56) specimens.

140

PLATE 68A GW37 CR 310 mm Y117-61 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0030

Ependyma

.0152

Gray matter

2.3167

White matter

6.0916

141

PLATE 68B See the matched myelin stained section in Plates 67A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Dorsal root

Dorsal horn Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract Sp

in oc

er eb ell

Intermediate interneurons Central canal with surrounding ependyma

Ventral horn interneurons

Ventral white commissure

Ventral gray commissure

middle trunk?

Ventral corticospinal tract Ventral horn motoneurons

Ventral funiculus Ventral

Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

ts?

Lateral funiculus

Central autonomic area

Clarke's column?

trac

Lateral horn motoneurons

ar

Lateral corticospinal tract

142

PLATE 69A GW37 CR 310 mm Y117-61 Middle Thoracic Myelin stain

Areas (mm2) Central canal

.0029

Ependyma

.0069

Gray matter

1.5695

White matter

4.5747

143

PLATE 69B See the matched cell body stained section in Plates 70A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis

Dorsal median septum

Deep fasciculus cuneatus Dorsal intermediate septum

Superficial fasciculus cuneatus

Myelinating terminals of dorsal root collaterals

Dorsal root collateralization zone

Do

rsa

lr

oo

t

Dorsal root bifurcation zone

Lissauer's tract

Reticulated area Central canal surrounded by ependyma

Intermediate gray

t r a cts ?

Ventral gray commissure Ventral white commissure

Ventral funiculus

tra

a lic

cts

?

t

Ventral gray

I n trasp

l

ph

ina

ce

Ve corticos ntral pinal tra c

La cortic teral osp tract inal

Lateral funiculus

erebellar tracts?

Lateral reticulospinal tract?

inoc

Dorsal gray

Sp

Incipient subgelatinosal plexus

Sp

in

o

Vestibulospinal tract? Ventral median fissure

144

PLATE 70A GW37 CR 310 mm Y117-61 Middle Thoracic Cell body stain

Areas (mm2) Central canal

.0045

Ependyma

.0132

Gray matter

1.6976

White matter

4.9867

145

PLATE 70B See the matched myelin stained section in Plates 69A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Deep fasciculus cuneatus Superficial fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Dorsal root

Lamina I Dorsal Substantia horn gelatinosa Lissauer's tract

Laminae IV-V Spi

noc

Ventral horn interneurons

ar tracts?

Lateral funiculus

Intermediate interneurons

erebell

Lateral corticospinal tract Lateral horn motoneurons

Central autonomic area

Clarke's column?

Central canal with surrounding ependyma

middle trunk?

Ventral corticospinal tract

Ventral white commissure Ventral horn motoneurons

Ventral gray commissure

Ventral funiculus

Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

146

PLATE 71A GW37 CR 310 mm Y117-61 Lower Thoracic Myelin stain

Areas (mm2) Central canal

.0057

Ependyma

.0070

Gray matter

2.3538

White matter

4.8340

147

PLATE 71B See the matched cell body stained section in Plates 72A and B

Dorsal funiculus Dorsal median septum

Superficial fasciculus gracilis Deep fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Do

rsa

lr

oo

t

Myelinating terminals of dorsal root collaterals

Intermediate gray

ocerebellar tracts? Spin

La cortic teral ospi tract nal

Dorsal gray

Reticulated area

Lissauer's tract

Incipient subgelatinosal plexus Lateral reticulospinal tract? Central canal surrounded by ependyma Ventral gray commissure Ventral white commissure

Ventral gray

Ventral funiculus Ventral corticospinal tract Ventral median fissure

s? tr a

ct

In

ha

li c

tr a

c ts

?

Lateral funiculus

spin al t

ra

S

o pin

ce

Vestibulospinal tract?

p

148

PLATE 72A

GW37 CR 310 mm Y117-61 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0031

Ependyma

.0130

Gray matter

2.5442

White matter

5.3951

149

PLATE 72B See the matched myelin stained section in Plates 71A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis

Dorsal median septum

Dorsal root collateralization zone

Dorsal Lamina I horn Substantia

Dorsal root bifurcation zone

cor Later tico al trac spina l t

gelatinosa Laminae IV-V

Lateral horn motoneurons

Lissauer's tract

Central autonomic area Clarke's column?

Spinocerebellar tracts

Intermediate interneurons

Central canal with surrounding ependyma Ventral gray commissure Ventral white commissure

Ventral horn interneurons

middle trunk?

Ventral horn motoneurons

Ventral corticospinal tract

Ventral funiculus

Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

150

PLATE 73A GW37 CR 310 mm Y117-61 Upper Lumbar Myelin stain

Areas (mm2) Central canal

.0109

Ependyma

.0113

Gray matter

5.4305

White matter

6.0778

151

PLATE 73B See the matched cell body stained section in Plates 74A and B

Dorsal funiculus Superficial fasciculus gracilis

Dorsal median septum

Deep fasciculus gracilis Dorsal root collateralization zone

Myelinating terminals of dorsal root collaterals

Dorsal root bifurcation zone Lissauer's tract

Lateral corticospinal tract

Incipient subgelatinosal plexus

Dorsal gray Reticulated area

Lateral reticulospinal tract?

Intermediate gray

?

Ventral gray commissure cts

Ventral white commissure

Ventral gray

tra

Lateral funiculus

Central canal surrounded by ependyma

c t ce s ? ph a

li

c

Myelinated proximal axons of ventral horn motoneurons

Intraspin

al

Ventral funiculus Ventral rootlets Ventral root

Vestibulospinal tract? Ventral corticospinal tract Ventral median fissure

a t r no i Sp

152

PLATE 74A GW37 CR 310 mm Y117-61 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0087

Ependyma

.0178

Gray matter

5.7810

White matter

6.4894

153

PLATE 74B See the matched myelin stained section in Plates 73A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

co Lat rtic era o l tra spin ct al

Dorsal horn

Central canal with surrounding ependyma Central autonomic area Intermediate interneurons foot? ankle? thigh? leg?

thigh?

Ventral horn motoneurons

hip?

Ventral gray commissure Ventral white commissure

Ventral horn interneurons hip?

lower trunk?

Ventral corticospinal tract

Ventral funiculus

Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

154

PLATE 75A GW37 CR 310 mm Y117-61 Lumbar Enlargement Myelin stain

Areas (mm2) Central canal

.0043

Ependyma

.0087

Gray matter

6.6891

White matter

5.5948

155

PLATE 75B See the matched cell body stained section in Plates 76A and B

Dorsal funiculus Dorsal median septum

Superficial fasciculus gracilis Deep fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Myelinating terminals of dorsal root collaterals

l rsa

ot

Lissauer's tract

ro

Do

Incipient subgelatinosal plexus

Lateral corticospinal tract

Dorsal gray

Reticulated area

Intermediate gray Lateral reticulospinal tract?

Central canal surrounded by ependyma Ventral gray commissure

cts

Ventral white commissure

ic

tra

Myelinated proximal axons of ventral horn motoneurons

ce

ph

al

Lateral funiculus

?

Ventral gray

Ventral funiculus Ventral rootlets Ventral median fissure

? ts no c i a tr Sp l na Intraspi

Vestibulospinal tract? Ventral corticospinal tract?

156

PLATE 76A GW37 CR 310 mm Y117-61 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0036

Ependyma

.0140

Gray matter

6.7877

White matter

6.0663

157

PLATE 76B See the matched myelin stained section in Plates 75A and B

Dorsal funiculus Deep fasciculus gracilis Superficial fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

co Lat rtic era o l tra spin ct al

Dorsal horn

Central autonomic area Central canal with surrounding ependyma

Intermediate interneurons

foot?

Ventral horn interneurons

ankle? leg? leg?

Ventral gray commissure Ventral white commissure

thigh?

hip?

Ventral corticospinal tract

thigh?

Ventral horn motoneurons

lower trunk?

Ventral funiculus Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

158

PLATE 77A GW37 CR 310 mm Y117-61 Sacral/Coccygeal Myelin stain

Areas (mm2) Central canal

.0050

Ependyma

.0160

Gray matter

4.3017

White matter

2.1841

159

PLATE 77B See the matched cell body stained section in Plates 78A and B

Dorsal funiculus Fasciculus gracilis?

Dorsal median septum Dorsal root

Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract

Lateral corticospinal tract?

Dorsal gray

Intermediate gray Central canal surrounded by ependyma Ventral gray commissure

cts

?

Ventral white commissure

tra

Ventral gray

ic

Lateral funiculus

I

nt

Ventral funiculus Ventral median fissure

l s ? pha t c e r a s p i n a l t r a noc i Sp

Vestibulospinal tract?

160

PLATE 78A GW37 CR 310 mm Y117-61 Sacral/Coccygeal Cell body stain

Areas (mm2) Central canal

.0044

Ependyma

.0271

Gray matter

4.4850

White matter

2.2665

161

PLATE 78B See the matched myelin stained section in Plates 77A and B

Dorsal funiculus Fasciculus gracilis?

Dorsal median septum

Dorsal root bifurcation zone Lissauer's tract

Central autonomic area Central canal with surrounding ependyma

Lateral funiculus

Intermediate interneurons Ventral gray commissure Ventral white commissure

er

tru nk ?

Ventral horn interneurons low

Dorsal horn

Dorsal root collateralization zone

Lamina I Substantia gelatinosa Laminae IV-V

Ventral funiculus Ventral horn motoneurons

Ventral median fissure

In this section, the dorsal funiculus still shows some myelination gliosis, indicating that this level of the spinal cord is more immature than rostral levels. The lateral funiculus has a very sparse population of proliferating glia throughout, and the border of the lateral corticospinal tract is not visible. Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

162

V. The Early Postnatal Period: Infants at 4 days, 4 weeks, and 4 months

A. Matched myelin and cell body stained sections in the spinal cord of a 4-day-old infant Plate 79 is a survey of matched myelin stained and cell body stained sections from Y299-62, a specimen in the Yakovlev Collection with a crown rump length of 350 mm. All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area of the section in square millimeters (mm2). Fullpage normal contrast photographs of each specimen are in Plates 80A-91A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 80B-91B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without any section numbers. Twenty-two myelin stained sections and 22 cell body stained sections were photographed ranging from the cervical enlargement to sacral/coccygeal levels. The 44 photographic prints were intuitively arranged in order from upper cervical to sacral/ coccygeal levels, using internal features such as the size of the corticospinal tracts and the width of the ventral horn. Then myelin and cell body stained sections were matched and six different levels were analyzed. Since many sections in the middle thoracic region are damaged, that level is not illustrated. As in the previous specimens, the cross-sectional area of a myelin stained section is smaller than the matching cell body stained section in all cases. Using the total areas of the myelin stained sections, the overall size differences between levels indicate the following comparisons: the cervical enlargement is the level with the largest crosssectional area, being larger than the lumbar enlargement by 65%. The sacral/coccygeal level has the smallest crosssectional area and is 37% smaller than the lower thoracic level. Myelination in this specimen is slightly advanced compared to the GW37 specimen (columns 2 and 3, Table V A-1) because there is no longer a gradient between superficial and deep parts of the gracile and cuneate fasciculi, and more myelinated axons are in the subgelatinosal plexus. The ventral corticospinal tract is smaller in this specimen than the one at GW37. For a discussion of individual differences between specimens regarding the corticospinal tracts, see Chapter 7, Section 7.2.2 in Altman

and Bayer, 2001). The corticospinal tracts in the 4-dayold infant indicate more advanced myelination than in the GW37 fetus by having at least a sparse concentration of reactive glia and a high concentration of proliferating glia (columns 3 and 4, Table V A-1). Another indication of maturation is the sparse (medial) to very sparse (lateral) gradient of reactive glia in the lateral corticospinal tract. Myelination follows a medial to lateral gradient in that tract (Altman and Bayer, 2001). There is an overall gradient of sparse reactive glia (superficial fibers in the vestibulospinal and spinocephalic tracts) to the beginnings of myelination (deep fibers in the intraspinal tracts) in the ventral and lateral funiculi that cuts across the borders of the fiber tracts.

Table V A-1: Glia types and concentration in the white matter in a 4-day-old Infant

Name

Proliferating glia Reactive glia Myelination

DORSAL ROOT

Myelinated

---

Sparse**

VENTRAL ROOT

Myelinated

---

---

DORSAL FUNICULUS: dorsal root bif. zone

Many fibers*

---

Sparse

dorsal root col. zone

Myelinated

---

Sparse

fas. gracilis

Myelinated

---

Sparse

fas. cuneatus

Myelinated

---

Sparse

---

None

Sparse

Lissauer's tract

LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract --Very sparse ven. cortricospinal tract --Very sparse spinocerebellar tracts Myelinated --ven. commissure Myelinated --intraspinal tracts Many fibers --lat. reticulospinal tract Some fibers Very sparse spinocephalic tracts Some fibers† Very sparse• med. long. fasiculus Some fibers --vestibulospinal tract Some fibers --** * † •

Sparse Sparse Sparse Dense Dense Sparse Dense Sparse Sparse

dense band in a sparse field (part of the boundary cap?) intermingled in a bed of nonreactive glia few fibers at the cervical level peripheral fibers at the cervical level

163

PLATE 79 INFANT, 4 DAYS, CR 350 mm, Y299-62 MYELIN STAIN

CELL BODY STAIN

Plates 80A, 80B Cervical Enlargement Total Area: 20.063 mm2

Plates 81A, 81B Cervical Enlargement Total Area: 21.370 mm2

Plates 82A, 82B Upper Thoracic Total Area: 10.177 mm2

Plates 83A, 83B Upper Thoracic Total Area: 10.514 mm2

Plates 84A, 84B Lower Thoracic Total Area: 7.7961 mm2

Plates 85A, 85B Lower Thoracic Total Area: 8.2168 mm2

Plates 86A, 86B Upper Lumbar Total Area: 11.735 mm2

Plates 87A, 87B Upper Lumbar Total Area: 12.730 mm2

Plates 88A, 88B Lumbar Enlargement Total Area: 12.152 mm2

Plates 89A, 89B Lumbar Enlargement Total Area: 12.162 mm2

Plates 90A, 90B Sacral/Coccygeal Total Area: 5.6792 mm2

Plates 91A, 91B Sacral/Coccygeal Total Area: 5.9052 mm2

164

PLATE 80A Infant, 4 Days CR 350 mm Y299-62 Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0029

Ependyma

.0119

Gray matter

6.9875

White matter

13.0610

165

PLATE 80B See the matched cell body stained section in Plates 81A and B

Dorsal funiculus Dorsal median septum Dorsal intermediate septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Examples of myelination and concentrations of reactive glia Myelinated Very sparse reactive glia

Dorsal root Lissauer's tract

Sparse reactive glia

p

in

o c

Subgelatinosal plexus

llar ebe er

Dorsal gray

Intermediate gray

Lateral funiculus

tracts?

Myelinated terminals of dorsal root collaterals

m y gr elin ad at ien ion t

Reticulated area Lateral reticulospinal tract? Central canal surrounded by ependyma Ventral gray commissure

Myelinated axons of ventral horn motoneurons

a

Ventral white commissure

ct s ? ac ts?

La ter al co tra rtic ct os pi na l

S

r

Ventral root Sparse reactive glia Few myelinated fibers Many myelinated fibers Examples of myelination and concentrations of reactive glia

in h sp a ep I n t r c o in Sp

Ventral funiculus

Ventral median fissure

my gr elina ad ti ien on t

l

al

Ventral rootlets

tr

t

a

ic

Ventral gray

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus? Ventral corticospinal tract

The fine lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar and the lateral corticospinal tracts. Other lines run across fiber tracts, such as the area surrounding the ventral horn that includes the medial and central parts of the intraspinal tracts, and the internal fibers of the spinocephalic tracts. Most tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The ventral part of the spinocephalic tracts overlaps with the vestibulospinal tract; its central part includes fibers in the ventrolateral reticulospinal tract (not labeled), the spinotectal tract (not labeled), and the spino-olivary tract (not labeled). The intraspinal tracts overlap with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract.

166

PLATE 81A

Infant, 4 Days CR 350 mm Y299-62 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0019

Ependyma

.0167

Gray matter

7.8633

White matter

13.4880

167

PLATE 81B See the matched myelin stained section in Plates 80A and B

Dorsal funiculus

Dorsal median septum Dorsal intermediate septum

Dorsal horn

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root with dense glia in boundary cap Lissauer's tract

Central canal surrounded by ependyma Lateral cervical nucleus

Intermediate interneurons

digits? wrist?

Central autonomic area

digits? hand?

Ventral entral gray commissure V

Central cervical nucleus? Ventral horn interneurons

arm and forearm?

Lateral funiculus

Ventral V entral white commissure

skull and neck?

shoulder?

Ventral horn motoneurons

Ventral root

Ventral funiculus

Ventral median fissure

Dense Sparse

Examples of concentrations of proliferating glia

The corticospinal tracts no longer stand out as clear areas with very sparse proliferating glia in the white matter. There is a large area of slightly more dense proliferating glia in the ventral and lateral funiculi that corresponds partly to the regions that are still myelinating. Refer to the matching myelin stained section for the approximate locations of both corticospinal tracts, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

168

PLATE 82A Infant, 4 Days CR 350 mm Y299-62 Upper Thoracic Myelin stain

Areas (mm2) Central canal

.0016

Ependyma

.0081

Gray matter

2.8495

White matter

7.3181

169

PLATE 82B See the matched cell body stained section in Plates 83A and B

Dorsal median septum Dorsal intermediate septum Myelinated terminals of dorsal root collaterals

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root Lissauer's tract

S pi no

ce

Subgelatinosal plexus

Reticulated area Lateral reticulospinal tract?

ts?

ts?

Central canal surrounded by ependyma Ventral gray commissure Ventral white commissure

ac l

tr

ac

tr

Ventral gray

Ventral funiculus

t In

Sp

ic

in

al

ra

sp

in

e oc

ph

m y gr elin ad ati ien on t

Intermediate gray

a

Lateral funiculus

Vestibulospinal tract? Ventral median fissure

Ventral corticospinal tract?

This section and Plate 80 show the gradient of sparse to very sparse reactive glia in the lateral corticospinal tract (separated by a dashed line within the lateral corticospinal tract). On the right side, the circular area of more dense reactive glia may be the rubrospinal tract.

lar tracts?

Rubrospinal tract?

Dorsal gray

bel

my graelina die tio nt n

re

Lateral corticospinal tract

170

PLATE 83A Infant, 4 Days CR 350 mm Y299-62 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0019

Ependyma

.0141

Gray matter

2.9713

White matter

7.5265

171

PLATE 83B See the matched myelin stained section in Plates 82A and B

Dorsal funiculus Dorsal median septum Dorsal intermediate septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone

Dorsal root with dense glia in boundary cap

Dorsal root bifurcation zone

Lamina I

Dorsal Substantia horn gelatinosa

Lissauer's tract

Laminae IV-V

Central canal surrounded by ependyma

Lateral horn motoneurons

Central autonomic area Intermediate interneurons Ventral horn interneurons

Ventral gray commissure

Lateral funiculus

Ventral white commissure

middle trunk?

Ventral horn motoneurons

Ventral funiculus

Ventral median fissure

The white matter in this section has a uniformly sparse population of proliferating glia, and no fiber tracts in the ventral and lateral funiculi stand out with a different glial concentration. This suggests that myelination gliosis is finished. Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal, ventral corticospinal, rubrospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

172

PLATE 84A

Infant, 4 Days CR 350 mm Y299-62 Lower Thoracic Myelin stain

Areas (mm2) Central canal

.0005

Ependyma

.0047

Gray matter

1.9740

White matter

5.8169

173

PLATE 84B See the matched cell body stained section in Plates 85A and B

Dorsal funiculus

Dorsal median septum Fasciculus gracilis

Myelinated terminals of dorsal root collaterals

Dorsal root collateralization zone

Dorsal root bifurcation zone Lissauer's tract

Dorsal gray

Sp

Subgelatinosal plexus

Reticulated area

Clarke's column

Ventral funiculus

sp I n tra in Sp

ac

ts

?

s?

Ventral gray

Central canal surrounded by ependyma Ventral gray commissure Ventral white commissure

tr

Intermediate gray

i nal tracts?

Lateral funiculus

erebellar tr act

Lateral reticulospinal tract?

inoc

Lateral corticospinal tract

oc

e

a ph

li

c

Vestibulospinal tract? Ventral corticospinal tract? Ventral median fissure

174

PLATE 85A Infant, 4 Days CR 350 mm Y299-62 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0015

Ependyma

.0148

Gray matter

2.1221

White matter

6.0785

175

PLATE 85B See the matched myelin stained section in Plates 84A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis

Dorsal root collateralization zone Lamina I

Dorsal root bifurcation zone

Dorsal Substantia horn gelatinosa Laminae IV-V

Lissauer's tract

Lateral corticospinal tract?

Central canal surrounded by ependyma

Lateral horn motoneurons

Clarke's column Intermediate interneurons

Lateral Central autonomic funiculus area Ventral white commissure

Ventral horn interneurons

Ventral gray commissure

middle trunk?

Ventral horn motoneurons

Ventral funiculus

Ventral median fissure This caudal section shows a slight reduction in the concentration of proliferating glia in the lateral corticospinal tract; that suggests a rostral to caudal gradient in the myelination of the lateral corticospinal tract. There is also a region of slightly greater density in the ventral and lateral funiculi. Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

176

PLATE 86A Infant, 4 Days CR 350 mm Y299-62 Upper Lumbar Myelin stain

Areas (mm2) Central canal

.0019

Ependyma

.0087

Gray matter

4.7867

White matter

6.9382

177

PLATE 86B See the matched cell body stained section in Plates 87A and B

Dorsal funiculus

Dorsal median septum Clarke's column (displaced neurons) Myelinated terminals of dorsal root collaterals Dorsal root

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract

Dorsal gray

Reticulated area Lateral reticulospinal tract?

Clarke's column

Intermediate gray

Spinocerebellar tracts

Central canal surrounded by ependyma Ventral gray commissure

Lateral funiculus

Ventral white commissure

?

?

Ventral gray

Intr

Ventral funiculus

in asp

al

no Spi Ventral median fissure

ce

s

ts

ct

ac

a tr

tr

corti Lateral cosp inal trac

t

Subgelatinosal plexus

p

l ha

ic

Vestibulospinal tract?

178

PLATE 87A Infant, 4 Days CR 350 mm Y299-62 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0014

Ependyma

.0147

Gray matter

5.2589

White matter

7.4545

179

PLATE 87B See the matched myelin stained section in Plates 86A and B

Dorsal funiculus Clarke's column (displaced neurons)

Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root with dense glia in boundary cap

Lissauer's tract

Dorsal horn Lamina I Dorsal horn Substantia gelatinosa Laminae IV-V

Central canal surrounded by ependyma

Clarke's column Intermediate interneurons

Lateral funiculus

Lateral corticospinal tract? Central autonomic area

leg?

Ventral gray commissure Ventral white commissure

Ventral horn interneurons thigh and hip? lower trunk?

Ventral horn motoneurons

Ventral funiculus

Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

180

PLATE 88A Infant, 4 Days CR 350 mm Y299-62 Lumbar Enlargement Myelin stain

Areas (mm2) Central canal

.0020

Ependyma

.0104

Gray matter

8.0120

White matter

4.1273

181

PLATE 88B See the matched cell body stained section in Plates 89A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis

Myelinated terminals of dorsal root collaterals

Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root

Lissauer's tract Subgelatinosal plexus

Dorsal gray Reticulated area Lateral corticospinal tract

Lateral reticulospinal tract?

Intermediate gray

Central canal surrounded by ependyma Ventral gray commissure

c tr ac ali

Myelinated axons of ventral horn motoneurons

? ts

Ventral funiculus Ventral root

no

ce

ph

Lateral funiculus

Ventral median fissure

Intraspi

l na

tr

Vestibulospinal tract?

ac

pi

Ventral gray

ts?

Ventral white commissure

S

182

PLATE 89A Infant, 4 Days CR 350 mm Y299-62 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0046

Ependyma

.0302

Gray matter

7.9475

White matter

4.1796

183

PLATE 89B See the matched myelin stained section in Plates 88A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifuracation zone

Dorsal root with band of dense glia in boundary cap

Lissauer's tract Lamina I

Dorsal Substantia horn gelatinosa Laminae IV-V

Central autonomic area

Intermediate interneurons

Central canal surrounded by ependyma

foot and toes?

ankle and foot? leg?

Ventral gray commissure Ventral horn interneurons

Ventral white commissure

hip? thigh? hip?

Ventral horn motoneurons

Ventral root

lower trunk?

Ventral funiculus Ventral Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

184

PLATE 90A Infant, 4 Days CR 350 mm Y299-62 Sacral/Coccygeal Myelin stain

Areas (mm2) Central canal

.0036

Ependyma

.0228

Gray matter

3.8397

White matter

1.8131

185

PLATE 90B See the matched cell body stained section in Plates 91A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Myelinated terminals of dorsal root collaterals

Dorsal root Incipient subgelatinosal plexus

Lissauer's tract

Dorsal gray

Lateral funiculus

Central canal surrounded by ependyma

ts

?

Ventral gray commissure Ventral white commissure

Ventral gray

ac

Region of very sparse reactive glia in the lateral funiculus (autonomic fibers?)

Intermediate gray

tr

Lateral corticospinal tract?

Ventral funiculus Ventral median fissure

Intr

as

n pi

al

Spinocephalic tracts?

Vestibulospinal tract?

186

PLATE 91A

Infant, 4 Days CR 350 mm Y299-62 Sacral/Coccygeal Cell body stain

Areas (mm2) Central canal

.0037

Ependyma

.0281

Gray matter

4.0451

White matter

1.8283

187

PLATE 91B See the matched myelin stained section in Plates 90A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Dorsal root collateralization zone

Dorsal root with band of dense glia in boundary cap

Dorsal root bifurcation zone Lissauer's tract

Lamina I Dorsal horn Substantia gelatinosa Laminae IV-V Central autonomic area Intermediate interneurons

Central canal surrounded by ependyma

Ventral gray commissure Ventral horn interneurons

Ventral white commissure

Ventral funiculus Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

188

Part V: The Early Postnatal Period (continued) B. Matched myelin and cell body stained sections in the spinal cord of a 4-week-old infant Plate 92 is a survey of matched myelin stained and cell body stained sections from Y23-60, a specimen in the Yakovlev Collection with a crown rump length of 410 mm. All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post-fixation) of the section in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 93A-102A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 93B-102B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without consecutive section numbers. In addition, some sections were flipped over before they were placed on the plates. Sixteen myelin stained sections and 16 cell body stained sections were photographed ranging from upper cervical to lumbar enlargement levels. The 32 photographic prints were intuitively arranged in order from upper cervical to lumbar levels, using internal features such as the reduced size of the corticospinal tracts from rostral to caudal levels. Then, myelin and cell body stained sections were matched and 6 different levels are analyzed. Only myelin stained sections are illustrated at the cervical enlargement and upper lumbar levels because there are no matching cell body stained sections.

ated axons from the ventral corticospinal tract (see p. 198 in Ranson and Clark, 1959). Prior to this time, axons in the ventral corticospinal tract are present, but they may not have grown decussating collaterals.

As in the previous specimens, the cross-sectional area of a myelin stained section is smaller than the matching cell body stained section with the exception of the upper thoracic level. At that level, the cell body and myelin stained sections are not perfectly matched; the cell body stained section is probably 3 to 4 sections below the myelin stained section. Using the total areas of the myelin-stained sections, the cervical enlargement has the largest cross-sectional area, being larger than the lumbar enlargement by 20%; the two thoracic levels have smaller cross-sectional areas, averaging 74% smaller than the cervical enlargement and 45% smaller than the lumbar enlargement.

Table V B-1: Glia types and concentration in the white matter in a 4-week-old infant

Myelination in this specimen has two different characteristics (Table V B-1) than in the 4-day-old infant. First, there are myelination gradients in two fiber tracts. (1) The lateral corticospinal tract has a gradient of medial (dense) to lateral (sparse) reactive glia at cervical levels; myelinated fibers will first appear medially and last laterally (see Chapter 9, Figures 9-25 through 9-32 in Altman and Bayer, 2001). (2) There is some indication of a myelination gradient in the spinocephalic tracts at both cervical levels, because the outer fibers appear to myelinate later than the inner fibers (those closer to the intraspinal tracts). Second, the ventral commissure, which is either myelinating or is myelinated at GW37 and the 4-day-old infant, appears to be less advanced in this specimen, especially at cervical levels. That is probably due to the crossing of unmyelin-

The gray matter in the myelin stained sections shows heavy fascicles of myelinated fibers penetrating the dorsal horn from above and medially where it joins the intermediate gray. There is a large subgelatinosal plexus of myelinated fibers in the myelin stained sections. Fine fascicles of myelinated fibers are also scattered throughout the ventral horn, only labeled in the lumbar enlargement where there is a high ratio of gray matter to white matter. Many of these myelinated axons are from the large neurons in the dorsal root ganglion penetrating the dorsal horn from above and below (in the subgelatinosal plexus), arborizing in the intermediate gray (especially around Clarke’s column), and in the ventral horn. Other myelinated axons are from the motoneurons in the ventral horn that later organize into fascicles to form the ventral rootlets of spinal nerves. The intraspinal tract is one of the earliest myelinating; see the dense reactive glia in this tract at GW26 (Plates 47, 49, and 51).

Name

Proliferating glia Reactive glia Myelination

DORSAL ROOT

Myelinated

---

Sparse•

VENTRAL ROOT

Myelinated

---

--Sparse

DORSAL FUNICULUS: dorsal root bif. zone

Many fibers*

---

dorsal root col. zone

Myelinated

---

Sparse

fas. gracilis

Myelinated

---

Sparse

fas. cuneatus

Myelinated

---

Sparse

---

None

Sparse

Lissauer's tract LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract ven. cortricospinal tract spinocerebellar tracts ven. commissure intraspinal tract spinocephalic tract med. long. fasiculus vestibulospinal tract • * †

--Gradient† Sparse --Sparse Sparse Myelinated --Sparse Myelinated** --Sparse Myelinated --Sparse Gradient†† Gradient†† Sparse Many fibers --Sparse Some fibers

---

Sparse

dense band in a sparse field in Plate 102 intermingled in a bed of nonreactive glia dense to sparse at cervical and thoracic levels, sparse at remaining levels ** some unmyelinated fibers at cervical levels †† at cervical levels: myelinating internally, reactive glia externally; at other levels: myelinated or myelinating

189

PLATE 92 INFANT, 4 WEEKS, CR 410 mm, Y23-60 MYELIN STAIN Plates 93A, 93B Upper Cervical Total Area: 15.186 mm2

Plates 95A, 95B Cervical Enlargement Total Area: 17.136 mm2

CELL BODY STAIN Plates 94A, 94B Upper Cervical Total Area: 15.947 mm2

MATCHING SECTION NOT AVAILABLE

Plates 96A, 96B Upper Thoracic Total Area: 10.017 mm2

Plates 97A. 97B Upper Thoracic Total Area: 9.8042 mm2

Plates 98A, 98B Lower Thoracic Total Area: 9.7045 mm2

Plates 99A, 99B Lower Thoracic Total Area: 10.856 mm2

Plates 100A, 100B Upper Lumbar Total Area: 11.213 mm2

Plates 101A, 101B Lumbar Enlargement Total Area: 14.299 mm2

MATCHING SECTION NOT AVAILABLE

Plates 102A, 102B Lumbar Enlargement Total Area: 14.478 mm2

190

PLATE 93A Infant, 4 Weeks CR 410 mm Y23-60 Upper Cervical Myelin stain

Areas (mm2) Central canal

.0054

Ependyma

.0115

Gray matter

3.5600

White matter

11.6090

191

PLATE 93B See the matched cell body stained section in Plates 94A and B

Note the lighter staining of the ventral commissure in this section. This commissure is myelinated in the GW37 fetus and 4-day-old infant. It is possible that the unmyelinated ventral corticospinal tract (present in each of the specimens at the cervical level) is growing decussating collaterals in the 4-week-old infant that dilute the number of myelinated fibers in the commissure.

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Fasciculus cuneatus

Dorsal intermediate septum

Dorsal root collateralization zone

Myelinated terminals of dorsal root collaterals

Dorsal root bifurcation zone Lissauer's tract S p i n o c e

Dorsal gray

Lateral reticulospinal tract?

Subgelatinosal plexus

Intermediate gray

ts?

Ventral gray commissure Ventral white commissure

I

n tr

Ventral funiculus sparse reactive glia many myelinating fibers dense reactive glia some myelinating fibers myelinated

Examples of staining densities representing degrees of myelination

a aspin

r l t

Sp

Ventral median fissure

tr

ac

ac

ts

Ventral gray

Rubrospinal tract?

Reticulated area Central canal surrounded by ependyma

t r a c t s

Lateral funiculus

e l l a r e b

my e gra lina die tion nt

r

Lateral corticospinal tract

in

e oc

a ph

li

c

n tio na ent i l e i mygrad

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus? Ventral corticospinal tract

The fine dashed lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fiber tracts. These lines may correspond to the borders of some fiber tracts, such as the spinocerebellar tracts. Other lines run across fiber tracts, such as the area surrounding the ventral horn that includes the medial and central parts of the intraspinal tracts and the internal fibers of the spinocephalic tracts. Most tracts in the ventral and lateral funiculi overlap and do not have distinct borders. The lateral corticospinal tract overlaps with the rubrospinal tract. The spinocephalic tracts (outer curved arrow) overlap with the vestibulospinal tract and include fibers in the ventrolateral reticulospinal tract (not labeled), the spinotectal tract (not labeled), and the spino-olivary tract (not labeled). The intraspinal tracts (inner curved arrow) overlap with the medial longitudinal fasciculus, the tectospinal tract, and the lateral reticulospinal tract. Straight arrows in the lateral and ventral funiculi indicate myelination gradients (see comments on these gradients in Plate 95).

192

PLATE 94A Infant, 4 Weeks CR 410 mm Y23-60 Upper Cervical Cell body stain

Areas (mm2) Central canal

.0057

Ependyma

.0151

Gray matter

3.6685

White matter

12.2580

193

PLATE 94B See the matched myelin stained section in Plates 93A and B

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone

Dorsal median septum Dorsal intermediate septum

Dorsal horn

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root bifurcation zone Lissauer's tract

Lateral cervical nucleus? Intermediate interneurons

Central autonomic area

Lateral funiculus

Ventral gray commissure arm?

ph dia ra gm sku ll? ?

Ventral horn interneurons

Central canal surrounded by ependyma

Central cervical nucleus?

shoulder?

Ventral white commissure

Ventral horn motoneurons

Example of a sparse proliferating glial concentration (present throughout the white matter)

Ventral funiculus Ventral median fissure

The white matter in this cell body stained section and other sections in this specimen have a uniformly sparse population of proliferating glia. Refer to the matching myelin stained section for the approximate locations of both corticospinal tracts, the medial longitudinal fasciculus, the rubrospinal, lateral reticulospinal, vestibulospinal, tectospinal, intraspinal, and spinocephalic tracts.

194

PLATE 95A Infant, 4 Weeks CR 410 mm Y23-60 Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0062

Ependyma

.0143

Gray matter

4.7533

White matter

12.3620

195

PLATE 95B This Plate has no matched cell body stained section

As in Plate 93, there is lighter staining of the ventral commissure in this section, possibly due to the crossing of unmyelinated axons from the ventral corticospinal tract.

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal intermediate septum Myelinated terminals of dorsal root collaterals

Lissauer's tract S p i n o c e r

my gr elin ad ati ien on t

e l l a r e b

Lateral corticospinal tract

Reticulated area

Dorsal gray

Rubrospinal tract?

Subgelatinosal plexus

s t r a c t

Central canal surrounded by ependyma

Intermediate gray

Ventral gray commissure Ventral white commissure

Lateral funiculus

Lateral reticulospinal tract?

Ventral funiculus

Spi Ventral median fissure Myelinated ventral rootlets

ts

e noc

tr

c I ntraspinal tr a

ac

ts

Ventral gray

a ph

li

c

n aio lin ient e my grad

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus? Ventral corticospinal tract

There are two myelination gradients (straight arrows) in the ventral and lateral funiculi in this section and also in the section shown in Plate 93. The arrow within the lateral corticospinal tract starts ventromedially in the area with a greater density of reactive glia and stops dorsolaterally in the area with sparse reactive glia. Ventromedial fibers enter the gray matter at rostral levels and will be the first to myelinate. Dorsolateral fibers enter the gray matter at caudal levels and will be the last to myelinate. The arrow cutting across the intraspinal and spinocephalic tracts starts close to the ventral gray in a region that contains many myelinating fibers and stops in an area that contains sparse reactive glia just beneath the pial membrane. Fibers in the intraspinal tract myelinate first, then internal fibers in the spinocephalic tracts, then outer fibers in the spinocephalic tracts.

196

PLATE 96A Infant, 4 Weeks CR 410 mm Y23-60 Upper Thoracic Myelin stain

Areas (mm2) Central canal

.0032

Ependyma

.0094

Gray matter

2.3627

White matter

7.6419

197

PLATE 96B See the matched cell body stained section in Plates 97A and B

Note the asymmetry in the size of the ventral corticospinal tract in this section and in Plates 95 and 97.

Dorsal median septum Dorsal intermediate septum Myelinated terminals of dorsal root collaterals Dorsal root

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract

t

Subgelatinosal plexus

Dorsal gray

in

Lateral reticulospinal tract?

oc

e

Reticulated area Central canal surrounded by ependyma

Intermediate gray

Ventral gray commissure

s

Lateral funiculus

llar tract

Rubrospinal tract?

be

c

Sp

re

l ac ra al tr e t La spin o tic or

Ventral white commissure

ac

ac

ts

ts

Ventral gray

Ventral funiculus Ventral median fissure

Intr

aspina

r l t

oc Spin

ep

h

ic al

tr

Vestibulospinal tract? Ventral corticospinal tract

The ventral commissure stains darkly in this section, even though there is a small ventral corticospinal tract. The still unmyelinated axons in the ventral corticospinal tract may not have crossed the midline at this level, indicating a rostral (first) to caudal (last) gradient of maturation. The myelination gradient in the lateral corticospinal tract is not evident in this section, but note the lighter staining of the superficial fibers in the ventral and lateral funiculi compared to the darker staining of the deep fibers.

198

PLATE 97A Infant, 4 Weeks CR 410 mm Y23-60 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0026

Ependyma

.0113

Gray matter

2.2909

White matter

7.4994

199

PLATE 97B See the matched myelin stained section in Plates 96A and B

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Dorsal root Lamina I

Dorsal horn Substantia

Lissauer's tract

gelatinosa Laminae IV-V

Clarke's column

Lateral horn motoneurons

Central canal surrounded by ependyma

Intermediate interneurons

Lateral funiculus

central trunk?

Ventral horn interneurons

Central autonomic area

central trunk?

Ventral gray commissure Ventral white commissure

Ventral horn motoneurons Ventral median fissure

Ventral funiculus

Refer to the matching myelin stained section for the approximate locations of both corticospinal tracts, the rubrospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

200

PLATE 98A Infant, 4 Weeks CR 410 mm Y23-60 Lower Thoracic Myelin stain

Areas (mm2) Central canal

.0029

Ependyma

.0083

Gray matter

2.9576

White matter

6.7358

201

PLATE 98B See the matched cell body stained section in Plates 99A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone

Myelinated terminals of dorsal root collaterals

Dorsal root bifurcation zone Dorsal root Lissauer's tract

ticoLater spi al nal tra

ct

Subgelatinosal plexus

Dorsal gray Reticulated area Lateral reticulospinal tract?

cor

Clarke's column

Rubrospinal tract? Central canal surrounded by ependyma

Intermediate gray

Spinocerebellar tracts

Lateral funiculus

Ventral gray commissure Ventral white commissure

s Intra

Ventral funiculus

pi

Sp

Ventral median fissure

c ino

ct

tr

ra

l na

s

ac

ts

Ventral gray

ep

h

i al

c

Vestibulospinal tract?

The ventral commissure stains darkly in this section, just as in Plate 96. A small ventral corticospinal tract may be present on the right side in the unlabeled circular area bordering the ventral median fissure. All axons in the commissure appear to be myelinated in this section. The ventral part of the lateral funiculus has a lightly stained area (containing late myelinating fibers in the external parts of the spinocephalic tracts) sandwiched by dark staining medially (containing early myelinating fibers in the intraspinal and internal parts of the spinocephalic tracts) and dark staining laterally (containing the early myelinating spinocerebellar fibers).

t

202

PLATE 99A Infant, 4 Weeks CR 410 mm Y23-60 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0045

Ependyma

.0132

Gray matter

3.5220

White matter

7.3162

203

PLATE 99B See the matched myelin stained section in Plates 98A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I

Dorsal Substantia horn gelatinosa

Lissauer's tract

Laminae IV-V

Displaced Clarke's column neuron Central canal surrounded by ependyma Clarke's column

Lateral horn motoneurons

Intermediate interneurons Ventral horn interneurons

Central autonomic area central trunk?

Ventral gray commissure

central trunk?

Ventral horn motoneurons

Ventral white commissure

Ventral funiculus Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the rubrospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

204

PLATE 100A Infant, 4 Weeks CR 410 mm Y23-60 Lumbar Myelin stain

Areas (mm2) Central canal

.0070

Ependyma

.0070

Gray matter

4.1280

White matter

7.0713

205

PLATE 100B This Plate has no matched cell body stained section

Dorsal funiculus Dorsal median septum Myelinated terminals of dorsal root collaterals

Fasciculus gracilis Dorsal root collateralization zone Dorsal root bifurcation zone Lissauer's tract

Dorsal gray tic Late osp ral ina l tr a

ct

Subgelatinosal plexus

cor

Lateral reticulospinal tract?

Clarke's column

Intermediate gray

Rubrospinal tract?

Ventral gray commissure Ventral white commissure

I ntraspinal tract

s?

no Spi

Ventral funiculus Ventral median fissure

tr

ac

ts

Ventral gray

ce

li

c

Lateral funiculus

Reticulated area Central canal surrounded by ependyma

a ph

Vestibulospinal tract?

The ventral commissure stains darkly in this section (the light area is a blood vessel). Apparently, all axons in the commissure are myelinated. In contrast to rostral sections, all fibers in the spinocephalic tracts are darkly stained, indicating myelination is well on its way to completion at this level. The external axons in the spinocephalic tracts at this level are more proximal to their cell bodies of origin than at rostral levels, indicating a proximal (first) to distal (second) gradient of myelination.

206

PLATE 101A Infant, 4 Weeks CR 410 mm Y23-60 Lumbar Enlargement Myelin stain

Areas (mm2) Central canal

.0090

Ependyma

.0086

Gray matter

7.8739

White matter

6.4072

207

PLATE 101B See the matched cell body stained section in Plates 102A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum Myelinated terminals of dorsal root collaterals

Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal root

Lissauer's tract

Subgelatinosal plexus

Dorsal gray Lateral corticospinal tract

Rubrospinal tract? Reticulated area

Intermediate gray

Central canal surrounded by ependyma

Lateral reticulospinal tract?

Ventral gray commissure

Ventral gray

al

ic

tra

cts

Myelinated axons of ventral horn motoneurons

c oc ts ep h

Lateral funiculus

Ventral white commissure

Ventral funiculus

ra l t in a Intraspin Sp

Ventral median fissure

Vestibulospinal tract?

The ventral commissure stains darkly in this section indicating that all axons in the commissure are myelinated. Fibers in the spinocephalic tracts are even more darkly stained than those in Plate 100, indicating myelination is more advanced at this level. The axons in the spinocephalic tracts at this level are more proximal to their cell bodies of origin than at rostral levels; hence, myelination follows a proximal (first) to distal (second) gradient.

208

PLATE 102A Infant, 4 Weeks CR 410 mm Y23-60 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal

.0075

Ependyma

.0148

Gray matter

7.9194

White matter

6.5366

209

PLATE 102B See the matched myelin stained section in Plates 101A and B

Dorsal funiculus Fasciculus gracilis Dorsal root collateralization zone

Dorsal median septum

Dorsal root bifurcation zone

Dorsal horn

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root Lissauer's tract

Central canal surrounded by ependyma

Intermediate interneurons

Central autonomic area Ventral gray commissure

toes? ankle, foot, and toes?

Ventral horn interneurons

Ventral white commissure

thigh and leg? hip? thigh? hip?

Ventral horn motoneurons Ventral median fissure

Ventral funiculus Ventral

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the rubrospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

210

Part V: The Early Postnatal Period (concluded) C. Matched myelin and cell body stained sections in the spinal cord of a 4-month-old infant Plate 103 is a survey of matched myelin stained and cell body stained sections from Y286-62, a specimen in the Yakovlev Collection with a crown rump length of 440 mm. All sections are shown at the same scale. The boxes enclosing each section list the approximate level and the total area (post-fixation) of the section in square millimeters (mm2). Full-page normal contrast photographs of each specimen are in Plates 104A-117A. Low contrast photographs with superimposed labels and outlines of structural details are in Plates 104B-117B. In this specimen, the myelin stained and cell body stained sections were preserved on separate large glass plates without any section numbers. Forty-one myelin stained sections and 38 cell body stained sections were photographed ranging from upper cervical to lumbar enlargement levels. Sections beyond the lumbar enlargement were damaged. The 79 photographic prints were intuitively arranged in order from upper cervical to lumbar enlargement levels, using internal features such as the size of the corticospinal tracts, and the width of the ventral horn. Then, myelin and cell body stained sections were matched and seven different levels were analyzed. As in the previous specimens, the cross-section area of most myelin stained sections is smaller than the matching cell body stained sections. Using the total areas of the myelin stained sections, the middle thoracic level has the smallest cross-sectional area, 54% smaller than the cervical enlargement and 52% smaller than the lumbar enlargement. The cervical enlargement has the largest cross-sectional area, but it is larger than the lumbar enlargement by only 5%. That difference is much smaller than at earlier ages. The cervical enlargement is 65% larger than the lumbar enlargement in the 4-day-old infant (Page 162) and 20% larger in the 4-week-old infant (Page 188). Growth of the lumbar enlargement lags behind the cervical enlargement indicating a rostral to caudal gradient in maturation. Myelination in this specimen is nearing completion, and is very similar to that of adults (see Table V C-1). All parts of the dorsal funiculus are myelinated except Lissauer’s tract, which remains unmyelinated in maturity. Myelination in the lateral corticospinal tract still shows a gradient in the mediolateral and rostrocaudal directions, but it has generally advanced from that seen in the 4-week-old infant. The ventral corticospinal tract may or may not exist in this specimen. If it is present, it is completely myelinated throughout its length (see Chapter 9, Figures 9-25 through 9-32, Altman and Bayer, 2001). The myelination gradient in the spinocephalic tracts is now restricted to the upper cervical level; the outer fibers are surrounded by

dense reactive glia, and the inner fibers (those closer to the intraspinal tracts) are myelinating. Caudally, the spinocephalic tract is myelinated. It is important to note that the rostrocaudal myelination gradients in the lateral corticospinal tract and the spinocephalic tracts give clear evidence that axons myelinate first proximal to their site of origin at the cell body and progressively later distally. In addition, the mediolateral myelination gradient in the lateral corticospinal tract is most probably related to the retarded entry of axons from later generated neurons in medial parts of the primary motor cortex (destined to terminate in the ventral horn at lumbar and sacral areas). The gray matter in the myelin stained sections shows heavy fascicles of myelinated fibers penetrating the dorsal horn that were pointed out in the previous specimen. Myelination in the subgelatinosal plexus and the reticulated area continues to progress.

Table V C-1: Glia types and concentration in the white matter in a 4-month-old infant

Name

Proliferating glia Reactive glia Myelination

DORSAL ROOT

Myelinated

---

Sparse

VENTRAL ROOT

Myelinated

---

---

DORSAL FUNICULUS: dorsal root bif. zone

Many fibers*

---

Sparse

dorsal root col. zone

Myelinated

---

Sparse

fas. gracilis

Myelinated

---

Sparse

fas. cuneatus

Myelinated

---

Sparse

---

none

Sparse

Lissauer's tract LATERAL and VENTRAL FUNICULI: lat. cortricospinal tract ven. cortricospinal tract spinocerebellar tracts ven. commissure intraspinal tract spinocephalic tract med. long. fasiculus vestibulospinal tract * †

Gradient† Gradient† Sparse Myelinated** --Sparse Myelinated --Sparse Myelinated --Sparse Myelinated --Sparse Gradient†† Gradient†† Sparse Myelinated --Sparse Myelinated

---

Sparse

intermingled with unmyelinated fibers myelinated to myelinating at cervical and thoracic levels, dense reactive glia to sparse reactive glia at upper lumbar level, sparse reactive glia at lumbar enlargement ** the presence of this tract is assumed. If present, it is myelinated. †† at upper cervical level: myelinating internally, dense reactive glia externally; at other levels: myelinated or myelinating

211

PLATE 103 INFANT, 4 MONTHS, CR 440 mm, Y286-62 MYELIN STAIN

CELL BODY STAIN

Plates 104A, 104B Upper Cervical Total area: 27.489 mm2

Plates 105A, 105B Upper Cervical Total area: 28.327 mm2

Plates 106A, 106B Cervical Enlargement Total area: 33.314 mm2

Plates 107A, 107B Cervical Enlargement Total area: 33.211 mm2

Plates 108A, 108B Upper Thoracic Total area: 22.398 mm2

Plates 109A, 109B Upper Thoracic Total area: 24.760 mm2

Plates 110A, 110B Middle Thoracic Total area: 15.291 mm2

Plates 111A, 111B Middle Thoracic Total area: 16.112 mm2

Plates 112A, 112B Lower Thoracic 17.098 mm2

Plates 113A, 113B Lower Thoracic 17.300 mm2

Plates 114A, 114B Upper Lumbar 24.774 mm2

Plates 115A, 115B Upper Lumbar 24.914 mm2

Plates 116A, 116B Lumbar Enlargement 31.708 mm2

Plates 117A, 117B Lumbar Enlargement 32.275 mm2

212

PLATE 104A Infant, 4 Months CR 440 mm Y286-62 Upper Cervical Myelin stain

Areas (mm2) Central canal

.0066

Ependyma

.0082

Gray matter

6.0358

White matter

21.4380

213

PLATE 104B See the matched cell body stained section in Plates 105A and B The myelinating axons in the most superficial region of the lateral corticospinal tract extend to lumbosacral levels where they are unmyelinated (see Plate 116). The lighter staining in the most superficial region of the spinocephalic tracts is occupied by axons having cells of origin at lumbosacral levels where these axons are myelinated (see Plate 116). Both gradients indicate that myelination proceeds from proximal to distal in axons.

Dorsal funiculus Dorsal median septum Dorsal intermediate septum

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone

Dorsal root Lateral corticospinal tract (myelinated) (myelinating)

Dorsal root bifurcation zone Lissauer's tract

Subgelatinosal plexus

Spin

Dorsal gray

m gra yelin die ati nt on

Reticulated area

oc

Myelinated terminals of dorsal root collaterals

Ventral gray commissure Ventral white commissure

cts

Intermediate gray

Lateral funiculus

ebellar tracts

Central canal surrounded by ependyma

er

Lateral reticulospinal tract?

c

tra

Ventral gray

Ventral median fissure

my e gra lina die tion nt

ep oc

al tr

in

p

Myelinated axons from dorsal root afferents, intraspinal tract afferents, and ventral motoneuron efferents

Ventral funiculus

ac

as

ts

ha

li

Intr

in

Sp

Vestibulospinal tract? Tectospinal tract?

Ventral corticospinal tract?

Medial longitudinal fasciculus?

As stated in the previous specimens, the fine dashed lines in the lateral and ventral funiculi segregate regions of differing densities of the myelin stain, not the borders of fiber tracts. In this 4-month-old infant, the ventral and lateral funiculi contain dense staining indicative of advanced myelination. There are three lightly stained areas: sparse reactive glia in the most superficial part of the spinocephalic tracts, a myelinating region in the superficial part of the lateral corticospinal tract, and a thin unidentified myelinating region just medial to the spinocerebellar tracts. Most of the lateral corticospinal tract cannot be distinguished from the rest of the lateral funiculus by its myelination pattern.

214

PLATE 105A Infant, 4 Months CR 440 mm Y286-62 Upper Cervical Cell body stain

Areas (mm2) Central canal

.0045

Ependyma

.0145

Gray matter

6.0685

White matter

22.2390

215

PLATE105B See the matched myelin stained section in Plates 104A and B

Dorsal funiculus Dorsal median septum

Fasciculus gracilis Fasciculus cuneatus

Dorsal intermediate septum

Dorsal root collateralization zone

Lamina I Dorsal horn Substantia

Dorsal root bifurcation zone

gelatinosa Laminae IV-V

Lissauer's tract

Edge artifact staining Central cervical nucleus?

Lateral cervical nucleus? Intermediate interneurons

Central autonomic area

Ventral gray commissure Ventral white commissure

m? rag

dia ph

sho

uld

er?

Central canal surrounded by ependyma

skull?

Lateral funiculus

Ventral horn interneurons

Ventral horn motoneurons

Ventral funiculus Ventral median fissure

Note the uniform light staining of the white matter in this and all other cell body stained sections in this specimen. That indicates that myelination gliosis is completely finished, even in the lateral corticospinal tract, because nearly all tracts are either myelinated or myelinating. Refer to the matching myelin stained section for the approximate locations of both corticospinal tracts, the medial longitudinal fasciculus, the tectospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

216

PLATE 106A Infant, 4 Months CR 440 mm Y286-62 Cervical Enlargement Myelin stain

Areas (mm2) Central canal

.0124

Ependyma

.0160

Gray matter

10.2870

White matter

22.9990

217

PLATE 106B See the matched cell body stained section in Plates 107A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Fasciculus cuneatus

Dorsal intermediate septum

Dorsal root collateralization zone

Lateral corticospinal tract (myelinating) (myelinaed)

Dorsal root bifurcation zone Lissauer's tract

Damaged area in section

Subgelatinosal plexus

S

p

Dorsal gray

in

o

ce re

Lateral reticulospinal tract?

Reticulated area

tracts

Intermediate gray

lar

Myelinated terminals of dorsal root collaterals

bel

m gra yelin die ati nt on

Central canal surrounded by ependyma Ventral gray commissure Ventral white commissure

Lateral funiculus

ac

ts

Ventral gray

Ventral funiculus Myelinated axons from dorsal root afferents, intraspinal tract afferents, and ventral motoneuron efferents

Ventral median fissure

a traspinal tr

Spin

o

s

h cep

Vestibulospinal tract? Tectospinal tract? Medial longitudinal fasciculus? Ventral corticospinal tract?

tr

ct

ic

In

al

218

PLATE 107A Infant, 4 Months CR 440 mm Y286-62 Cervical Enlargement Cell body stain

Areas (mm2) Central canal

.0010

Ependyma

.0256

Gray matter

10.1850

White matter

22.9900

219

PLATE 107B See the matched myelin stained section in Plates 106A and B

Dorsal funiculus

Dorsal median septum Dorsal intermediate septum

Dorsal horn

Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I Substantia gelatinosa Laminae IV-V

Lissauer's tract

Damaged area in section

Lateral cervical nucleus? Intermediate interneurons

Central cervical nucleus?

Central autonomic area

Lateral funiculus Edge artifact staining

hand?

Central canal surrounded by ependyma

wrist?

Ventral gray commissure

forearm? forearm?

Ventral white commissure

Ventral horn interneurons neck?

arm? shoulder?

Ventral horn motoneurons

Ventral funiculus Ventral median fissure Refer to the matching myelin stained section for the approximate locations of both corticospinal tracts, the medial longitudinal fasciculus, the tectospinal, lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

220

PLATE 108A Infant, 4 Months CR 440 mm Y286-62 Upper Thoracic Myelin stain

Areas (mm2) Central canal

.0088

Ependyma

.0136

Gray matter

5.6424

White matter

16.7340

221

PLATE 108B See the matched cell body stained section in Plates 109A and B

Dorsal funiculus Fasciculus gracilis Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Dorsal median septum Dorsal intermediate septum Myelinated terminals of dorsal root collaterals Lateral corticospinal tract (myelinating) (myelinated)

Dorsal root

Sp

in oc

Lissauer's tract

er

eb e

Subgelatinosal plexus

lla

Dorsal gray

r

Central canal surrounded by ependyma Ventral gray commissure Ventral white commissure

ac

tr

ts

ac

ts

Ventral gray

In

tr

Ventral funiculus Ventral median fissure

a spi

l na

tr

Sp

o in

ce

p

l ha

Vestibulospinal tract?

ic

cts

Reticulated area

Intermediate gray Lateral funiculus

Lateral reticulospinal tract?

tra

m gra yelin die ati nt on

222

PLATE 109A Infant, 4 Months CR 440 mm Y286-62 Upper Thoracic Cell body stain

Areas (mm2) Central canal

.0099

Ependyma

.0241

Gray matter

6.5545

White matter

18.1720

223

PLATE 109B See the matched myelin stained section in Plates 108A and B

Dorsal funiculus

Dorsal horn

Fasciculus gracilis

Dorsal median septum Dorsal intermediate septum Lamina I Substantia gelatinosa Laminae IV-V

Fasciculus cuneatus Dorsal root collateralization zone Dorsal root bifurcation zone

Lissauer's tract

Lateral horn motoneurons

Central autonomic area

Clarke's column Intermediate interneurons

Ventral horn interneurons

Central canal surrounded by ependyma

Ventral gray commissure Ventral white commissure

central trunk?

Ventral horn motoneurons

Ventral funiculus Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

224

PLATE 110A Infant, 4 Months CR 440 mm Y286-62 Middle Thoracic Myelin stain

Areas (mm2) Central canal

.0040

Ependyma

.0062

Gray matter

2.8387

White matter

12.4420

225

PLATE 110B See the matched cell body stained section in Plates 111A and B

Dorsal funiculus

Dorsal median septum Fasciculus gracilis

Myelinated terminals of dorsal root collaterals

Dorsal root collateralization zone

Lateral corticospinal tract (dense reactive glia) (myelinating) (myelinated)

Sp

Lis sa tracuer's t

Dorsal root bifurcation zone

inoc ere b

Subgelatinosal plexus

ll

m gra yelin die ati nt on

Lateral reticulospinal tract? Myelinated fibers in Clarke's column

Intermediate gray

Ventral gray commissure Ventral white commissure

ts

Central canal surrounded by ependyma

Ventral funiculus

Ventral median fissure

li

ac

ra

sp

c

ts

Int

tr

ac

Ventral gray

tr

Lateral funiculus

e

Reticulated area

in a l

i Sp

no

p ce

ha

Vestibulospinal tract?

ar tracts

Dorsal gray

226

PLATE 111A Infant, 4 Months CR 440 mm Y286-62 Middle Thoracic Cell body stain

Areas (mm2) Central canal

.0031

Ependyma

.0117

Gray matter

2.8799

White matter

13.2170

227

PLATE 111B See the matched myelin stained section in Plates 110A and B

Dorsal funiculus Fasciculus gracilis Dorsal median septum Dorsal root collateralization zone Dorsal root bifurcation zone

Lamina I Dorsal Substantia horn gelatinosa Laminae IV-V

Lissauer's tract

Clarke's column

Central autonomic area

Intermediate interneurons Lateral horn motoneurons

Lateral funiculus Central canal surrounded by ependyma

Ventral horn interneurons

Ventral gray commissure Ventral white commissure

central trunk?

Ventral horn motoneurons

Ventral funiculus Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

228

PLATE 112A Infant, 4 Months CR 440 mm Y286-62 Lower Thoracic Myelin stain

Areas (mm2) Central canal

.0090

Ependyma

.0091

Gray matter

4.0248

White matter

13.0550

229

PLATE 112B See the matched cell body stained section in Plates 113A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum Myelinated terminals of dorsal root collaterals

Dorsal root collateralization zone

Lateral corticospinal tract (dense reactive glia) (myelinating)

Dorsal root bifurcation zone

Li ss tra aue ct r's

Dorsal root

Spino

n tio na nt eli die my gra

Dorsal gray

Subgelatinosal plexus

Lateral reticulospinal tract?

Myelinated fibers in Clarke's column

Central canal surrounded by ependyma

Ventral gray commissure

tr

ac

Intr

Ventral gray

ts

Ventral white commissure

ic

as p

in

Ventral funiculus

s

al tr

act

S Ventral median fissure

n pi

oc

h ep

al

Vestibulospinal tract?

cts llat tra

Lateral funiculus

Intermediate gray

cerebe

Reticulated area

230

PLATE 113 A Infant, 4 Months CR 440 mm Y286-62 Lower Thoracic Cell body stain

Areas (mm2) Central canal

.0073

Ependyma

.0160

Gray matter

4.2476

White matter

13.0290

231

PLATE 113B See the matched myelin stained section in Plates 112A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum

Dorsal Dorsal horn horn

Dorsal root collateralization zone

Lamina I Substantia gelatinosa Laminae IV-V

Dorsal root bifurcation zone

Lissauer's tract

Clarke's column Lateral horn motoneurons

Intermediate interneurons Central canal surrounded by ependyma

Central autonomic area Ventral gray commissure Ventral white commissure

Ventral horn interneurons central trunk?

Ventral horn motoneurons

Ventral funiculus Ventral Ventral median fissure Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

232

PLATE 114A Infant, 4 Months CR 440 mm Y286-62 Upper Lumbar Myelin stain

Areas (mm2) Central canal

.0140

Ependyma

.0155

Gray matter

9.5065

White matter

15.2380

233

PLATE 114B See the matched cell body stained section in Plates 115A and B

Dorsal funiculus Dorsal median septum Myelinated terminals of dorsal root collaterals

Fasciculus gracilis Dorsal root collateralization zone

Lateral corticospinal tract (sparse reactive glia) (dense reactive glia)

Dorsal root bifurcation zone Li ss tra aue ct r's

Dorsal gray

Subgelatinosal plexus Reticulated area

n tio na nt eli die my gra

Myelinated fibers in Clarke's column

Intermediate gray

Central canal surrounded by ependyma

Lateral funiculus

Ventral gray commissure

Lateral reticulospinal tract?

Ventral white commissure

I

ra

ct

s

Ventral gray

n t raspina l tracts

Ventral funiculus Sp

Ventral median fissure

c ino

ep

Vestibulospinal tract?

Compare the low staining density of the lateral corticospinal tract in this caudal section with the intense staining in more rostral sections, for instance, Plate 104. The progressively decreasing staining intensity indicates a rostral to caudal gradient of myelination in the lateral corticospinal tract.

h

i al

c

t

234

PLATE 115A Infant, 4 Months CR 440 mm Y286-62 Upper Lumbar Cell body stain

Areas (mm2) Central canal

.0086

Ependyma

.0270

Gray matter

9.8606

White matter

15.0180

235

PLATE 115B See the matched myelin stained section in Plates 114A and B

Dorsal funiculus Fasciculus gracilis

Dorsal median septum Lamina I

Dorsal root collateralization zone

Dorsal Dorsal Substantia horn gelatinosa horn

Dorsal root bifurcation zone

Laminae IV-V

Li ss tra auer ct 's

Clarke's column Intermediate interneurons

Central autonomic area

Central canal surrounded by ependyma

Ventral gray commissure

Ventral horn interneurons leg?

Ventral white commissure

hip? lower trunk?

thigh? hip?

Ventral horn motoneurons

Ventral funiculus Ventral median fissure

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Lateral funiculus

236

PLATE 116A Infant, 4 Months CR 440 mm Y286-62 Lumbar Enlargement Myelin stain

Areas (mm2) Central canal

.0180

Ependyma

.0187

Gray matter

17.7370

White matter

13.9340

237

PLATE 116B See the matched cell body stained section in Plates 117A and B

The axons in the lateral corticospinal tract at this caudal level are unmyelinated. These fibers are more distal to their cells of origin than they are at cervical and thoracic levels where they are myelinating (Plates 104, 106, and 108). The lateral corticospinal tract is small at this level because most of the axons have terminated at more rostral levels.

Dorsal funiculus Fasciculus gracilis Dorsal root collateralization zone

Dorsal median septum

Dorsal root bifurcation zone

Myelinated terminals of dorsal root collaterals Dorsal root

Lissauer's tract

Dorsal gray

Lateral corticospinal tract

Subgelatinosal plexus

Reticulated area

Lateral reticulospinal tract?

Intermediate gray

Central canal surrounded by ependyma

Ventral gray

Ventral gray commissure

ts

Ventral white commissure

li

c

tr

ac

Lateral funiculus

I

n

Ventral funiculus Myelinated axons from dorsal root afferents, intraspinal tract afferents, and ventral motoneuron efferents

Ventral median fissure

tr

aspinal

c tra

ts

i Sp

no

c

h ep

a

Vestibulospinal tract?

In contrast to the upper cervical level (Plate 104), the axons in the spinocephalic tracts at this level are more proximal to their cells of origin and are completely myelinated. Like the lateral corticospinal tract, the spinocephalic tract is smaller at this level because additional axons join the tract at more rostral levels.

238

PLATE 117A Infant, 4 Months CR 440 mm Y286-62 Lumbar Enlargement Cell body stain

Areas (mm2) Central canal Ependyma

.0115 .0231

Gray matter

18.3550

White matter

13.8850

239

PLATE 117B See the matched myelin stained section in Plates 116A and B

Dorsal funiculus Fasciculus gracilis Dorsal root collateralization zone

Dorsal median septum

Dorsal root bifurcation zone

Lamina I

Dorsal Substantia horn gelatinosa

's er au t ss ac Li tr

Laminae IV-V

Intermediate interneurons

Central autonomic area

Central canal surrounded by ependyma ankle and foot?

foot?

foot?

Ventral horn interneurons thigh and leg?

ankle? thigh?

Ventral gray commissure

Ventral white commissure

lower trunk?

hip?

thigh and leg?

hip and thigh?

lower trunk? hip?

Ventral horn motoneurons Ventral median fissure

Ventral funiculus

Refer to the matching myelin stained section for the approximate locations of the lateral corticospinal tract, the lateral reticulospinal, vestibulospinal, intraspinal, and spinocephalic tracts.

Dorsal root

Lateral funiculus

240

VI. Three-Dimensional Reconstructions of the Developing Spinal Cord A. The cervical level of eight first–trimester specimens Figures 2 through 9 feature computer reconstructed three-dimensional models of the cervical part of the developing spinal cord in eight first-trimester specimens ranging from crown rump (CR) lengths of 3.3 to 56 mm (see Table VI A–1). It is during this period that the spinal cord shows the most dramatic morphogenetic changes, starting out as a structure containing mostly neuroepithelium, and ending with a structure where most components of the neuroepithelium have disappeared, neurons have been generated, migration to the gray matter is largely completed, and most major components of the white matter appear and fill with growing axons. Table VI A–2 lists the colors used to distinguish 13 different structures in the developing spinal cord in all the figures. The reconstructed models in the figures are all shown in the same orientation but differ in the number of structures that are visible to demonstrate various features of spinal cord morphogenetic development.

Table VI A–1: Specimens Used for 3-D Reconstructions of the Cervical Spinal Cord

These figures demonstrate three important relationships during spinal cord development. First, before the appearance of a population of neurons in the gray matter, the neuroepithelial stem cells that will produce that population proliferate and expand. Those stem cells recede and ultimately disappear when all the neurons in that population are generated. Second, there is a ventral (first) to dorsal (last) gradient of neuroepithelial growth and decline. Ventral horn neurons, especially the large alpha motoneurons, are generated earlier than neurons in the intermediate gray and the latest neurons to be generated are in the dorsal horn. These morphogenetic observations confirm the neurogenetic gradient that has been well documented in the rat spinal cord with 3H–thymidine autoradiography (Altman and Bayer, 1984, 2001). Third, the sequential neuroepithelial gradient is followed by a sequential gradient of morphogenesis. The ventral horn appears first, followed by the intermediate gray and finally by the dorsal horn.

Table VI A–2: Color Key for Structures in the Spinal Cord STRUCTURE

COLOR

NAME

CR (mm)

GW

Entire outside edge

Transparent clear

C6144

3.3

3.5-4.0

Roof plate

Brown

C836

4.0

4.0

Floor plate

Brown

Dorsal funiculus

Pale blue

Ventral funiculus

Pale violet

Lateral funiculus

Pale purple

M2065

8.0

5.25

C6517

10.5

5.5

BOTH SIDES OR IN MIDLINE:

LEFT SIDE:

C8965

19.1

7.0

C8553

22.0

7.2

M2050

36.0

8.5

Undivided neuroepithelium

Dark cyan

Gray matter

Bright cyan

RIGHT SIDE:

Y380-62

56.0

14

Ventral neuroepithelium

Dark red

Ventral horn

Bright red

Intermediate neuroepithelium

Dark yellow-green

Intermediate gray

Bright yellow-green

Dorsal neuroepithelium

Dark yellow

Dorsal horn

Bright yellow

Figure 2. The neuroepithelium, gray matter, and the outer edge of the spinal cord is visible in all eight specimens. The specimens are not shown to scale because structures in the smaller ones (A-D) cannot be seen clearly after reduction to the scale of the largest one (H). At CR 3.3 mm (A), there is no gray matter. The ventral neuroepithelium is larger than the intermediate or dorsal neuroepithelia. At CR 4.0 mm (B) a thin sliver of gray matter, the ventral horn, appears adjacent to the ventral neuroepithelium. At CR 8.0 mm (C), there is gray matter adjacent to all parts of the neuroepithelium, but the ventral horn is largest. The intermediate and dorsal components of the neuroepithelium are growing larger. At CR 10.5 mm (D), the ventral neuroepithelium starts to recede, but the intermediate and dorsal components are still large. The ventral horn is still the largest gray matter component, but neurons are also accumulating in the intermediate gray and dorsal horn. That same process continues at CR 19.1 mm (E). The intermediate neuroepithelium is receding at CR 22.0 mm (F) but the dorsal neuroepithelium is still prominent. The dorsal horn is now larger than the intermediate gray. By CR 36 mm (G), the dorsal neuroepithelium considerably recedes and the dorsal horn is as large as the ventral horn. By CR 56 mm (H), the neuroepithelium has been replaced by an ependyma surrounding the shrinking central canal, and the dorsal and ventral horns are prominent components of the gray matter.

241

Overview of Spinal Cord Development – GW3.5 to GW14

A

B

Roof plate

C

m

CR 0.2

m

Floor plate

5m

CR 0.5

m

um

heli

Do

pit roe

eu

ln rsa

lium

the

pi roe

eu

te n

m eliu h t i p rm roe Inte mm eu 0 n al 8. ntr Ve CR

m

3.3

FIGURE 2

m 4.0

0.5

mm

a edi

mm

rn

l ho

sa Dor

D

ate edi

y gra

rm

Inte

orn al h r t Ven m

5m

0. R1

E

C 9.1

mm

1m

m

R1

C

G

1m

m

F 6.0 R3

C

m

CR

m 2.0

1m

2

m

1m

m rn

al ho

Dors

H Gray matter

gray diate e m r Inte

orn ral h Vent

CR 1m

m

0 56.

mm

Outside edge of the spinal cord (transparent envelope)

mm

242

The Neuroepithelium – GW3.5 to GW4.0

FIGURE 3 A

CR 3.3 mm, C6144

U n d i v i d ed n

ithelium euro ep

Roof plate

Dorsal Intermediate Central canal Floor plate

0.25

mm

B

Ventral

Subdivided neuroepithelium

CR 4.0 mm, C836

Gray matter

0.5 mm

Figure 4. Enlarged views of the CR 8.0 mm (A) and CR 10.5 mm (B) specimens (note different scales). The top views do not show the gray matter on the right side so that surface features of the neuroepithelium can be observed. In A, the three components of the neuroepithelium are roughly the same size, and their outer surfaces are smooth. The outer surface represents the basal aspect of the neuroepithelium; the apical aspect (inner surface) forms the edge of the central canal. The ventral horn bulges outward. There is an intermediate gray that is thicker at its junction with the ventral horn and thinner at its junction with the few postmitotic neurons accumulating in the dorsal horn. The early dorsal horn neurons may be the large Waldeyer cells (Altman and Bayer, 2001). In B, the ventral neuroepithelium is beginning to recede in comparison with the still growing intermediate and dorsal neuroepithelia. The dashed line indicates a depression in the ventral neuroepithelium that may indicate the region where the greatest concentration of stem cells of large motoneurons are located at earlier stages. Possibly, this depression in the neuroepithelium signals the end of the neurogenetic period for motoneuron generation. The downward arrowheads point to undulations in the outer surface of the intermediate and dorsal parts of the neuroepithelium that may be “sojourn zones” of postmitotic neurons. Short survival 3H–thymidine autoradiagraphic studies in the rat spinal cord show no label uptake in these zones, even though cell density indicates that they are still within the neuroepithelium (Altman and Bayer, 2001). It is postulated that the sojourn zones are accumulations of premigratory neurons destined to settle within the intermediate gray, dorsal horn, and possibly the ventral horn. All components of the gray matter are growing larger when B is compared to A. The ventral horn is still the most prominent gray matter component and features a smooth outer surface. There is a shallow concave area in the ventral part of the outer surface of the dorsal horn. That shallow depression is the region where axons from large cells in the dorsal root ganglion accumulate in the oval bundle of His, or what we call the dorsal root bifurcation zone (see Figure 9).

Ventral horn Figure 3. An enlarged view of the specimens at CR 3.3 mm (A) and CR 4.0 mm (B) with only the neuroepithelium, roof plate, floor plate, and gray matter visible (note scale differences). The specimen in A has no gray matter. That indicates that postmitotic neurons have not yet been generated. The neuroepithelium is full of mitotically active stem cells that will eventually produce neurons. The top reconstruction in B does not show any gray matter on the right side. The bottom view in B shows a sliver of postmitotic neurons, presumably motoneurons, adjacent to the central part of the ventral neuroepithelium. Notice that the surface of the neuroepithelium is smooth in both specimens. In addition, the ventral neuroepithelium is larger than intermediate or dorsal neuroepithelia in both specimens.

0.5 mm

243

Neuroepithelium and Gray Matter – GW5.25 to GW5.5

FIGURE 4 CR 8.0 mm, M2065

euroepithelium ded n

A

Dorsal Intermediate

U n d i vi

Ventral

Subdivided neuroepithelium

0.5 mm Roof plate Central canal Gray matter Floor plate

Dorsal horn

Ventral horn

Intermediate gray

B

Subdivided gray matter

CR 10.5 mm, C6517 ▼ ▼ ▼

1 mm

▼

244

FIGURE 5 A

Neuroepithelium and Gray Matter – GW7.0 to GW7.2

CR 19.1 mm, C8965

▼

▼

Dorsal

▼

Intermediate

▼

Ventral

Subdivided neuroepithelium

1 mm Undivided neuroepithelium

▲

▲ ▲

B

CR 22.0 mm, C8553 ▼

▼ ▼

▼

1 mm

Dorsal horn Intermediate gray

Subdivided gray matter Ventral horn

Roof plate Central canal Floor plate Gray matter

▲

▲

▲

Figure 5. Enlarged views of the CR 19.1 mm (A) and CR 22 mm (B) specimens (note scale differences). The top views show only the surface features of the neuroepithelium on the right side; bottom views show the adjacent gray matter on the right side. In A, the ventral neuroepithelium has receded but the intermediate and dorsal neuroepithelia are still active (now with many undulations, downward arrowheads). The outer surface of the posterior ventral horn has bulges and depressions (upward arrowheads) representing the segregation of the ventral motoneurons into columns in the cervical enlargement. In B, the same features are present, but now the intermediate neuroepithelium is starting to recede, and the dorsal horn is growing rapidly.

245

Neuroepithelium and Gray Matter – GW8.5 to GW14 Roof plate

A ▼

FIGURE 6 CR 36 mm, M2050

▼

▼

Dorsal Intermediate Ventral

Subdivided neuroepithelium Floor plate Gray matter

1 mm Undivided neuroepithelium ▲ Central canal

▲

▲

B

CR 56 mm, Y380-62

Ependyma

1 mm ▲ Ependyma

▲ Dorsal horn

Intermediate gray

Ventral horn

▲

Subdivided gray matter

Figure 6. Enlarged views of the CR 36 mm (A) and CR 56 mm (B) specimens (note scale differences). The top views show only the surface features of the neuroepithelium on the right side; bottom views show the adjacent gray matter on the right side. In A, all parts of the neuroepithelium are relatively smaller than in previous specimens but the dorsal neuroepithelium (still with many undulations, downward arrowheads) is active. The bulges in the outer surface of the ventral horn (upward arrowheads) extend throughout its entire length representing the advancing segregation of the ventral motoneurons into columns at all cervical levels. In B, the neuroepithelium has been transformed into an ependyma that surrounds a considerably smaller central canal. The outer surface of the ependyma is smooth. The dorsal horn has grown relatively larger and reaches the same size as the ventral horn.

246

Neuroepithelium, Roof and Floor Plates – GW3.5 to GW5.5 FIGURE 7 CR 3.3 mm, C6144 A Figure 7. The undivided neuroepithelium, roof plate, floor plate, and gray matter on the Roof plate

left side of the spinal cord in the following specimens: CR 3.3 mm (A), CR 8 mm (B), and CR 10.5 mm (C). Note the scale differences between specimens. The dashed lines run through the sulcus limitans. Notice that the sulcus is closer to the roof plate in A and shifts downward in B and C. That coincides with the expansion of the dorsal neuroepithelium, which is located above the sulcus limitans. In these young specimens, the roof plate is the uppermost structural feature of the spinal cord, and is always above the level of the dorsal horn. In addition, the roof plate has a smooth domed surface with no indication of a cleft in the midline. The vertical bars show the actual distance (in millimeters) between the roof and floor plates in the approximate center section of each model. The distances continually increase from A to C indicating that the neuroepithelium has a net growth during this period, even though many young neurons have already been generated and their stem cells are no longer in the neuroepithelium.

Dorsal ca

nal

anal

Central c

0.25 mm

anal

al c

r Vent

Floor plate 0.25 m

m

B

CR 8.0 mm, M2065

Dorsal canal

Central canal

0.59 mm

l

a Ventral can

0.5 mm

Anterior cut edge of the neuroepithelium

C

Medial face of the neuroepithelium

CR 10.5 mm, C6517 Dorsal canal

Central canal

0.74 mm

anal

c Ventral

Gray matter 1 mm

247

Neuroepithelium, Roof and Floor Plates – GW7.0 to GW14 Figure 8. A continuation of the series in the following specimens: CR 19.1 mm (A), CR 22 mm (B), CR 36 mm (C), and CR 56 mm (D). Note the scale differences between specimens. The sulcus limitans (dashed line) gets closer and closer to the floor plate from A to D, reflecting the regression of the ventral and intermediate neuroepithelia. On the other hand, the dorsal neuroepithelium is expanding between A and B, indicating that it is still producing neurons. Note that the distance between the roof and floor plates (vertical bars with measurements in millimeters) reaches its maximum between A and B as the dorsal neuroepithelium stretches in the dorsoventral plane. The dorsal neuroepithelium declines between B and C as neurogenesis wanes, and the distance between the roof and floor plates gets shorter. The dorsal neuroepithelium disappears between C and D, marking the end of neurogenesis in the cervical spinal cord. By this time, the distance between the roof and floor plates shrinks to a smaller value than the youngest specimen (A, Figure 7). In A, B, and C, the roof plate is positioned at the same level as the most dorsal part of the gray matter, then it sinks downward in D, and the gray matter begins to arch dorsally, as neurons settle and start to differentiate in the dorsal horn. During that downward movement, the roof plate changes its surface features. In all panels of Figure 7, the roof plate surface is a single arch spanning the midline. In A of this series, the surface is a shallow double arch with a cleft in the midline. The roof plate still has a double arch in the dorsal midline in B, but there is a noticable lengthening in the dorsoventral plane. In C and D, the double arch is replaced by a sharp vertical spike in the midline that starts out elongated (C) and then moves downward (D) as the dorsal canal completely disappears to leave only the central canal. Concurrent with these changes in the roof plate, the dorsal funiculus fills with axons in its most medial parts and marks the beginning of the fasciculus gracilis and fasciculus cuneatus in the cervical spinal cord. It is postulated that the roof plate provides some guidance or structural support for axonal recruitment in these fasciculi (Altman and Bayer, 2001).

FIGURE 8

A Dorsal canal

0.91 mm l canal

Centra

Ventral canal Roof plate (short double arch)

Floor plate 1 mm

CR 19.1 mm, C8965

B Dorsal canal

1.01mm Central canal Ventral canal Roof plate (elongated double arch)

1 mm

CR 22.0 mm, C8553

C Dorsal canal

0.49 mm al Central canal n a Ventral c

Roof plate (elongated vertical spike)

1 mm

CR 36.0 mm, M2050 0.16 mm

D Central canal

Roof plate (contracting vertical spike)

1 mm

Gray matter

CR 56.0 mm, Y380-62

248

Growth of the White Matter – GW5.25 to GW14

FIGURE 9 A

RP

*

*

*

*

* *

*

*

D

**

IN

DN

DF

VN

CR

mm

8.0

FP

* VF * * 0.5 * * mm * * * B * * *

.0 R 22

1m

mm

C

m

E

CC

CR 1m

mm

1

VC

*

* * * * * * * * * *

m

C

0.5

DH

m

1m

m

CR

m 36.0

F

LF IG

GM

VH

1m

m

m

CR

m 9.1

1

1m

m

.0 R 56

mm

C

Figure 9. The emergence and growth of the white matter (see Table VI A–2 for color code) in the following specimens: CR 8 mm (A), CR 10.5 mm (B), CR 19.1 mm (C), CR 22 mm (D), CR 36 mm (E), and CR 56 mm (F). Note scale differences between specimens. All parts of the gray matter and neuroepithelium are visible in the models. Asterisks (A to D) indicate where the top of the roof plate (RP) is visible in the dorsal midline. The dorsal funiculus (DF) and ventral funiculus (VF) appear simultaneously in A; the lateral funiculus (LF) is definite in C. The first axons in the ventral funiculus are most likely those of the early generated commissural neurons that accumulate in the space between the lateral edges of the floor plate (FP), the most ventral part of the ventral neuroepithelium (VN), and the ventromedial edge of the ventral horn (VH). In some sections of the model in A, the axons are even closer to the midline, so the ventral funiculus first has a medial direction of growth (bottom arrows in A pointing toward the midline). In B, these fibers cross the midline in the ventral commissure (VC), and the ventral funiculus increases in size and fills out laterally as well as medially. The ventral funiculus continues to enlarge throughout the remainder of spinal cord development and contains several descending tracts (medial longitudinal fasciculus, tectospinal tract, and vestibulospinal tract) as well as some of the ascending spinocephalic tracts and the collaterals of axons from neurons throughout the ventral and intermediate gray in the intraspinal tract (also known as the propriospinal tract). The dorsal funiculus has a complex pattern of growth that features three different gradients. In A, the dorsal funiculus contains axons from large neurons in the dorsal root ganglia that enter the spinal cord and bifurcate into ascending and descending branches. This is the dorsal root bifurcation zone (also known as the oval bundle of His). That zone is more obvious at lower than upper cervical levels because the highest dorsal root ganglion is below the highest cervical level of the spinal cord, and axons have not yet reached there. The forward-facing arrows in A and B indicate the advance of the dorsal root bifurcation zone to upper cervical levels, which are filled with axons in C. Axons from the dorsal root continue to fill the bifurcation zone, and the next gradient of growth is from ventrolateral to dorsomedial (arrows arching upward toward the midline in C and D). During this time, the bifurcating fibers are growing collaterals (the dorsal root collateralization zone) that will eventually penetrate the gray matter. In E and F, the dorsal funiculus reaches the midline and expands downward (downward facing arrows). These axons are segregated in the midline by a band of glia that extends to the pia from the top of the roof plate. This medial part of the dorsal funiculus contains the long ascending branches of the dorsal root axons in the fasciculus gracilis and fasciculus cuneatus (see Chapter 5, Figures 5-28 and 5-49 in Altman and Bayer, 2001).

249

Part VI: Three-Dimensional Reconstructions (concluded)

B. The progressive segregation of ventral horn motoneurons into columns This section features computer reconstructed threedimensional models of the entire spinal cord in three specimens: CR 36 mm (GW8.5), CR 56 mm (GW10.5), and CR 108 mm (GW14). Throughout the ventral horn, individual sections contain clumps of motoneurons. When the sections are aligned, these clumps form longitudinal columns. Each distinguishable motoneuron column has been reconstructed in the three specimens. Figures 10 and 11 provide an overview of the sections included in the GW8.5 model; the model is shown in Figures 12 through 15. Figures 16 through 18 provide an overview of the sections included in the GW10.5 model; the model is shown in Figures 19 through 22. Figures 23 through 25 provide an overview of the sections included in the GW14 model; the model is shown in Figures 26 through 31. The segregation of motoneuron columns is a progressive developmental process. On each side of the ventral horn, there are 7 motor columns in the GW8.5 specimen, 9 in the GW10.5 specimen, and 15 in the GW14 specimen. Experimental studies in animals show that motoneuron segregation is related to dendritic differentiation (see Chapter 1, Section 1.6 and Chapter 3, Section 3.3 in Altman and Bayer, 2001). When there are fewer columns containing larger groups of neurons, dendrites are growing in many directions. As development progresses, the dendrites are reshaped into well-defined bundles that extend longitudinally in the spinal cord (see Figures 1–68, 1–69, 3–23 and 3–24 in Altman and Bayer, 2001). That process produces progressively better defined motoneuron columns in cell body stained sections. Possibly, motoneuron dendritic bundles develop similarly in humans. Compare, for example, the progressive segregation of motoneurons in the cervical enlargement in Plates 15, 23, and 31.

All parts of the spinal cord contain at least one motoneuron column located in the ventromedial part of the ventral horn. In the thoracic cord, that is the only column present. Progressively more lateral columns are found at the cervical and lumbar levels, especially in their respective enlargements. Traditionally, these columns are named according to their position. In our book on development of the human spinal cord (Altman and Bayer, 2001), that locational terminology has been put into a grid system containing up to four panels (medial, central, lateral, and far–lateral) and three tiers (ventral, dorsal, and retrodorsal). The positional labeling system is functionally neutral because little is known about the exact groups of muscles that are innervated by motoneurons in the human spinal cord. The overview figures that introduce each model contain the positional labeling for each reconstructed motoneuron column. However, there is a wealth of experimental anatomical evidence in animals, including primates, that the locations of motoneuron columns in the ventral horn have functional significance (see Chapter 1, Section 1.6 and Chapter 2, Section 2.3.2 in Altman and Bayer, 2001). The ventromedial columns at all levels (medial panel, ventral tier) contain motoneurons that innervate the axial muscles. More laterally placed columns at upper cervical and upper lumbar levels contain motoneurons that innervate proximal limb muscles (central, lateral, and far-lateral panels, ventral to dorsal tiers). Lateral columns in the cervical and lumbar enlargements contain motoneurons that innervate distal limb muscles (especially those located in the far-lateral panel and retrodorsal tier). The overview figures preceding each model contain a table listing the possible innervations of each three-dimensionally reconstructed motor column.

250

GW8.5, CR 36 mm, M2050

FIGURE 10

Cervical Sections of 3-D Model

section number

edge of white matter edge of gray matter

730

750 801

2

d v

1

2

1

897

m c

1

UPPER CERVICAL

5 4 3

rd d v

1

m c l

MOTOR COLUMN KEY Labeled Number (also in Figure 13E) Color Column (Panel and Tier)

921

Muscle Innervation (?) medioventral (mv), mediodorsal (md) (all sections)

AXIAL

2

centrodorsal (cd) (sections 730-750)

SHOULDER GIRDLE?

3

1ateroventral (lv), centroventral (cv) (sections 897-1113)

PROXIMAL UPPER LIMB

4

laterodorsal (ld), centrodorsal (cd) (sections 897-1113)

PROXIMO-DISTAL UPPER LIMB (ARM AND FOREARM)

5

lateroretrodorsal (lrd) (sections 897-1113)

DISTAL UPPER LIMB (WRIST AND HAND)

1

5 1

4 3

954

5

1

4

3

GRID KEY (based on Altman and Bayer, 2001) TIERS

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral)

1006 5

*This panel includes the lateral and far lateral subdivisions

1 4

LOWER CERVICAL 1170

1053 1113 5 4

5

3

3

1

5 4

1

3

1

Figure 10. This specimen from the Minot Collection contains 10-µm thick sections that are consecutively numbered on the slides. All outlines are profiles of the sections used in the cervical spinal cord. Three of these sections are illustrated in the Atlas. Section 730 is in Plate 14; section 921 is in Plate 15, and section 1113 is in Plate 16. After fixation, the total length of the cervical part of the model (shown in Figure 13) extends 4.4 mm.

251

GW8.5, CR 36 mm, M2050

section number

1225

FIGURE 11 Thoracic, Lumbar and Sacral Sections of 3-D Model

1281 1593

edge of white matter

1702 1

mc

d v

edge of gray matter

1

1761 1 1

THORACIC 1

1884

MOTOR COLUMN KEY Labeled Number (also in Figures 14E, 15E) Color Column (Panel and Tier) Muscle Innervation (?) 1

mv (all sections) md (section 2287) cv (section 2452

AXIAL

6

lv (sections 2008-2134) cv, lv (sections 2287-2452)

PROXIMAL LOWER LIMB

1

2008

7

d v

6 1

mc l

2134 7

ld (sections 2008-2452)

DISTAL LOWER LIMB

GRID KEY (based on Altman and Bayer, 2001)

7

TIERS

1

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral)

6

LUMBAR

2287

*This panel includes the lateral and far lateral subdivisions 7 6

1

2452 SACRAL 2607

7 6 1

2674 1 1

Figure 11. A continuation of the previous figure showing outlines of the sections used in the thoracic, lumbar, and sacral spinal cord. Six of these sections are illustrated in the Atlas. Section 1593 is in Plate 17; section 1884 is in Plate 18, section 2134 is in Plate 19, and sections 2607 and 2674 are in Plate 20. After fixation, the total length of the thoracic part of the model (Figure 14) is 6.99 mm and the combined lumbar and sacral regions is 6.66 mm (Figure 15).

252

Top Front View GW8.5, CR 36 mm, M2050

A

Posterior edge of white matter

Anterior edge of white matter

B Sacral area

Posterior edge of gray matter Lumbar Enlargement

FIGURE 12

Anterior edge of gray matter

Cervical Enlargement

Thoracic area

Anterior edge of gray matter

C Lateral motor columns (lumbar)

Central canal (defines midline)

Lateral motor columns (cervical)

Right

Left

Medial motor columns

Figure 12. The entire spinal cord in specimen M2050 (GW8.5, CR 36 mm) as viewed from the front and top (A, B, C, this page) or the upper right side (D, E, F, facing page). The front edge of the reconstruction is section 730, the back edge is section 2674. The total length after fixation is 19.44 mm. In this and the following reconstructions, the spinal cord has been enlarged 3 times more crosswise than lengthwise. Otherwise, the model is too long to see the various motoneuron columns clearly. Solid outlines define the outer edges of the white matter (transparent envelope, A and D); dashed outlines define the outer edges of the gray matter (less transparent envelope, B and E). The gray-white solid in panels A-E is the central canal. The colored solids in panels A-E are ventral horn motor columns. There are seven distinguishable pairs of motor columns in this specimen. The medial motor columns (cyan) extend throughout the entire length of the spinal cord. There are four lateral motor columns in the cervical region, a short one (light brown) in the upper cervical and three (yellow, orange, red) in the cervical enlargement. The ventral to dorsal stacking of the lateral motor columns is more easily observed in the side views (especially panel F). At this stage of development, the lumbar enlargement is not prominent and contains only two lateral motor columns (purple and pink).

253

GW8.5, CR 36 mm, M2050 Side View D

FIGURE 12

DORSAL

VENTRAL

Sacral area

E Thoracic area

Lumbar enlargement

Cervical enlargement

F

Central canal (defines midline)

Lateral motor columns (cervical)

Lateral motor columns (lumbar)

Medial motor column (left) Medial motor column (right)

254

GW8.5, CR 36 mm, M2050

FIGURE 13 Top View A

Side View B

White matter outlined

C

D

Gray matter outlined

E

F Central canal (defines midlline)

5 4

3

2 1 Medial motor columns

Lateral motor columns

Figure 13. The cervical region in specimen M2050 (GW8.5, CR 36 mm). The individual sections (730–1170) in this model are diagrammed in Figure 10. The reconstructed length in the specimen is 4.4 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined outer transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and motor columns (colored solids). Panels C and D show the only gray matter (outlined outer transparent envelope) around the central canal and motor columns. Panels E and F show the central canal and motor columns alone. Motor column 1 (cyan, axial muscle motoneurons?) is located in the medial panel, ventral and dorsal tiers. Motor column 2 (light brown, shoulder girdle motoneurons?) is located in the central panel, dorsal tier at high cervical levels. Motor column 3 (red, shoulder muscle motoneurons?) in the cervical enlargement is located in the lateral and central panels, ventral tier. Motor column 4 (orange, arm and forearm muscle motoneurons?) in the cervical enlargement is located in the lateral and central panels, dorsal tier. Motor column 5 (yellow, wrist and hand muscle motoneurons?) in the cervical enlargement is located in the lateral panel, retrodorsal tier.

255

GW8.5, CR 36 mm, M2050

FIGURE 14

Top View A

Side View B

White matter outlined

C

D

Gray matter outlined

E

Central canal (defines midlline)

1

F

Medial motor columns

Figure 14. The thoracic region in specimen M2050 (GW8.5, CR 36 mm). The individual sections in this model are diagrammed in Figure 11 and include sections 1225-1884. The reconstructed length in the specimen is 6.99 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined outer transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and the medial motor columns (cyan). Panels C and D show the only gray matter (outlined outer transparent envelope) around the central canal and the medial motor columns. Panels E and F show the central canal and the medial motor columns alone. This region of the spinal cord is distinguished from other regions by having only one motor column (1) that probably innervates axial muscles associated with the thoracic vertebrae, rib cage and dorsal abdominal wall.

256

GW8.5, CR 36 mm, M2050

FIGURE 15 Top View A

Side View B

White matter outlined

C

D

Gray matter outlined

E

Central canal (defines midlline)

F

Medial motor column extending into the sacral level

7 6

1 Medial motor columns

Lateral motor columns

Figure 15. The lumbar and sacral regions in specimen M2050 (GW8.5, CR 36 mm). The individual sections in this model are diagrammed in Figure 11 and include sections 2008–2674. The reconstructed length in the specimen is 6.66 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined outer transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and motor columns (colored solids). Panels C and D show the gray matter (outlined outer transparent envelope) around the central canal and motor columns. Panels E and F show the central canal and motor columns alone. Motor column 1 (cyan, axial muscle motoneurons?) is located in the medial and central panels, ventral and dorsal tiers. Motor column 6 (purple, proximal lower limb muscle motoneurons?) in the lumbar enlargement is located in the central and lateral panels, ventral tier. Motor column 7 (pink, distal lower limb muscle motoneurons?) in the lumbar enlargement is located in the lateral panel, dorsal tier.

257

GW10.5, CR 56 mm, Y380-62

FIGURE 16 Cervical Sections of 3-D Model

361

edge of white matter edge of gray matter

section number

391 rd d v

2 1

2

431

1

m c l 4 3

UPPER CERVICAL

1 2

MOTOR COLUMN KEY Labeled Number (also in Figure 20E) Color Column (Panel and Tier) Muscle Innervation (?) 1

mv, md (all sections)

AXIAL

2

cd (sections 361-391) lv, lc (sections 431-551) ld (sections 581-611)

DIAPHRAGM AND PROXIMAL UPPER LIMB (SHOULDER)

3

1d (sections 431-581)

PROXIMO-DISTAL UPPER LIMB (ARM AND FOREARM)

4

lrd (sections 431-551)

DISTAL UPPER LIMB (WRIST AND HAND)

5

cv (sections 551-581) mv, cv (section 611)

A medial subdivision of motor column 2? A lateral subdivision of motor column 1?

461 4

1

3 2

491 4 3

1 2

521 GRID KEY (based on Altman and Bayer, 2001)

4

TIERS

1

3 2

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral)

551

*This panel includes the lateral and far lateral subdivisions 4 3 2

5

1

LOWER CERVICAL 581 3 2

611 2

5

1

rd d v

5 1

m c l Figure 16. This specimen from the Yakovlev Collection contains serially numbered 35-µm thick sections. All outlines are profiles of the sections used in the cervical region of the spinal cord. These profiles were constructed from the right side of the section, then copied to the left side to make the model symmetrical. After fixation, the cervical region of the model is 8.79 mm long. Two of these sections are illustrated in the Atlas. Section 361 is in Plate 22; section 521 is in Plate 23. The cervical model is shown by itself in Figure 20.

258

GW10.5, CR 56 mm, Y380-62

FIGURE 17

Thoracic Sections of 3-D Model

section number

671 UPPER THORACIC

edge of white matter 1

m c

edge of gray matter

d v

706 MOTOR COLUMN KEY Labeled Number (also in Figure 21E) Color Column (Panel and Tier) Muscle Innervation (?) 1

mv (all sections)

1

731

AXIAL 1

GRID KEY (based on Altman and Bayer, 2001) TIERS

781

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral) 1

846

*This panel includes the lateral and far lateral subdivisions

1

881 LOWER d THORACIC

1

m c

v

Figure 17. A continuation of the previous figure showing outlines of the sections used in the thoracic region of the spinal cord. Two of these sections are illustrated in the Atlas. Section 671 is in Plate 24; section 881 is in Plate 25. After fixation, the total length of the thoracic part of the model is 7.35 mm. The thoracic model is shown by itself in Figure 21.

259

GW10.5, CR 56 mm, Y380-62

FIGURE 18 Lumbar and Sacral Sections of 3-D Model

section number

911

941 971 8

rd d v

76 1

m c l

edge of white matter edge of gray matter

8 7

6

1

8 7

LUMBAR

6

1

1001

8 9 7

MOTOR COLUMN KEY

1

6

Labeled Number (also in Figure 22E) Color Column (Panel and Tier) Muscle Innervation (?)

1031

8 9

1

mv, md (all sections)

6

cv, lv (sections 911-1091)

PROXIMAL LOWER LIMB (HIP AND THIGH)

7

1d (sections 911-1091)

PROXIMO-DISTAL LOWER LIMB (KNEE AND LEG)

8

lrd, ld (sections 911-1091)

DISTAL LOWER LIMB (ANKLE AND FOOT)

7

AXIAL

1

6

1066

8 1

9

7

6

9

cd, cv (sections 1001-1091)

PROXIMAL LOWER LIMB (HIP AND THIGH)

1091 GRID KEY (based on Altman and Bayer, 2001) TIERS

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral)

8 7

1 9

6

*This panel includes the lateral and far lateral subdivisions

1151

SACRAL

1

m c

1181

1211

1

m

d v

d v

1

Figure 18. A continuation of the previous figure showing outlines of the sections used in the lumbar (911-1091) and sacral (1151-1211) regions of the spinal cord. Three of these sections are illustrated in the Atlas. Section 911 is in Plate 26; section 1006 is in Plate 27, section 1211 is in Plate 28. After fixation, the total length of the lumbosacral part of the model is 10.5 mm. The lumbosacral part of the model is in Figure 22.

260

GW10.5, CR 56 mm, Y380-62

FIGURE 19 A

B Sacral area

Posterior edge of gray matter Lumbar Enlargement

Posterior edge of white matter

Anterior edge of gray matter

Thoracic area

Cervical Enlargement

Anterior edge of white matter

Anterior edge of gray matter

C

Central and lateral motor columns Central canal (lumbar) (defines midline)

Central and lateral motor columns (cervical)

❉

❉

Right Left Medial motor columns

Figure 19. The entire spinal cord in specimen Y380-62 (GW10.5, CR 56 mm) as viewed from the front and top (A, B, C, this page) or the upper right side (D, E, F, facing page). The front edge of the reconstruction is section 361, the back edge is section 1211. The total length after fixation is 29.79 mm. In this and the following reconstructions, the spinal cord has been enlarged 3 times more crosswise than lengthwise. Otherwise, the model is too long to see the various motoneuron columns clearly. Solid outlines define the outer edges of the white matter (transparent envelope, A and D); dashed outlines define the outer edges of the gray matter (less transparent envelope, A, B and D, E). The gray-white solid in panels A-E is the central canal. The colored solids in panels A-E are ventral horn motor columns. There are nine pairs of motor columns on each side of the ventral horn. The medial motor columns (cyan) extend throughout the entire length, including sacral levels. There are four lateral motor columns in the cervical region, a long one (light brown) extending throughout the entire region and three (red, orange, yellow) that are limited to the cervical enlargement. The lumbar enlargement is more prominent than at GW8.5, and contains four central and lateral motor columns (purple, pink, violet, pale violet). The asterisks in C indicate the motor columns in the cervical enlargement that are not visible in the side view. The ventral to dorsal stacking of the lateral motor columns is more easily seen in the side views (especially panel F).

261

GW10.5, CR 56 mm, Y380-62

D

FIGURE 19

DORSAL

VENTRAL

Sacral area

E

Thoracic area

Lumbar enlargement

Cervical enlargement

F Central and lateral motor columns (cervical)

Central canal (defines midline)

Central and lateral motor columns (lumbar) Medial motor column (left) Medial motor column (right)

262

GW10.5, CR 56 mm, Y380-62

FIGURE 20 Top View A

Side View B

White matter outlined

C

D

Gray matter outlined

E

Central canal (defines midlline)

F

5 4 3 2 1

Medial motor columns

Central and lateral motor columns

Figure 20. The cervical region in specimen Y380-62 (GW10.5, CR 56 mm). The individual sections in this model are diagrammed in Figure 16 and include sections 361-611. The reconstructed length is 8.79 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and motor columns (colored solids). Panels C and D show the gray matter (outlined transparent envelope) around the central canal and motor columns. Panels E and F show the central canal and motor columns alone. Motor column 1 (cyan, axial muscle motoneurons?) is located in the medial panel, ventral and dorsal tiers. Motor column 2 (light brown, proximal upper limb muscle motoneurons?) is located in the central and lateral panels, ventral and dorsal tiers. Motor column 3 (red, arm and forearm muscle motoneurons?) is located in the cervical enlargement in the lateral panel, dorsal tier. Motor column 4 (orange, wrist and hand muscle motoneurons?) is located in the cervical enlargement in the lateral panel, retrodorsal tier. Motor column 5 (yellow, forearm muscle motoneurons?) in the posterior cervical enlargement (sections 551-611) is located in the medial and central panels, ventral tier.

263

GW10.5, CR 56 mm, Y380-62

FIGURE 21

Top View A

Side View B

White matter outlined

C

D

Gray matter outlined

E

Central canal (defines midlline)

F

1 Medial motor columns

Figure 21. The thoracic region in specimen Y380-62 (GW10.5, CR 56 mm). The individual sections in this model are diagrammed in Figure 17 and include sections 671-881. The reconstructed length is 7.35 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and the medial motor columns (cyan). Panels C and D show the gray matter (outlined transparent envelope) around the central canal and the medial motor columns. Panels E and F show the central canal and the medial motor columns alone. This region of the spinal cord is distinguished from other regions by having only one motor column (1) that probably innervates axial muscles associated with the thoracic vertebrae, rib cage, and dorsal abdominal wall.

264

GW10.5, CR 56 mm, Y380-62

FIGURE 22 Top View A

Side View B

White matter outlined

C

D

Gray matter outlined Medial motor column extending into the sacral level

E

F

Central canal (defines midlline) Central and lateral motor columns

9 8 7

6

1 Medial motor columns

Figure 22. The lumbar and sacral regions in specimen Y380-62 (GW10.5, CR 56 mm). The individual sections (911-1211) in this model are diagrammed in Figure 18. The reconstructed length of the specimen is 10.5 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and motor columns (colored solids). Panels C and D show the gray matter (outlined transparent envelope) around the central canal and motor columns. Panels E and F show the central canal and motor columns alone. Motor column 1 (cyan, axial muscle motoneurons?) is located in the medial and central panels, ventral and dorsal tiers. The remaining motor columns (6-9) are limited to lumbar levels (sections 911-1091). Column 6 (purple, motoneurons controlling hip movements?) is located in the central and lateral panels, ventral tier. Column 7 (pink, motoneurons controlling knee movements?) is located in the lateral panel, dorsal tier. Column 8 (violet, motoneurons controlling ankle and foot movements?) is located in the lateral panel, dorsal and retrodorsal tiers. Column 9 (pale violet, motoneurons controlling hip movements?) is located in the central panel, dorsal and ventral tiers.

265

GW14, CR 108 mm, Y68-65

FIGURE 23 Cervical Sections of 3-D Model

section number

701

741 rd d v

2 1

mc

4

edge of white matter

2

edge of gray matter

781

1

l

3

UPPER CERVICAL

4 32

1

821 MOTOR COLUMN KEY Labeled Number (also in Figures 27C-28C) Color

2

5 4

1

3

Column (Panel and Tier) Muscle Innervation (?)

861 mv (all sections)

1 2

cv, cd (sections 701-821)

3

cv, cd (sections 701-821)

4

cd (sections 741-781) ld, lv (sections 821-1061)

5

ld (sections 821-981) 1v (section 1021) lrd (section 1061)

7 8

AXIAL

DIAPHRAGM, PROXIMAL UPPER LIMB (SHOULDER)

9

5 6

4

PROXIMO-DISTAL UPPER LIMB (ARM AND FOREARM)

1

901

7 8

1

941

7 8

9

ld (sections 901-1061)

6

9

5 4

5 6

1 4

7

lrd (sections 861-1061)

8

lrd (sections 861-1061)

9

lrd (sections 861-1061)

DISTAL UPPER LIMB (WRIST AND HAND)

9

6

5

1

4

GRID KEY (based on Altman and Bayer, 2001) 7

TIERS

1021

8

9 6

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral) *This panel includes the lateral and far lateral subdivisions

981

7 8

5

4

LOWER CERVICAL

7

1

1061

8

9

6

5 4

rd d v

1

mc l Figure 23. This specimen from the Yakovlev Collection contains serially numbered 35-µm thick sections. All outlines are profiles of the sections used in the cervical region of the spinal cord. These profiles were constructed from the right side of the section, then copied to the left side to make the model symmetrical. Two of these sections are illustrated in the Atlas. Section 741 is in Plate 30; section 981 is in Plate 31. The cervical model is shown by itself in Figures 27 and 28. After fixation, the cervical region of the model is 12.6 mm long.

266

GW14, CR 108 mm, Y68-65

FIGURE 24 1181 section number

1221

Thoracic Sections of 3-D Model

1141 aut

1

1

aut

edge of white matter edge of gray matter

1261

aut

1301

aut

1

1

1101

1341

aut 1

aut

aut 4 1

mc

UPPER d v THORACIC

1381

1

aut 1

1421

MOTOR COLUMN KEY Labeled Number (also in Figure 31E)

aut

Color Column (Panel and Tier) Innervation (?) 1

mv, cv (all sections)

AXIAL MUSCLES

AUT

Lateral horn (all sections)

POSTGANGLIONIC MOTONEURONS

cd (section1101)

UPPER LIMB MUSCLES ?

4

1

1461 aut 1

1501 aut 1

GRID KEY (based on Altman and Bayer, 2001) TIERS

1541

l (lateral)* c (central) m (medial)

PANELS

rd (retrodorsal) d (dorsal) v (ventral)

aut 1

*This panel includes the lateral and far lateral subdivisions

1581 LOWER THORACIC

1621

1

1701

1741

aut

1661 aut 1 aut

aut

aut

1

d v

1

1

mc

Figure 24. A continuation of the previous figure showing outlines of the sections used in the thoracic region of the spinal cord. Three of these sections are illustrated in the Atlas. Section 1141 is in Plate 32; section 1581 is in Plate 33; section 1741 is in Plate 34. After fixation, the total length of the thoracic part of the model is 22.4 mm. The thoracic model is shown by itself in Figure 31.

267

GW14, CR 108 mm, Y68-65 Lumbar and Sacral Sections of 3-D Model

FIGURE 25 section number

LUMBAR

1781 aut

d v

10 1

MOTOR COLUMN KEY

mc l

1821

edge of gray matter edge of white matter

aut

Labeled Number (also in Figures 29C-30C)

11

Color Column (Panel and Tier) Innervation (?) mv (sections 1781-1901)

1

1

10

aut

Lateral horn (sections 1781-1821)

10

ld, lv (sections 1781-1981) cd (section 2021)

11

1d, lv (sections 1821-1981)

12

ld (sections 1861-1901) 1rd (sections 1941-1981)

13

ld, lrd (sections 1861-2061)

14

lrd (sections 1861-1981)

15

cd, ld (sections 1861-1981)

1861

AXIAL MUSCLES

rd d v

13 14 12 10

POSTGANGLIONIC MOTONEURONS

15 11

mc

1901 PROXIMAL LOWER LIMB MUSCLES (HIP AND THIGH)

1

13 12

14

10

11

l

15 1

1941

DISTAL LOWER LIMB MUSCLES (ANKLE AND FOOT)

12

PROXIMO-DISTAL LOWER LIMB MUSCLES (KNEE AND LEG)

13 14 15 11 10

1981

GRID KEY (based on Altman and Bayer, 2001) TIERS

l (lateral)* c (central) m (medial)

PANELS

13 12 14 11 15 10

rd (retrodorsal) d (dorsal) v (ventral)

2021

*This panel includes the lateral and far lateral subdivisions

13 10

2061 13

2101 mc

d v

SACRAL Figure 25. A continuation of the previous figure showing outlines of the sections used in the lumbar (1781-1981) and sacral (2021-2101) regions of the spinal cord. Three of these sections are illustrated in the Atlas. Section 1821 is in Plate 35; section 1901 is in Plate 36, section 2061 is in Plate 37. After fixation, the total length of the lumbosacral part of the model is 11.2 mm. The lumbosacral part of the model is in Figures 29-30.

268

GW14, CR 108 mm, Y68-65

FIGURE 26 Posterior edge of white matter

A Anterior edge of white matter

Anterior edge of gray matter

Posterior edge of gray matter Lumbar Enlargement

B

Sacral area

Cervical Enlargement

Thoracic area

Anterior edge of gray matter

C

Central canal Central and (defines midline) lateral motor columns (cervical)

Central and lateral motor columns (lumbar)

Autonomic motor column

Right Left Medial motor columns

Figure 26. The entire spinal cord in specimen Y68-65 (GW14, CR 108 mm) as viewed from the front and top (A, B, C, this page) or the upper right side (D, E, F, facing page). The front edge of the reconstruction is section 701, the back edge is section 2101. The total length after fixation is 49 mm. In this and the following reconstructions, the spinal cord has been enlarged 3 times more crosswise than lengthwise. Otherwise, the model is too long to see the various motoneuron columns clearly. Solid outlines define the outer edges of the white matter (transparent envelope, A and D); dashed outlines define the outer edges of the gray matter (less transparent envelope, A, B and D, E). The gray-white solid in panels A-E is the central canal. The colored solids in panels A-E are ventral horn motor columns. There are 15 pairs of motor columns on each side of the ventral horn in this specimen. The medial motor columns (cyan) are split in the thoracic region and extend into the lumbar enlargement up to section 1901 and then disappear. The remaining motor columns are in the ventral, central, and lateral sectors. There are eight central and lateral motor columns in the cervical region (light brown, orange, yellow shades). The lumbar enlargement is large in this specimen and contains six central and lateral motor columns (purple, violet, magenta shades). The top and side views in panels C and F show the intertwined arrangement of the motor columns in the cervical and lumbar enlargements. The autonomic motor columns (green) in the lateral horn of the thoracic and upper lumbar regions are also reconstructed in this specimen.

269

GW14, CR 108 mm, Y68-65

FIGURE 26

D DORSAL

VENTRAL

E

Sacral area Thoracic area

Lumbar enlargement

Cervical enlargement

F

Central and lateral motor columns (cervical)

Central canal (defines midline)

Autonomic motor columns Medial motor column (left) Medial motor column (right)

Central and lateral motor columns (lumbar)

270

GW14, CR 108 mm, Y68-65

FIGURE 27 A

White matter outlined

Gray matter outlined

B

C

D

Columns 1 and 2 Central canal

E

Columns 1 and 3

F

Columns 1 and 4

G

Columns 1 and 5

H

Columns 1 and 6

I

Columns 1 and 7, 8, 9

Central canal (defines midlline)

7 8 6 9 5 4

2 3 1

Medial motor columns

Central and lateral motor columns

Figure 27. The top front view of the cervical region in specimen Y68-65 (GW14, CR 108 mm). The individual sections (701-1061) in this model are diagrammed in Figure 23. The length of the cervical region is 12.6 mm after fixation. All panels show the entire model or parts of it from the top front. Panel A shows both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and motor columns (colored solids). Panel B shows only the gray matter (outlined transparent envelope) around the central canal and motor columns. Panel C shows the central canal and all motor columns alone. Subsets of motor columns are shown in the small side panels D-I.

271

GW14, CR 108 mm, Y68-65

FIGURE 28 D

A

Columns 1 and 2 Central canal

E

Columns 1 and 3

F

Columns 1 and 4

G

Columns 1 and 5

H

Columns 1 and 6

I

Columns 1 and 7, 8, 9

White matter outlined

B

Gray matter outlined

Central canal (defines midlline)

C Central and lateral motor columns

9 5

8 7 6

4 2 Medial motor columns

1

3

Figure 28. A continuation of the previous figure in the same specimen, except that all panels show either the entire model or parts of it from the side view. Motor column 1 (cyan, axial muscle motoneurons?) is located in the medial panel, ventral tier. Motor columns 2 and 3 (yellow, orange) at upper cervical levels (sections 701-821) may innervate proximal upper limb muscles, and are located in the central and lateral panels, ventral and dorsal tiers. Motor columns 4 to 6 (yellow brown, red, orange) are located in the cervical enlargement in the central and lateral panels, ventral, dorsal, and retrodorsal tiers and may innervate arm and forearm muscles. Motor columns 7 to 9 (dark red orange, light red orange, yellow orange) are located in the cervical enlargement in the lateral panel, retrodorsal tier and may innervate wrist and hand (digit) muscles.

272

GW14, CR 108 mm, Y68-65

FIGURE 29 White matter outlined

A

D

Columns 1 and Autonomic Central canal

E

Columns 1 and 10

F

Columns 1 and 11

G

Columns 1 and 12

H

Columns 1 and 13

I

Columns 1 and 14

J

Columns 1 and 15

Gray matter outlined

B

C Central canal (defines midlline)

13 14 12 11 10

15 1

Medial motor columns

Autonomic motor columns

Central and lateral motor columns

Figure 29. The lumbar and sacral regions in specimen Y68-65 (GW14, CR 108 mm). The individual sections (1781-2101) in this model are diagrammed in Figure 25. The reconstructed length is 11.2 mm after fixation. All panels show the model from the top front. Panel A shows both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (graywhite solid) and motor columns (colored solids). Panel B shows the gray matter (outlined transparent envelope) around the central canal and motor columns. Panel C shows the central canal and all the motor columns. The small side panels (D-J) show various subsets of motor columns with the central canal.

Figure 30. A continuation of the previous figure in the same specimen except that all panels show either the entire model or parts of it in a side view. Motor column 1 (cyan, medial panel, ventral tier) may innervate axial muscles. It is bifurcated in the first section (1781) and single in sections 1821 to 1861. The autonomic motor column (green) is in the first two sections of the model (1781-1821) and contains the most posterior neurons that innervate the sympathetic ganglia. The remaining motor columns (10-15) are present only at lumbar levels or upper sacral levels (sections 1781-2061). Columns

273

GW14, CR 108 mm, Y68-65

FIGURE 30

A

D Central canal

Columns 1 and Autonomic

E

Columns 1 and 10

F White matter outlined

B Columns 1 and 11

G

Columns 1 and 12

H Gray matter outlined

C

Autonomic motor columns Central canal (defines midlline)

Columns 1 and 13

I

Central and lateral motor columns

Columns 1 and 14 13 14 11 10

J

12

15 1

Medial motor columns

Columns 1 and 15

10 (pale violet) and 11 (dark violet) may innervate proximal lower limb muscles controlling the hip joint and are located in the lateral panel, ventral and dorsal tiers. Column 12, 13, and 14 (pale violet, pink, purple) are located in the lateral panel, dorsal and retrodorsal tiers. They may innervate lower limb muscles controlling the ankle and foot. Column 15 (reddish-magenta) is located in the central and lateral panels, dorsal tier and may innervate leg muscles.

274

GW14, CR 108 mm, Y68-65

FIGURE 31 Top View

A

B

Side View

White matter outlined

C

D The recessed dorsal midline "Domes" of gray matter in the dorsal horn

Gray matter outlined

E

F Central canal (defines midlline)

Autonomic motor columns 11

Posterior cap of motor column 4 in cervical region Medial motor columns (split)

Figure 31. The thoracic region in specimen Y68-65 (GW14, CR 108 mm). The individual sections (1101-1741) in this model are diagrammed in Figure 24. The reconstructed length is 22.4 mm after fixation. Panels A, C, and E show the model from the top front, panels B, D, and F from the upper right side. Panels A and B show both the white matter (outlined transparent envelope) and gray matter (inner transparent envelope) around the central canal (gray-white solid) and the medial motor columns (1, cyan), autonomic motor columns (green), and the posterior cap of motor column 4 (yellowbrown) from the cervical region that is present only in the first section (1101). Panels C and D show only the gray matter (outlined transparent envelope) around the central canal and the motor columns. Panels E and F show the central canal and the motor columns alone. This region of the spinal cord is distinguished from other regions by having a bifurcated motor column 1 that probably innervates axial muscles associated with the thoracic vertebrae, rib cage, and dorsal abdominal wall. The autonomic motor columns in the lateral horn are prominent and innervate neurons in the sympathetic ganglia.

275

VII. Summary and Conclusions This section features a quantitative summary of the major developmental events that have been qualitatively demonstrated in photographs of the spinal cord at immature through mature stages. In order to compare the complete timetable of development, the cervical level is dealt with most completely because that level is illustrated at all ages. First, absolute changes in area are discussed, followed by relative changes in area. Finally, a semi-quantitative approach using myelin staining density is used to determine myelination sequences in various fiber tracts. Figures 32 and 33 show progressive changes in area of various components of the spinal cord in absolute values (mm2). It is important to note that the absolute values are based on a single specimen at each age; these specimens were collected over a period of several years, were preserved with different fixatives, and were not all cut at the same thickness or in exactly the same plane. All of these factors contribute to “noise” in the data, and the graphs are not smooth progressions from one age to another. The time when most noise in the data exists is between GW4.0 and GW6.8 in the first trimester. Nine specimens from this period are shown in Plates 2 through 10. Fortunately, six of these specimens are from the Minot Collection, where the fixative is constant and the cutting plane is more regular between specimens. When all of the specimens in the Carnegie Collection are removed from the data, the changes in absolute areas from one age

to the next indicate developmental changes that allow a more accurate interpretation of real events (the first 6 data points in Figure 32 graphs). These quantitative data, when combined with the qualitative evidence in the photographs, allow the construction of timetables of developmental events among neuronal populations in the spinal cord as shown in Table VII-1. Figures 34 through 37 feature pie-charts showing relative changes in the areas of varous components of the spinal cord throughout its development. The problems associated with fixative, cutting planes, section thickness, and staining types are much less relevant here. Figures 34 and 35 look at the entire range of development from GW4.0 to the 4th postnatal week. Figures 36 and 37 look at the changes in maturation of cervical through sacral levels in 6 specimens from GW8.5 to the 4th postnatal month. Finally, Table VII-2 shows a semi-quantitative analysis of the sequential steps in myelination in several of the major fiber tracts in the spinal cord at the level of the cervical enlargement. These events can be used to make some generalizations about myelination sequences in fiber tracts, not only in the spinal cord, but throughout the central nervous system. These conclusions are explained in the caption of Table VII-2.

276

Changes in Area During the First Trimester

FIGURE 32 Age in Gestational Weeks (GW)

A

6

7

9

10

11 0.25

Neuroepithelium

6

5

7

0.20

0.20

0.15

0.15 2

0.10 0.05

0.10

3 4 8

0.05

1

9

0.00

0.00

Area (mm2)

B

Spinal Canal

0.20

0.20

0.15

0.15 7

0.10

0.10

5 8

3

0.05 1 0.00

0.05

6 9

4

2

0.00

C

Gray Matter

1.00

Area (mm2)

Area (mm2)

Area (mm2)

8

Area (mm2)

5

9 8

0.80

0.80

7

6

1.00

0.60

0.60

0.40

0.40

Area (mm2)

0.25

4

5 0.20

0.20 3 1 2

4

0.00

D

White Matter 9

0.80

1.00 0.80

8

0.60

0.60 7 6

0.40

0.40

5

0.20

0.20

1 23 4 0.00

4

5

6

7

8

9

10

Age in Gestational Weeks (GW)

0.00 11

Area (mm2)

1.00

Area (mm2)

0.00

Figure 32. The total area of the neuroepithelium (A), spinal canal (B), gray matter (C), and white matter (D) at cervical levels during the first trimester. The italicized numbers in each graph indicate data points. The first six data points in each graph are from Minot specimens only because the same fixative was used and there is a more constant cutting plane between specimens. Thus, quantitative data more reliably indicate developmental trends. The red line marks the peak area of the neuroepithelium (A) and serves as a dividing line. First consider what happens before the peak. Between GW4 and GW4.5 (data points 1 and 2, A), the neuroepithelium expands, especially its ventral part (see Plates 2 and 3). Neuroepithelial area reaches a plateau (single red arrow, A) between GW4.75 and GW5.25 (data points 3 and 4, A). That plateau occurs at the time when the area of the ventral neuroepithelium remains stable during the production of the large motoneurons in the ventral horn (see Plates 4 and 5). The most rapid growth spurt in the neuroepithelium occurs between GW5.25 and GW5.5 (data points 4 and 5, A) when the spinal canal reaches its peak area (red arrow, B) as the dorsal neuroepithelium rapidly expands about the dorsal part of the spinal canal. There is a simultaneous growth spurt in the area of the gray matter (C) as neurons accumulate in the ventral horn and in the area of the white matter (D) as axons accumulate in the ventral funiculus, ventral commissure, and dorsal root bifurcation zone of the dorsal funiculus. These growth spurts are qualitatively visible by looking at the changing appearance of the spinal cord in Plates 5, 6, and 7. After GW5.5, the area of the neuroepithelium grows more slowly to reach a peak at GW6.8 (data point 6, A). That slow growth is due to concurrent shrinking of the ventral neuroepithelium and expansion of the dorsal neuroepithelium (see Plates 8-10). Now consider what happens after the peak. The neuroepithelial area gradually declines by GW7.25 (data point 7, A), then plummets by GW8.5 (data point 8, A). During the rapid decline (double red arrows, A; see Plates 11 and 12) the dorsal neuroepithelium shrinks dramatically as late-generated neurons migrate into the dorsal horn and the dorsal and ventral parts of the spinal canal disappear to leave only a central canal (B). During the decline in the neuroepithelium, the gray matter (C) and white matter (D) continue to increase. However, throughout the entire first trimester, the area of the gray matter is greater than the area of the white matter.

277

TABLE VII-1 and FIGURE 33 Table VII–1: Developmental Events in Neuronal Populations during the First Trimester NAME Alpha Motoneurons

GW4.0-4.5 Stem cells proliferate

GW4.75-5.25

First neurons generated

Most neurons generated

GW5.5-6.8

GW7.25-8.5

Migration and settling

Ventral Horn Interneurons

Stem cells proliferate

Intermediate Gray

Stem cells proliferate

Dorsal Horn Layers I,IV,V

Stem cells proliferate

Dorsal Horn Layers II,III

Stem cells dormant?

Stem cells proliferate

Central Autonomic Area

Stem cells dormant?

Stem cells proliferate

Neurons generated Neurons generated Neurons generated

GW10.5 on

Segregation into columns and maturation Migration and settling Migration and settling Migration and settling Neurons generated Neurons generated

Maturation

Maturation

Maturation Migration and settling Migration and settling

Maturation

Maturation

Table VII-1. The major events that take place in neuronal populations in the cervical spinal cord during the first trimester. Dark green indicates the first step in development when the stem cells rapidly proliferate just prior to neurogenesis. Light green indicates the approximate period of neurogenesis. Pink indicates the time during which neurons migrate away from the neuroepithelium and settle in the gray matter. Orange indicates the period of maturation, which continues throughout the second and third trimesters and well after birth. Light brown indicates the postulated time when the stem cells of neuronal populations that are generated late are either dormant or have not yet appeared. The time periods for the various events in each population are based on the data in Figure 32 and the qualitative changes visible in Plates 2 through 12.

Changes in Area from GW14 to the 4th Postnatal Month

Area (mm2)

30

30

BIRTH

WHITE MATTER

25

35 25

GRAY MATTER

20

20

15

15

10

10

5

5 14

19

26

Age (Gestational Weeks)

37 NB

4

Area (mm2)

35

16

Age (Postnatal Weeks)

Figure 33. The area of the gray matter (dark gray) and white matter (light gray) at the cervical enlargement from the beginning of the second trimester (GW14), birth (NB-newborn), to 16 weeks after birth. From GW26 on, data of the myelin stained section area is used. The area graphs are stacked so that the scale on the Y axis indicates the total area of the spinal cord. These areas are measured in fixed tissue, so the actual values in the living spinal cord will be higher by an unknown amount. The areas of the ependyma and spinal canal are so small that they do not register above the baseline. Since neurogenesis finishes in the first trimester (see Figure 32), the gradual increase in the area of the gray matter throughout this period is due to the growth and differentiation of the neurons themselves. The white matter increases more rapidly, especially between GW19 and GW26 (compare Plates 40 and 47). There is gradual growth from GW 26 to the perinatal period (between GW37 and postnatal week 4), then more rapid growth between postnatal weeks 4 and 16 (compare Plates 95 and 106). The first growth spurt is probably due to the accumulation of axons in all of the major fiber tracts, including both parts of the corticospinal tract. The second growth spurt is probably due to the completion of myelination throughout the cervical enlargement at the 4th postnatal month. Only the outermost crescent of the corticospinal tract is still myelinating at this time (see Plate 106). From GW26 on, the area of the white matter exceeds the area of the gray matter.

278

Proportional Areas at the Cervical Enlargement GW 3% 4% 2% 2% GW 7%

FIGURE 34

4.0

4.5

7% 1.5% 2%

15%

GW

18%

5 4.7

17%

71% 65% 6%

47%

12% 20.5%

1%

1%

GW

5.5

13% 28% 19% 38%

1.5%

.5%

GW 6

.5

7%

.6%

.3%

21%

40.7%

2.8%

8.5 W G

2.6%

24%

53%

46%

WHITE MATTER

SPINAL CANAL

ROOF PLATE

GRAY MATTER

NEUROEPITHELIUM

FLOOR PLATE

Figure 34. A series of pie-shaped graphs showing the proportional areas of the white matter (white), gray matter (gray), spinal canal (light green), neuroepithelium (dark green), roof plate (orange), and floor plate (red) at the cervical enlargements of specimens from gestational week (GW) 4.0 (upper right graph) to GW8.5 (lower right graph). The section measured from the GW4 specimen is illustrated in Plate 2, GW4.5 in Plate 3, GW4.75 in Plate 4, GW5.5 in Plate 7, GW6.5 in Plate 8, and GW8.5 in Plate 12. The neuroepithelium dominates from GW4-4.75 when stem cells are rapidly increasing in preparation for successive waves of neurogenesis. Even though the neuroepithelium declines in its proportion throughout the entire period, it is actually increasing its absolute area up to GW6.5 (see Figure 32). In contrast, the peak proportional areas of the spinal canal (GW4.75-5.5) occur at the same time as the largest absolute area and their decline at GW6.5-8.5 is associated with dramatic shrinkage of the neuroepithelium (Figure 32). When neurons are produced and migrate out of the neuroepithelium, the area of the gray matter proportionally increases. The first proportional increase between GW4.75-5.5 is almost exclusively due to growth of the ventral horn. Later proportional increases are due to continual growth of the ventral horn, the emerging intermediate gray, and the rapid expansion of the dorsal horn. The white matter measured at GW4.0-4.75 is primordial and contains only a few axons; that area is the same as the mantle layer described in traditional developmental studies. From GW5.5 onward, the proportional area increases are due to accumulating axons, mainly in the dorsal funiculus, ventral funiculus, and ventral commissure at GW5.5 and GW6.5. By GW8.5 the proportional area of the white matter sharply increases. The dorsal funiculus greatly expands when long ascending collaterals form the gracile and cuneate fasciculi. The roof and floor plates have their largest proportional areas early (GW4.0-4.75) supporting the animal experimental evidence of a promint role in the overall dorsal–to–ventral differentiation of the gray matter and growth of axons parallel to the dorsal midline and across the ventral midline (see Chapter 4 in Altman and Bayer, 2001).

279

Proportional Areas at the Cervical Enlargement

FIGURE 35 4TH .12%

W

K EE

CENTRAL CANAL AND EPENDYMA

27.74%

72.14%

.14%

GW 37

CENTRAL CANAL AND EPENDYMA

62.12%

.2% .4%

.5%

.7%

45%

GW

GW

.5%

CENTRAL CANAL AND EPENDYMA

58.5% 49%

GW 26

19

.4%

.5 10

.7%

.2%

.1%

37.75%

41.3%

49.8%

52.7%

WHITE MATTER

CENTRAL CANAL

ROOF PLATE

GRAY MATTER

EPENDYMA

FLOOR PLATE

Figure 35. A continuation of the previous figure in specimens at GW10.5 (see section measured in Plate 23), GW19 (Plate 40), GW26 (Plate 47), GW37 (Plate 65), and the 4th postnatal week (Plate 95). During this period, the proportional areas of the gray and white matter make up most of the spinal cord. The roof and floor plates disappear by GW26, and the proportional areas of the central canal and ependyma are so small that they appear as a slightly thicker black line separating the gray matter and white matter at the 12:00 position of each pie graph. The gray matter continually loses proportional area throughout this period but the absolute area does increase gradually as neurons grow dendrites and some interneurons grow elaborate terminal axon arbors (especially in the dorsal horn). The proportion of the white matter continually increases during this time, and it is the largest proportional area from GW26 onward.

280

Level Differences in Proportional Areas: GW8.5-GW19

FIGURE 36 GW 8.5 .78%

.57%

2.95%

CERVICAL ENLARGEMENT

GW10.5 3.41%

.7%

.4%

.5%

.7%

53.89% 38.41%

52.6% 45.1%

See Plate 15 1%

1%

THORACIC

2%

7%

1% 1%

50% 40%

49%

1% 2%

1%

3%

See Plate 40 FP 0.20% RP 1.14% CC 0.17%

52% 43%

2%

EP .5%

50%

See Plate 23

See Plate 17

LUMBAR ENLARGEMENT

GW19 FP .2% RP .2% CC .1%

EP 0.69%

53.6% 44.2%

See Plate 25 .8%

.5%

.7%

1%

See Plate 42 FP .1% RP .6% CC .3%

EP .6%

7%

29.4%

54%

33%

41%

56% 69%

See Plate 19 2%

1%

See Plate 27 3% 2%

3%

1% 5%

See Plate 44 FP 0.1% RP 1.0% CC 0.1%

EP 0.9%

SACRAL

10%

31%

26%

53% 40%

49% 72%

See Plate 20

See Plate 28

See Plate 45

WHITE MATTER

CENTRAL CANAL (CC)

ROOF PLATE (RP)

GRAY MATTER

EPENDYMA (EP)

FLOOR PLATE (FP)

Figure 36. Pie charts showing the proportional areas of the white matter (white), gray matter (gray), central canal (light green), ependyma (dark green), roof plate (orange), and floor plate (red) at the cervical enlargement level (row 1), thoracic level (row 2), lumbar enlargement level (row 3), and the sacral level (row 4). Each column contains measured sections from the same specimen: column 1 is M2050, GW8.5; column 2 is Y380–62, GW10.5; and column 3 is Y52–61, GW19. The plate numbers in each square refer to the illustration of the measured section in previous parts of this atlas. By following the shifting values of the proportional areas within a column, a gradient of maturation is evident between cervical enlargement to sacral levels. The size of the ependyma increases in the first two columns, indicating that the greatest degree of maturation is at the cervical enlargement level followed by declining levels of maturation at thoracic, lumbar enlargement, and sacral levels. By GW19, the central canal, ependyma, roof plate, and floor plate have very small proportional areas indicating the greater maturation of the spinal cord throughout its length at this time. The proportion of gray matter to white matter generally increases from cervical enlargement to sacral levels at all ages. That is because descending fiber tracts from the brain arrive first at cervical levels and progressively later at successively lower levels. Thus from GW8.5 to GW19, the white matter at the lumbar enlargement and sacral levels is less mature than it is at the cervical enlargement level.

281

Level Differences in Proportional Areas: GW37-4th Month GW 37

CERVICAL ENLARGEMENT

CC .05%

4-DAY-OLD INFANT EP .09%

CC .01%

37.75% 62.12%

65.10%

THORACIC

EP .11%

CC .01%

LUMBAR ENLARGEMENT

45.50%

EP .06%

54.40%

EP .25%

CC .03%

CC .02%

EP .09%

33.96%

18.56%

See Plate 110 CC .06%

EP .06%

43.95%

See Plate 88 CC .06%

EP .04%

81.37%

65.93%

CC .08%

See Plate 106

See Plate 84

See Plate 75

SACRAL

69.04%

74.61%

EP .07%

EP .05%

30.88%

25.32%

See Plate 69 CC .04%

CC .04%

See Plate 80

25.50%

74.34%

4-MONTH-OLD INFANT

34.83%

See Plate 65 CC .05%

EP .06%

FIGURE 37

55.94%

See Plate 116

EP .40%

CENTRAL CANAL (CC)

33.57%

31.93% 66.11%

WHITE MATTER

67.61%

GRAY MATTER EPENDYMA (EP)

See Plate 77

See Plate 90

Figure 37. A continuation of the previous figure in specimens at GW37 (column 1, Y117–61), a 4-day-old infant (column 2, Y299–62), and a 4-monthold infant (column 3, Y23–60). A section at the sacral level is not available in the 4-month-old infant. The roof and floor plates are absent at these ages, and the proportional areas of the central canal and ependyma appear on the graphs as slightly thicker black lines separating the gray matter and white matter at the 12:00 position in each chart. The smaller proportional areas of the white matter at the sacral level perinatally (bottom row, first two columns) indicate immature white matter. In contrast, the cervical enlargement (row1) and thoracic levels (row 2) have high to very high proportions of white matter to gray matter indicating that the white matter is quite mature in all these specimens. The very high proportion of white matter at the thoracic level is because the ventral horn is quite small at these levels, just as it is in the adult spinal cord. At the ages shown here, the lower proportional area of white matter at the lumbar enlargement (row 3) does not indicate immaturity. That is because all axons from the brain that terminate at lumbar and sacral levels pass through cervical and thoracic parts, increasing the proportional area of the white matter at those levels.

282

Myelination Sequences

TABLE VII-2

Table VII–2: Myelination Sequences in Major Fiber Tracts (Cervical Enlargement) TRACT

GW26

GW37

Birth

4th Week

4th Month

Sup. Fas. Gracilis

Sparse reactive glia

Some myelinating fibers

Myelinated

Myelinated

Myelinated

Deep Fas. Gracilis

Dense reactive glia

Myelinated

Myelinated

Myelinated

Myelinated

Sup. Fas. Cuneatus

Sparse reactive glia

Some myelinating fibers

Myelinated

Myelinated

Myelinated

Deep Fas. Cuneatus

Dense reactive glia

Myelinated

Myelinated

Myelinated

Myelinated

D.R. Col. Zone

Dense reactive glia

Myelinated

Myelinated

Myelinated

Myelinated

D.R. Bif. Zone

Some myelinating fibers

Some myelinating fibers

Myelinated

Myelinated

Myelinated

Spinocerebellar

Dense reactive glia

Some myelinating fibers

Many myelinating fibers

Myelinated

Myelinated

Intraspinal

Dense reactive glia

Some myelinating fibers

Myelinated

Myelinated

Myelinated

Spinocephalic

Sparse reactive glia

Dense reactive glia

Sparse reactive glia

Some myelinating fibers

Myelinated

Ventral Commissure

Some myelinating fibers

Many myelinating fibers

Many myelinating fibers

Some myelinating fibers

Myelinated

Ventral Corticospinal

No reactive glia

No reactive glia

Sparse reactive glia

Dense reactive glia

Myelinated

Lateral Corticospinal

No reactive glia

No reactive glia

Sparse reactive glia

Dense reactive glia

Many myelinating fibers

Table VII-2. Steps in the myelination of some major fiber tracts in the cervical enlargement of the spinal cord from the second trimester to the fourth postnatal month. The unmyelinated Lissauer’s tract and some descending tracts (tectospinal, medial longitudinal fasciculus, and vestibulospinal) are not shown. Pale violet indicates the first step when glial cells react with the myelin stain prior to the production of true myelin sheaths; these areas in the myelin stained sections contain a light dusting of punctate stain. Dark violet indicates the second step when the punctate staining is more plentiful, presumably due to more glia reacting with the myelin stain; however, the overall density of the stain is still light and few, if any, fibers are myelinating. Pale purple indicates step 3 when the density of the stain increases and glia produce myelin sheaths around some fibers. Medium purple indicates step four when the density of the stain increases yet again and more glia produce myelin sheaths around many fibers. Dark purple indicates the fifth and final step when glia produce myelin sheaths around nearly all the fibers in the tract; the truly myelinated areas appear solid black in the stained sections. Both the fasciculus gracilis and the fasciculus cuneatus show gradients in the myelination steps so that deep parts of each fasciculus myelinate before superficial parts. If the sequence of myelination is linked to the time when axons first enter these fasciculi, the conclusion can be drawn that the first axons in the tract occupy the most deep areas, and fibers that enter the tract later occupy progressively more superficial parts. The spinocephalic tract does not have an even sequence of myelination because the dense reactive glial staining at GW37 is bracketed by periods of sparse reactive glial staining. That may be due to the dilution of myelinated axons in the cervical level by the invasion of unmyelinated or less myelinated axons from parts of the spinal cord that are less mature, such as sacral and lumbar levels. The ventral commissure also has an uneven sequence of myelination. From the very first appearance of myelin staining, there are always some fibers that are myelinated. However, more fibers are myelinated at GW37 and postnatal day 4 than at the 4th postnatal week. That apparent regression is possibly due to the growth of unmyelinated axons from the ventral corticospinal tract into the commissure, thus diluting the number of myelinated fibers. The lateral corticospinal tract is the last fiber tract to myelinate. Indeed, some parts of it are still myelinating during the 4th postnatal month, especially the outer crescent closest to the spinocerebellar tracts. That part of the tract contains fibers that will terminate at sacral levels of the cord. The lateral corticospinal tract in thoracic, lumbar, and sacral levels is progressively less mature at the 4th postnatal month. That gradient of maturation suggests that cortical axons myelinate first proximal to the cell body and myelinate last distal to the cell body. This analysis departs from most previously published accounts of myelination sequences which describe various fiber tracts as either “not myelinated” or “myelinated.” It is a novel idea that myelination is a progressive event rather than an “all or none” event within a fiber tract. For a review of the literature on myelination in the spinal cord, see Chapter 8 and Chapter 9, Section 9.4 in Altman and Bayer (2001).

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GLOSSARY Alar plate––According to His’ bipartite division of the neural tube, that part of the lateral plate neuroepithelium situated above the sulcus limitans. It is hypothesized to be the source of dorsal horn sensory processing neurons and some intermediate gray neurons. In the tripartite division of the spinal cord neuroepithelium in this Atlas, this term is not used. See Dorsal neuroepithelium and Intermediate neuroepithelium. Autonomic motoneurons––Preganglionic sympathetic motoneurons in the lateral horn at thoracic and upper lumbar levels; preganglionic parasympathetic motoneurons in the lateral part of the intermediate gray at sacral levels. See Lateral horn motoneurons. Basal plate–– According to His’ bipartite division of the neural tube, that part of the lateral plate neuroepithelium situated below the sulcus limitans. It is hypothesized to be the source of ventral horn motoneurons, ventral horn interneurons, and some intermediate gray neurons. In the tripartite division of the spinal cord neuroepithelium in this Atlas, this term is not used. See Ventral neuroepithelium and Intermediate neuroepithelium. Cauda equina––Elongated roots of spinal nerves in the vertebral canal that exit from the spine below the level of their points of attachment to the spinal cord. Cellular roof plate––Cells in the roof plate that have fibrous-like projections into the roof of the spinal canal. Initially, the cellular roof plate caps the midline of the spinal cord, but it starts to sink downward at the end of the first trimester, apparently leaving behind a string of non-neural cells that form the dorsal median septum. See Roof plate. Central autonomic area––Small- and medium-sized neurons that surround the central canal and provide a bridge between the two wings of the gray matter on either side of the midline. This area is homologous to the central gray surrounding the cerebral aqueduct and fourth ventricle in the brain. It contains the dorsal and ventral gray commissures that bridge the midline. These neurons are activated by nociceptive afferents from small cells in the dorsal root ganglion.

Cervical region––The part of the spinal cord that is continuous with the medulla of the brain through the foramen magnum of the skull. Eight spinal nerves exit from this region; all nerves except C1 have a dorsal root ganglion. C1 exits above the first vertebra; C8 exits in the transverse foramen of the C7 vertebra. Clarke’s column––A group of large neurons dorsal to the central canal in the intermediate gray at thoracic and upper lumbar levels. Its axons form the ipsilateral dorsal spinocerebellar tract. Conus medullaris––The cone-shaped terminus of the spinal cord. Dorsal canal––A transient part of the spinal canal that first expands then recedes in relation to the changing size of the dorsal neuroepithelium during the first trimester. Dorsal funiculus––Dorsal white matter that caps and fills in the U-shaped space between the paired dorsal horns. Most of the axons in this funiculus are from large dorsal root ganglion cells in four subdivisions: fasciculus gracilis, fasciculus cuneatus, dorsal root bifurcation zone, and dorsal root collateralization zone. Dorsal horn––Dorsal wings of the butterfly-shaped gray matter that contain sensory processing neurons in laminae I-V. This is the latest differentiating region of the spinal cord gray matter. Dorsal horn, lamina I––A thin sheet of gray matter that caps the dorsal surface of the dorsal horn. It contains the large Waldeyer cells that process nociceptive sensory information. Axons of the Waldeyer cells cross the midline in the ventral white commissure and enter the ventral and lateral spinothalamic tracts, or the spinocephalic tracts. Dorsal horn, laminae II-III––See Substantia gelatinosa.

Central cervical nucleus––A cluster of large neurons in the intermediate gray near the central canal at cervical levels that is the continuation of Clarke’s column. Axons of these neurons enter the ipsilateral dorsal spinocerebellar tract.

Dorsal horn, laminae IV-V––A collection of large and small neurons; also called the nucleus proprius. These neurons process touch, pressure, and nociceptive input. At lumbar and thoracic levels, axons of some of the neurons in lamina IV travel to the medulla in the dorsal funiculus as a postsynaptic–dorsal column system. At all levels, some neurons in lamina IV travel in the dorsolateral region of the lateral funiculus (the spinocervicothalamic pathway) and terminate in a topographic pattern in the lateral cervical nucleus (Haines, 2000).

Cervical enlargement––Includes that part of the spinal cord from which the nerves of the brachial plexus arise, from C4 to T1. This part of the spinal cord has a larger diameter because of the greater amount of gray and white matter required to innervate the upper limb.

Dorsal intermediate septum––Aligned non-neural cells that separate the fasciculus gracilis and fasciculus cuneatus. There is a slight depression on the surface of the spinal cord to mark the spot where this septum joins the pia.

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Glossary Dorsal median septum––Aligned non-neural cells attached to the pia that extend downward in the dorsal midline. This septum separates the two gracile fasciculi on either side of the midline and may be formed by the receding cellular portion of the roof plate during the late first trimester and early second trimester. Dorsal neuroepithelium––Neuroepithelium surrounding the dorsal canal that produces dorsal horn neurons. This neuroepithelium still produces neurons in the late first trimester. Dorsal root––Part of a spinal nerve that is formed by axons of dorsal root ganglion cells that bring sensory input to the dorsal funiculus. Dorsal root bifurcation zone––Region in the dorsal funiculus where incoming axons from the dorsal root divide into ascending and descending branches. Dorsal root boundary cap––Specialized glial cells that accumulate at the entry points of the dorsal roots of the spinal nerve. These cells may act as targets that direct the growth of dorsal root axons into the spinal cord. Dorsal root collateralization zone––Region of the dorsal funiculus that contains the local collateral branches of dorsal root ganglion cell axons. These axons curve around the dorsomedial edge of the dorsal horn before penetrating the spinal gray matter. This region also contains axons of the intraspinal tracts.

Dorsal root ganglion––Primary sensory neurons embedded in the dorsal root of a spinal nerve that bring exteroceptive (touch, pressure), proprioceptive (stretch receptors in muscles and tendons), visceroceptive (receptors in the vital internal organs), and nociceptive (pain) input to the spinal cord. Dorsal spinocerebellar tract––Axons of neurons in Clarke’s column and the central cervical nucleus that extend ipsilaterally to the cerebellum in the peripheral dorsal lateral funiculus. Dorsolateral fasciculus––See Lissauer’s tract. Ependyma––Cells lining the central canal in the mature spinal cord. During the late first trimester and early second trimester, these cells gradually appear as the spinal neuroepithelium recedes. Fasciculus cuneatus––Central fiber tract in the dorsal funiculus above the 6th thoracic level (lateral to the fasciculus gracilis and medial to the dorsal root collateralization zone). Its main component is ipsilateral axons from large neurons in the dorsal root ganglia above the 6th thoracic level that carry exteroceptive information from the upper part of the body to the nucleus cuneatus in the lower medulla. A smaller component is axons of sensory relay neurons in laminae III–V that terminate in the nucleus cuneatus as part of the postsynaptic–dorsal column system.

Fasciculus gracilis––Medial fiber tract in the dorsal funiculus. Its main component is ipsilateral axons from large neurons in the dorsal root ganglia below the 6th thoracic level that carry exteroceptive information from the lower part of the body to the nucleus gracilis in the lower medulla. A smaller component is axons of sensory relay neurons in laminae III–V that terminate in the nucleus gracilis as part of the postsynaptic– dorsal column system. Fibrous roof plate––The end feet of roof plate cells that extend into the spinal canal in the dorsal midline. Floor plate––Specialized cells that bridge the ventral midline of the spinal canal during the first trimester. Initially, it is the most ventral structure in the embryonic spinal cord, lying directly above the notochord. The floor plate moves upward at the end of the first trimester, with the receding ventral neuroepithelium. It may be involved in ventral motoneuron induction and may provide directional cues for growing commissural axons. Intermediate gray––Spinal gray matter that forms a bridge between the dorsal and ventral horns (laminae VI–VII). Intermediate interneurons––Neurons in the intermediate gray whose axons do not extend to the brain or enter the ventral roots. These neurons may be part of the intrinsic locomotive pattern generator system in the spinal cord. Many of their axons travel up and down the spinal cord in the intraspinal tracts. Intermediate neuroepithelium––Neuroepithelium surrounding the sulcus limitans that produces intermediate gray neurons. Intraspinal tracts––Axons of spinal interneurons that travel in the white matter adjacent to the gray matter of the dorsal horn, intermediate gray, and ventral horn; also called the propriospinal tract or the spinospinal tract. These tracts overlap with the medial longitudinal fasciculus, the tectospinal tract, the ventral and lateral reticulospinal tracts, and the dorsal root collateralization zone. Laminae VI–VII––Gray matter of the intermediate gray. Lamina VIII––Gray matter of the ventral horn interneurons. Lamina IX––Gray matter of the ventral horn motoneurons. Lateral cervical nucleus––Large neurons scattered adjacent to and within the reticulated area at cervical levels. These neurons get input from lamina IV cells at all levels of the spinal cord. Their axons cross the midline in the ventral white commissure and travel to the thalamus in the medial lemniscus.

Glossary Lateral corticospinal tract––Contralateral axons in the dorsal part of the lateral funiculus from upper motoneurons in layer V of the primary motor cortex (pyramidal cells of Betz in area 4) and other cortical areas. These axons are somatotopically arranged (cervical terminating axons medial, lumbosacral terminating axons lateral). Axons from area 4 terminate primarily in laminae VI-IX in the ventral horn. Lateral funiculus––White matter between Lissauer’s tract and the ventral funiculus. The border between the lateral and ventral funiculi is indistinct. The lateral funiculus contains the following tracts: dorsal and ventral spinocerebellar, lateral corticospinal, rubrospinal, lateral reticulospinal, dorsolateral spinocephalic, and dorsolateral intraspinal. Lateral horn motoneurons––Preganglionic (visceral) motoneurons of the autonomic nervous system. These neurons are generated in the ventral neuroepithelium during the early first trimester and migrate dorsolaterally during the late first trimester to settle in the lateral horn. There are no lateral horn motoneurons at cervical levels. At thoracic and upper lumbar levels, the axons of these motoneurons leave the spinal cord in the ventral roots and terminate in the chain of sympathetic ganglia located adjacent to the anterolateral vertebral column. At sacral levels, the axons of these motoneurons leave the spinal cord in the ventral roots and terminate in parasympathetic ganglia located in the abdominopelvic cavity. Lateral plate––Dorsal, intermediate, and ventral parts of the spinal cord neuroepithelium. Lateral reticulospinal tract––Axons from large neurons in the medullary reticular formation that terminate primarily in laminae VII at all levels of the spinal cord. Axons in this tract travel close to the gray matter in the lateral funiculus and overlap with axons in the intraspinal tracts. Lateral spinothalamic tract––See Spinocephalic tracts. Lissauer’s tract––Unmyelinated axons of small neurons in dorsal root ganglia that carry pain input to the dorsal horn of the spinal cord; also called the dorsolateral fasciculus. This tract forms the boundary between the dorsal funiculus and the lateral funiculus.

285 Medial longitudinal fasciculus––Axons from neurons in the medial vestibular nucleus; also called the medial vestibulospinal tract. This fasciculus is most prominent at cervical levels adjacent to the medial ventral horn below the ventral white commissure in the ventral funiculus. Axons in this fasciculus overlap with those in the intraspinal tracts. Medulla––also called medulla oblongata. The part of the brain that is continuous with the spinal cord and contains the posterior part of the fourth ventricle. Motoneurons––A general term for neurons whose axons leave the spinal cord in the ventral roots of a spinal nerve. Axons of these neurons terminate either on skeletal muscle (somatic motoneurons) or on postganglionic neurons in autonomic ganglia (preganglionic visceral motoneurons). Neuroepithelium––The primordial nervous system composed of neural stem cells (NEP cells). The ultimate source of neurons, neuroglia, ependymal cells, tanycytes, and other specialized cells in the central nervous system. Notochord––A solid core of cells that lie in the midline beneath the neural tube floor plate that will form the spinal cord. These cells provide “induction” signals that establish “ventralization” of the neural tube so that motoneurons are only generated by stem cells in the ventral neuroepithelium. Oval bundle of His––See Dorsal root bifurcation zone. Primordial gray matter––A thin layer of young neurons between the spinal cord neuroepithelium and the primordial white matter present only during the early first trimester. The primordial gray matter differentiates into specific parts of the spinal gray. This area is traditionally called the mantle zone or layer. Primordial white matter––A thin cell-sparse layer beneath the pial membrane in the embryonic spinal cord. The main components of this layer are the end feet of specialized glial cells and spaces where axons of spinal cord fiber tracts will form the dorsal, lateral, and ventral funiculi. This area is traditionally called the marginal zone or marginal layer. Propriospinal tract––See Intraspinal tracts.

Lumbar enlargement––The part of the spinal cord that has a larger diameter and contains the gray matter and white matter associated with the lumbosacral plexus, the nerves that innervate the lower limb. Lumbosacral region––The most caudal part of the spinal cord that contains the lumbar enlargement. There are five lumbar spinal nerves, five sacral spinal nerves, and one coccygeal spinal nerve. Mantle zone or layer––See Primordial gray matter. Marginal zone or layer––See Primordial white matter.

Reticulated area––A region in the lateral funiculus adjacent to the intermediate gray where white matter and gray matter intermingle. Roof plate––Non-neural cells that cap the dorsal midline of the spinal canal during the first trimester. Initially, it is the most dorsal structure in the embryonic spinal cord, but it sinks downward at the end of the first trimester as the dorsal neuroepithelium recedes. As that happens, the dorsal funiculus fills with axons separated in the midline by aligned non-neural cells that form the dorsal median septum.

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Glossary Rubrospinal tract––Somatotopically arranged axons of the contralateral red nucleus in the lateral funiculus that intermingle with ventral fibers of the lateral corticospinal tract and terminate in laminae V-VIII of the spinal gray. Medial axons terminate at cervical levels, lateral axons terminate at lumbosacral levels.

Spinoreticular tract––Axons in the spinocephalic tracts that terminate in the reticular formation of the medulla and pons. Spinotectal tract––Axons in the spinocephalic tracts that terminate in the deep layers of the superior colliculus.

Sojourn zone––The peripheral parts of the dorsal and intermediate neuroepithelia that contain dense accumulations of premigratory young neurons that stay (“sojourn”) in the neuroepithelium prior to entering the gray matter. These zones have ruffled edges that are difficult to distinguish from the immediately adjacent gray matter. The ventral neuroepithelium may also contain a sojourn zone, but its ruffled outer edges are less obvious.

Subgelatinosal plexus––Intermingled axon terminal branches (lightly myelinated) and dendrites at the base of the substantia gelatinosa.

Somite––Paired oval clusters of cells outside the embryonic spinal cord that contain a dermatome (precursor of the dermis), myotome (precursor of axial skeletal musculature), and sclerotome (precursor of the axial skeleton).

Sulcus limitans––A shallow depression in the spinal neuroepithelium traditionally a landmark that separates the alar and basal plates. This sulcus marks the area of the spinal canal that will be retained as the central canal in the maturing and adult spinal cord.

Spinal canal––The enlarged fluid-filled core of the spinal cord present during the embryonic and early fetal period. It is surrounded laterally by the dorsal, intermediate, and ventral parts of the neuroepithelium. It is sealed dorsally by the roof plate and ventrally by the floor plate.

Tectospinal tract––Contralateral axons from neurons in the deep layers of the superior colliculus that travel below and intermingle with the medial longitudinal fasciculus in the ventral funiculus. All axons in this tract terminate at cervical levels in laminae VI–VIII in the intermediate gray. This tract overlaps with the intraspinal tracts.

Spinal cord––That part of the central nervous system that is continuous with the medulla of the brain and occupies the vertebral canal in the spine. Spinal nerve––A mixed nerve that separates into dorsal and ventral roots at its points of entry (dorsal root) or exit (ventral root) from the spinal cord. The incoming axons are those of the dorsal root ganglion. The outgoing axons are those of the ventral horn somatic motoneurons and lateral horn visceral motoneurons. A total of 31 spinal nerves are connected to the spinal cord. Spinocephalic tracts––A complex of several tracts occupying a crescent-shaped area in the peripheral ventral and lateral funiculi. The traditional anatomists named this region the ventral and lateral spinothalamic tracts. Modern research indicates that the majority of these axons terminate in the brainstem reticular formation (spinoreticular tract), the inferior olive nucleus (spino– olivary tract), the deep layers of the superior colliculus (spinotectal tract), the central gray in the midbrain, and the hypothalamus. Less than 50% of these axons terminate in the thalamus in the ventroposterolateral nuclear complex and in the intralaminar nuclei. Spinocerebellar tracts––Combined dorsal and ventral spinocerebellar tracts in the peripheral lateral funiculus. Spino-olivary tract––Axons in the spinocephalic tracts that terminate in the accessory nuclei associated with the inferior olive complex in the lower medulla.

Substantia gelatinosa––Component of the dorsal horn that contains interneurons in laminae II and III. These neurons include the central cells, tufted cells, stalked cells, and islet cells that receive sensory input from dorsal root ganglion cells.

Thoracic region––Part of the spinal cord that is the target and source of 12 pairs of spinal nerves associated with the 12 thoracic vertebrae. The ventral horn gray matter is narrow here because there are only axial muscles to innervate. Ventral canal––A transient part of the spinal canal that first expands then shrinks in relation to the changing size of the ventral neuroepithelium during the first trimester. Ventral corticospinal tract––Ipsilateral axons in the medial part of the ventral funiculus from upper motoneurons in layer V of the primary motor cortex (pyramidal cells of Betz in area 4). These axons are somatotopically arranged (cervical terminating axons dorsal, lumbosacral terminating axons ventral). Axons cross the midline in the ventral white commissure before terminating in laminae VI-IX in the ventral horn. Ventral funiculus––White matter between the lateral funiculus and the ventral white commissure. The border between the lateral and ventral funiculi is indistinct. The ventral funiculus contains the following tracts: medial longitudinal fasciculus, tectospinal, ventral corticospinal, ventral reticulospinal, vestibulospinal, ventromedial spinocephalic, and ventromedial intraspinal. Ventral gray commissure––A bridge of small neurons in the midline beneath the central canal.

Glossary Ventral horn––Ventral wings of the butterfly-shaped gray matter that contain motoneurons (lamina IX) and ventral horn interneurons (lamina VIII). This is the earliest differentiating region of the spinal cord gray matter. Ventral horn interneurons––Neurons in the ventral gray (lamina VIII) whose axons do not extend to the brain or enter the ventral roots. These neurons are part of the intrinsic locomotive pattern generator system in the spinal cord. Many of their axons travel up and down the spinal cord in the intraspinal tracts. Ventral horn motoneurons––Neurons in lamina IX whose axons leave the spinal cord in the ventral roots of the spinal nerves and innervate skeletal muscle. These neurons are generated in the ventral neuroepithelium during the early first trimester. During the late first trimester and second trimester, the motoneurons segregate into smaller and smaller clusters (in cross-sections) that are aligned longitudinally to form columns. Each column supplies specific groups of muscles. The most medial columns supply axial muscles. Ventrolateral columns supply proximal limb muscles in the cervical and lumbar enlargements. Dorsolateral columns supply distal limb muscles in the cervical and lumbar enlargements. Ventral median fissure––A deep cleft in the ventral midline that separates the medial walls of the ventral funiculus on the right and left sides of the spinal cord. The dorsal border of this fissure is formed by the ventral white commissure. Ventral neuroepithelium––Neuroepithelium surrounding the ventral canal that produces ventral horn motoneurons, autonomic motoneurons, and ventral horn interneurons. This is the first neuroepithelium to produce neurons, and the first to recede in the middle first

287 trimester. A portion of this neuroepithelium may be transformed to produce glia. Ventral root––Part of the spinal nerve that exits the central nervous system near the junction of the ventral and lateral funiculi. It is composed of the axons of ventral horn motoneurons that project to skeletal muscle and lateral horn motoneurons that project to autonomic ganglia. Ventral rootlets––Fine bundles of axons visible in the white matter of the ventral funiculus prior to coalescing and exiting the spinal cord in the ventral root. Ventral spinocerebellar tract––Axons of proprioceptive sensory processing neurons in the intermediate zone and spinal border cells in the intermediate gray that project to the cerebellum. Ventral spinothalamic tract––See Spinocephalic tract. Ventral reticulospinal tract––Axons from large neurons in the pontine reticular nucleus that terminate primarily in laminae VIII at all levels of the spinal cord. Axons in this tract travel close to the gray matter in the ventral funiculus and overlap with axons in the intraspinal tracts. Ventral white commissure––Axons that cross the midline beneath the ventral gray commissure. This is one of the earliest identifiable components of the white matter that contains early and late myelinating axons. Vestibulospinal tract––Axons of the lateral vestibular nucleus (Deiter’s) in the medial part of the peripheral ventral funiculus. These axons terminate in laminae VII and VIII throughout the entire length of the spinal cord. These axons overlap with medial axons in the spinocephalic tracts.