2. The Fine Structure of the Nervous System

3. The Fine Structure of the
Nervous System
Neurons and Their Supporting Cells
Third Edition
ALAN PETERS
Waterhouse Professor of Anatomy, Department of Anatomy and Neurobiology
Boston University School of Medicine, Boston, Massachusetts
SANFORD L. PALAY
Bullard Professor of Neuroanatomy, Emeritus
Harvard Medical School, Boston, Massachusetts
HENRY DBF. WEBSTER
Chief, Laboratory of Experimental Neuropathology
National Institute of Neurological Diseases and Stroke, Bethesda, Maryland
New York Oxford OXFORD UNIVERSITY PRESS 1991

4. Oxford University Press
Oxford New York Toronto Delhi Bombay
Calcutta Madras Karachi Petaling Jaya
Singapore Hong Kong Tokyo Nairobi Dar es
Salaam Cape Town Melbourne Auckland
and associated companies in
Berlin Ibadan
Copyright © 1970, 1976, 1991 by Alan Peters
Copyrighted under the International Copyright Union.
Published by Oxford University Press, Inc.,
200 Madison Avenue, New York, New York 10016
Oxford is a registered trademark of Oxford University Press
All rights reserved. No part of this publication may be reproduced, stored in
a retrieval system, or transmitted, in any form or by any means, electronic,
mechanical, photocopying, recording, or otherwise, without prior permission
of Oxford University Press.
Library of Congress Cataloging-in-Publication Data
Peters, Alan, 1929-
The fine structure of the nervous system : neurons and their supporting cells
Alan Peters, Sanford L. Palay, Henry deF. Webster. —3rd ed.
p. cm. Includes bibliographical references.
Includes index.
ISBN 0-19-506571-9
1. Nervous system—Ultrastructure.
I. Palay, Sanford L. II. Webster, Henry deF. III. Title.
[DNLM: 1. Nervous System—ultrastructure. WL 101 P481f]
QM575.P45 1990 611'.8—dc20 DNLM/DLC
for Library of Congress 90-14201
Various aspects of nerve cells in light microscopic preparations.
A. A small pyramidal cell from the visual cortex, Golgi method.
The axon (a) descends from the cell body.
B. A small neuron in the dentate nucleus of the cerebellum, Golgi
method. The axon (a) is represented only by its initial segment. c.
Protoplasmic (velate) astrocyte in the gray matter, Golgi method.
D. Oligodendrocyte in the white matter, Golgi method.
E. A motor neuron in the spinal cord showing the Golgi apparatus,
osmium tetroxide impregnation.
F. A motor neuron in the abducens nucleus showing the distribution
of mitochondria, Altmann-Kull method.
G. A motor neuron in the spinal cord showing the distribution
of neurofibrils, Cajal's silver stain. H. A motor neuron in the
abducens nucleus showing the disposition
of the Nissl bodies, thionin. .
i. A dorsal root ganglion cell, showing the axon coiled about the*
perikaryon and dividing into central and peripheral fibers, j. A
myelinated peripheral nerve fiber, showing a node of Ranvier,
Schmidt-Lanterman clefts, Schwann cell nucleus, and neurofibrils.
98765432
Printed in the United States of America
on acid-free paper

6. Preface
We wish to thank our many friends and colleagues who encouraged us to un-
dertake a third edition of this book on the fine structure of the nervous system.
This revision, like the previous editions (1970 and 1976), aims "to present in
words and pictures an account of the salient features of mammalian neurons
and neuroglial cells." We have thoroughly revised the text in order to bring it
up to date, and we have exchanged many of the original micrographs for ones
that we believe better show the characteristics of various structures. Through
the generosity of our colleagues, we have been able to add new freeze-fractured
material and some deep-etched preparations, as well as examples of various
labeling techniques. Consequently, the number of figures has increased from 118
to 137, and 51 of them are new illustrations.
Since the last edition was published there has been not only an information
explosion in neuroscience, but also a notable improvement in microtomes and
electron microscopes, so that the production of good electron micrographs poses
less of a challenge than it did even a decade ago. At the same time, however,
some of the "art" of electron microscopy has been lost. In the 1960s and early
1970s, when the technical demands of electron microscopy were greater, inves-
tigators devoted themselves wholeheartedly to acquiring the skills necessary for
producing electron micrographs that were both informative and esthetically at-
tractive. Sharp and clean images of well-fixed material were the aims of every
cytologist. Considerable effort was expended in the pursuit of the most complete
rendition of protoplasmic structure possible. Such images permitted neurocytol-
ogists to distinguish and describe all the components of the complex tissue that
the brain of any animal contains. Today, it is taken for granted that any study
that requires them can be illustrated with electron micrographs. But with the
increasing facility with the elementary techniques has come a decline in the ex-
acting criteria both for acceptable electron micrography and for credible inter-
pretation of the profiles displayed within them. Good examples of these changes
in standards can be found in the identification of synaptic junctions in tissues
taken from tracing experiments or from immunocytochemical studies. While this
vii

7. decline in standards is regrettable, it reflects the fact that electron microscopy of
the nervous system has passed the classic stage of exploration. Electron micro-
scopy is now being used to examine specific issues, such as the interconnections
among neurons and the locations of specific proteins or neuroactive substances.
Fifteen years ago we were using degeneration techniques and tracers, such as
horseradish peroxidase or radioactively labeled amino acids, in order to under-
stand how the nervous system is constructed. Although important information
was obtained through the use of these methods and they are still useful, their
place at the forefront has largely been ceded to intracellular filling techniques
and combined Golgi-electron microscopy. But the modern explosion in the neu-
romorphological sciences has been brought about by the use of antibodies to
identify the chemical signatures of neural pathways and individual neurons and
synaptic terminals. For all of these new approaches the appreciation of fine
structure is more pertinent than ever.
We have rewritten this book in the light of the information obtained through
the use of these newer methods because they have led to a much better under-
standing of the relationships between neuronal circuits and their functions. Con-
sequently we have extensively revised all of the chapters and added many new
references to the bibliography. We have, however, retained those references that
reflect the foundations upon which our new information is based. A reader fa-
miliar with the previous edition of this book will certainly recognize paragraphs
and descriptions that have not changed appreciably because no significant new
knowledge has come to our notice in that area. Other chapters, such as the
chapters on axons, synapses, sheaths, and the neuropil, have been almost en-
tirely rewritten. In the chapter on the neuropil we have tried to show the pos-
sibilities and limitations of the various techniques, so that this chapter has be-
come a vehicle for giving an account of the methods available. To a large extent
this strategy has allowed us to eliminate details of techniques from the other
chapters.
We hope that in this version of the book we have succeeded in correlating
structure and function and in providing a reference source of electron micro-
graphs and literature, in which both experienced neuroscientists and students
interested in the fine structure of the nervous system can find information be-
yond the scope of their immediate interests.
Although most of the illustrations come from our own collections, we have
relied on the generosity of many colleagues for illustrations of structures and
techniques that we have not explored ourselves. We gratefully acknowledge the
contributions of figures from J. Anders, D. J. Allen, Dennis Bray, Milton Bright-
man, Mary B. Bunge, Victoria Chan-Palay, M. W. Cloyd, Edward V. Famig-
lietti, Martin L. Feldman, James E. Hamos, C. K. Henrikson, John E. Heuser,
J. Hirokawa, James Kerns, Frank N. Low, Douglas L. Meinecke, Enrico Mug-
naini, Elio Raviola, Thomas S. Reese, Bruce Schnapp, Constantino Sotelo, Deb-
orah W. Vaughan, James E. Vaughn, Bruce W. Warr, and Raymond B. Wuer-
ker.
We are also grateful to Janet Harry, Mary Alba, Lilian Galloway and Joyce
Resil for typing the several versions of the manuscript and references, and to
Katherine Harriman, Karen Josephson and Claire Sethares for their expert tech-
nical assistance. In addition we wish to thank Dr. R. Hammer and Dr. V. J.
viii PREFACE

8. DeFeo for providing facilities for one of us (A.P.) to enjoy a session of quiet
writing at the University of Hawaii, and Dr. P. Hashimoto of Osaka University
for providing facilities for another of us (S.L.P.) during an extended visit.
By no means of least importance, we wish to pay tribute to our wives. With-
out their patience, understanding and support, we could not have completed
this revision.
Boston, Massachusetts Alan Peters
Concord, Massachusetts Sanford L. Palay
Bethesda, Maryland Henry deF. Webster
February 1990
PREFACE ix

9. Contents
List of Illustrations, xv
1 General Morphology of the Neuron, 3
2 The Neuronal Cell Body, 14
THE PERIKARYON, 14
The Nissl Substance, 14
The Agranular Reticulum, 22
The Golgi Apparatus, 26
Multivesicular Bodies, 33
Lysosomes, 33
Peroxisomes, 34
Lipofuscin Granules, 34
Mitochondria, 38
Microtubules and Neurofilaments, 40
Cilia and Centrioles, 41
Cytoplasmic Inclusions, 42
THE NUCLEUS, 48
General Morphology, 48
The Nuclear Envelope, 52
The Karyoplasm, 58
The Nucleolus, 59
Nuclear Inclusions, 60
THE PLASMA MEMBRANE, 64
3 Dendrites, 70
GENERAL MORPHOLOGY, 70
THE CYTOPLASM OF DENDRITES, 76
THE DENDRITIC SPINES, 82
MYELINATED DENDRITES, 96
GROWING TIPS OF DENDRITES, 98
xi

10. 4 The Axon, 101
AXON HILLOCK AND INITIAL AXON SEGMENT, 101
THE AXON BEYOND THE INITIAL SEGMENT, 108
Neurofilaments and Microtubules, 110
Membranous Components, 119
Cytoskeleton, 122
The Axonal Membrane, 124
AXOPLASMIC FLOW, 126
THE AXON GROWTH CONE, 132
THE IDENTIFICATION OF SMALL AXONS AND DENDRITES, 137
5 Synapses, 138
THE NEUROMUSCULAR SYNAPSE, 138
INTERNEURONAL CHEMICAL SYNAPSES, 147
The Synaptic Junction, 150
The Presynaptic Grid, 154
The Synaptic Cleft, 159
Potsysnaptic Densities, 160
Freeze-Cleavage, 166
Nonsynaptic Junctions Between Neurons, 168
Synaptic Vesicles With Clear Centers, 169
Shapes and Sizes of Vesicles, 169
Correlation Between Vesicle Shape and Function of Chemical Synapses, 176
Granular Vesicles, 178
Neurosecretory Vesicles, 184
Other Presynaptic Organelles, 186
Other Postsynaptic Organelles, 188
Synaptic Relations, 190
Axo-Dendritic Synapses, 190
Axo-Somatic Synapses, 191
Axo-Axonal Synapses, 192
Dendro-Dendritic Synapses, 195
Somato-Dendritic, Dendro-Somatic and Somato-Somatic Synapses, 196
Somato-Axonic Synapses, 198
Dendro-Axonic Synapses, 198
Synaptic Glomeruli, 199
ELECTROTONIC SYNAPSES, 203
MIXED SYNAPSES, 207
"SYNAPSES" INVOLVING NEUROGLIAL CELLS, 210
6 The Cellular Sheaths of Neurons, 212
THE SHEATHS OF UNMYELINATED GANGLION CELLS, 213
THE SHEATHS OF UNMYELINATED NERVE FIBERS, 218
THE SHEATHS OF MYELINATED FIBERS, 222
Internodal Peripheral Myelin, 224
The Formation of the Peripheral Myelin Sheath, 226
Internodal Central Myelin, 232
The Formation of the Central Myelin Sheath, 234
Identification of the Myelin-forming Cell of the Central Nervous System, 242
The Mechanism of Myelin Formation, 246
The Node of Ranvier, 250
The Schmidt-Lanterman Incisures, 261
xii CONTENTS

11. The Differences Between Peripheral and Central Myelin Sheaths, 262
The Proximity of Adjacent Sheaths, 262
The Thickness of Myelin Lamellae, 263
The Radial Component of the Central Sheath, 264
THE SHEATHS OF MYELINATED GANGLION CELLS, 265
MYELIN SHEATHS OF DENDRITES IN THE CENTRAL NERVOUS SYSTEM, 266
FUNCTIONS OF SATELLITE AND SCHWANN CELLS, 266
Early Development, 266
Axon Ensheathment and Myelin Formation, 267
Biochemical Relationships, 269
Breakdown of Myelin, 271
Other Functions, 272
7 The Neuroglial Cells, 273
THE DEVELOPMENT OF NEUROGLIA, 274
ASTROCYTES, 276
Fibrous Astrocytes, 277
Protoplasmic Astrocytes, 281
Functions of Astrocytes, 284
Structural Support, 284
Guidance for Neuroblast Migration and Axon Growth, 286
Graft Survival and Function, 288
Isolation of Receptive and Nodal Surfaces of Neurons, 288
Interactions with Oligodendroglia: Role in Myelination, 290
Blood-Brain Barrier, 290
Interactions with the Immune System, 293
Repair, 294
OLIGODENDROCYTES, 295
General Morphology, 295
Functions of Oligodendrocytes, 298
NEUROGLIAL CELLS INTERMEDIATE BETWEEN ASTROCYTES AND OLIGODENDROCYTES, 302
MICROGLIA, 304
General Morphology, 306
Functions, 308
Discussion, 308
8 The Ependyma, 312
THE MORPHOLOGY OF EPENDYMAL CELLS, 313
THE MORPHOLOGY OF TANYCYTES, 318
INTRAVENTRICULAR NERVE ENDINGS, 322
THE SUBEPENDYMA, 324
FUNCTIONS OF CELLS IN THE EPENDYMA, 325
Movements of Cerebrospinal Fluid, 325
Capture of Materials Present in the Cerebrospinal Fluid, 325
Proliferation, 325
Support, 326
Sensory Function, 326
Secretion, 326
Transport of Substances, 327
CONTENTS xiii

12. 9 Choroid Plexus, 328
THE CHOROIDAL EPITHELIUM, 330
THE VASCULARIZED CONNECTIVE TISSUE CORE, 336
FUNCTIONS OF THE CHOROID PLEXUS, 338
10 Blood Vessels, 344
CAPILLARIES, 344
ARTERIES AND ARTERIOLES, 350
VEINS, 354
11 The Neuropil, 356
THE IDENTIFICATION OF PROFILES IN THE NEUROPIL, 356
THE ORGANIZATION OF THE NEUROPIL AND SYNAPTIC CONNECTIONS, 364
Golgi—Electron Microscope Technique, 366
Intracellularly Injected Markers, 368
Reconstruction of Neurons and Their Processes, 370
Experimental Degeneration, 372
Intracellular Transport of Radioisotopes, 375
Antibodies to Neurotransmitters, 375
Antibodies to Neuropeptides, 380
Techniques Using Two Antibodies, 381
Combined Techniques, 382
12 Connective Tissue Sheaths of Peripheral Nerves, 384
EPINEURIUM, 384
PERINEURIUM, 385
ENDONEURIUM, 388
FUNCTIONS OF CONNECTIVE TISSUE SHEATHS, 392
13 The Meninges, 395
DURA MATER, 396
ARACHNOID MATER, 398
PIA MATER, 400
ENTRY OF PERIPHERAL NERVES INTO THE CENTRAL NERVOUS SYSTEM, 402
ARACHNOID VILLI, 404
References, 407 Index, 487
xiv CONTENTS

13. List of Illustrations
1-1 The Neuronal Cell Body, 11
2-1 The Cell Body of a Pyramidal Cell, 17
2-2 A Purkinje Cell, 19
2-3 Pyramidal Neuron, 21
2-4 Granule Cells of the Cerebellum, 23
2-5 The Cytoplasm of a Purkinje Cell, 25
2-6 The Cytoplasm of a Dorsal Root Ganglion Cell, 29
2-7 The Cytoplasm of a Dorsal Root Ganglion Cell, 31
2-8 Nissl Bodies in an Anterior Horn Cell, 35
2-9 The Golgi Apparatus and the Nissl Substance of a Purkinje Cell, 37
2-10 Golgi Apparatus of the Purkinje Cell, 39
2-11 Two Views of the Golgi Apparatus in a Freeze-Fractured Preparation, 43
2-12 Golgi Apparatus, Lysosomes, Nematosomes, and Fibrillary Inclusions, 45
2-13 Lipofuscin Granules, Cilia, and Centrioles, 47
2-14 Laminated Inclusion Body, 49
2-15 The Nuclear Envelope, Nissl Bodies, and Golgi Apparatus, 55
2-16 Nuclear Pores, 57
2-17 The Nucleolus, 61
2-18 Intranuclear Inclusions, 63
2-19 Diagram of Freeze Fracturing, 65
2-20 The Edge of a Purkinje Cell, Freeze-Fractured Preparation, 67
3-1 Pyramidal Neuron in Cerebral Cortex, 73
3-2 The Apical Dendrites of Pyramidal Cells, 75
3-3 Dendrite of a Purkinje Cell, 79
3-4 Dendrite of a Purkinje Cell, 81
3-5 Dendrites in Longitudinal and Transverse Section, 83
3-6 Dendrites in the Neuropil of the Anterior Horn: Transverse Section, 85
3-7 Dendrites in the Neuropil of the Cerebral Cortex, 87
3-8 Dendrites in Cerebellar and Cerebral Cortex, 89
3-9 A Spiny Branchlet of a Purkinje Cell Dendrite, 91
xv

14. 3-10 Olfactory Bulb, 93
3-11 Myelinated Dendrite in Olfactory Bulb, 97
3-12 Dendrite Growth Cones, 99
4-1 Axon Hillock and the Initial Axon Segment, 103
4-2 Axon Hillock and the Initial Axon Segment, 105
4-3 The Initial Axon Segment, Longitudinal Section, 107
4-4 The Initial Segment, Transverse Section, 109
4-5 The Initial Axon Segment and the Node of Ranvier Compared, 111
4-6 Axon Hillock and Initial Segment of a Trigeminal Ganglion Cell, 113
4-7 The Initial Segment of a Trigeminal Ganglion Cell, 115
4-8 Microtubules, Neurofilaments, and Neuroglial Filaments, 117
4-9 Axoplasmic Organelles, 121
4-10 Quick-frozen and Deep-etched Axoplasm, 123
4-11 Quick-frozen and Deep-etched Axoplasm, 125
4-12 Growth Cone from a Sympathetic Neuron in Tissue Culture, 129
4-13 Growth Cone from a Sympathetic Neuron in Tissue Culture, 131
4-14 Small Axons in the Molecular Layer of the Cerebellum, 133
4-15 Unmyelinated Axons Entering the Olfactory Bulb, 135
5-1 Motor End Plate, 141
5-2 Motor End Plate, 143
5-3 Freeze-Fractured Motor End Plates to Show Vesicle Release, 145
5-4 Puncta Adhaerentia, 149
5-5 Axon Terminal Emerging from the Myelin Sheath, 151
5-6 Synapses in the Cerebral Cortex, 153
5-7 Asymmetric Synapses, Cerebral Cortex, 155
5-8 Presynaptic Grid, 157
5-9 Synapses in the Cerebellum, 161
5-10 The Synaptic Junction Between an Axon and a Dendritic Thorn, 163
5-11 Asymmetric and Symmetric Synapses, 165
5-12 Synapses in the Anterior Horn of Spinal Cord, 167
5-13 Anterior Horn of Spinal Cord, 171
5-14 The Glomerulus, Cerebellar Cortex, 173
5-15 The Presynaptic Membrane, P face, 175
5-16 The Presynaptic Membrane, E face, 181
5-17 A Variety of Synapses, 183
5-18 Axo-axonic Synapse and Dense-cored Vesicles, 185
5-19 Dendro-dendritic and Somato-dendritic Synapses, 189
5-20 The Glomerulus, Lateral Geniculate Nucleus, 197
5-21 Electrotonic Synapses, 205
5-22 A Mixed Synapse, 209
6-1 The Sheath Surrounding a Dorsal Root Ganglion Cell, 215
6-2 The Sheath Surrounding a Trigeminal Ganglion Cell, 217
6-3 Unmyelinated Axons, Adult Peripheral Nerve, 219
6-4 Unmyelinated Axons, Adult Peripheral Nerve, 221
6-5 Myelinated Axon, Adult Peripheral Nerve, 227
xvi ILLUSTRATIONS

15. 6-6 Developing Schwann Cell Sheaths, 229
6-7 Developing Schwann Cell Sheaths, Later Stage, 231
6-8 Diagrammatic Representation of the Formation of Peripheral Myelin Sheaths, 233
6-9 Myelinated Nerve Fibers, Central Nervous System, 235
6-10 Myelin Sheaths: Central Nervous System, 237
6-11 Developing Myelin Sheaths, Central Nervous System, 239
6-12 Developing Myelin Sheaths, Central Nervous System, 241
6-13 Diagrammatic Representation of the Formation of Myelin in the Central Nervous System, 243
6-14 The Myelin Forming Cell, Central Nervous System, 245
6-15 The Node of Ranvier, Peripheral Nervous System, 249
6-16 The Node of Ranvier, Central Nervous System, 251
6-17 The Node and the Paranode, Central Nervous System, 253
6-18 Freeze-Fractured Myelin Sheaths, 255
6-19 Freeze-Fractured Myelin Sheaths, 257
6-20 Freeze-Fractured Myelin Sheaths, 259
6-21 Diagram of Membrane Particle Distribution at the Paranode, 260
7-1 Fibrous Astrocytes, 279
7-2 Protoplasmic Astrocytes, 283
7-3 Protoplasmic Astrocytes, 285
7-4 Protoplasmic Astrocyte, 287
7-5 Glial Limiting Membrane; Cerebral Cortex, 289
7-6 Orthogonal Assemblies and Gap Junctions of Astrocytes in Freeze-Fracture Preparations, 291
7-7 Perineuronal Oligodendrocytes, 297
7-8 An Oligodendrocyte, 299
7-9 Interfascicular Oligodendrocyte, 301
7-10 A Perineuronal Microglial Cell, 303
7-11 A Microglial Cell in a Senile Plaque, 307
8-1 The Ependyma, 315
8-2 Ependymal Surface, 317
8-3 The Cilia of Ependymal Cells, 319
8-4 Ependymal Cell Cytoplasm, 321
8-5 Ependymal Cell Junctions, 323
9-1 Scanning Electron Micrograph of the Choroid Plexus, 329
9-2 Epithelium and Stroma of the Choroid Plexus, 331
9-3 The Choroid Plexus, 333
9-4 Choroid Plexus, Intercellular Junctions, 335
9-5 Choroid Plexus, Intercellular Junctions, 337
9-6 Choroid Plexus, Surface Structures, 339
9-7 The Basal Ends of Choroidal Cells, 341
9-8 Kolmer Cells, 343
10-1 Capillary and Pericyte, 347
10-2 Capillaries, 349
10-3 Small Blood Vessel, 351
10-4 Intracerebral Arterioles, 353
10-5 An Arteriole, 355
ILLUSTRATIONS xvii

16. 11-1 The Neuropil, Anterior Horn, Spinal Cord, 359
11-2 The Neuropil, Cerebellar Cortex, 361
11-3 The Neuropil, Cerebral Cortex, 363
11-4 Lateral Geniculate Body Glomerulus, 365
11-5 Degenerating Boutons, 367
11-6 Filamentous Degeneration and Horseradish Peroxidase-labeled Neurons, 369
11-7 Golgi-Electron Microscope Technique, 371
11-8 Intracellular Horseradish Peroxidase Injection, 373
11-9 Glutamic Acid Decarboxylase Immunoreactive Axon Terminals, 377
11-10 Vasoactive Intestinal Polypeptide in the Cerebral Cortex, 379
12-1 Connective Tissue Sheaths of Nerves, 387
12-2 Epineurial and Perineurial Sheaths, 389
12-3 Perineurium and Endoneurium of Peripheral Nerve, 391
13-1 Meninges by Scanning Electron Microscopy, 397
13-2 Dura Mater, 399
13-3 Arachnoid Mater, 401
13-4 Pia Mater and Glia Limitans, 403
xviii ILLUSTRATIONS

17. Dedicated to the Memory of
Jan Evangelista Purkinje, 1787-1869
Louis-Antoine Ranvier, 1835-1922
Camillo Golgi, 1843-1926
Santiago Ramon у Cajal, 1852-1934

18. The Fine Structure of the Nervous System

19. 1
General Morphology of
the Neuron
Anyone who has studied the early history of cy- nervous system lay in the shape of the nerve cell
tology cannot fail to be impressed by the slow itself and to some extent in its size. The medusa-
development of the concept of the nerve cell. Most like nerve cell, with its corona of seemingly endless
types of cells do not have a history. Once the idea processes, was bizarre. Other cells had relatively
was grasped, in the theory of Schleiden (1838) simple shapes—globular, cylindrical, squamous,
and Schwann (1839), that cells are the architec- fusiform, and so on. Some fitted one into the other
tonic units of living things, it was fairly quick like pieces of a jigsaw puzzle to form an epithe-
work to recognize them in the various tissues and lium; others lay free and definable in the tissue
to proceed to the study of their contents, their fluids. Many, such as cartilage or certain epithelial
interrelations, and their functions. But the nerve cells, were clearly circumscribed by walls. Only
cell was more perplexing. It occasioned so much pigment cells, astrocytes, myoepithelial cells, and
difficulty for its students that almost a century a few others had shapes even roughly approxi-
passed before they could agree upon its shape. At mating those of nerve cells. But aside from the
first it was thought to be an independent globular fact that some of these examples were unknown
corpuscle suspended among nerve fibers, which in the early days, such cells could be easily encom-
looped and coiled about it and which it somehow passed in a single field or at least in a single
nourished (Valentine, 1836). Later, when the con- preparation under the microscope. The multipolar
tinuity between the perikaryon and the nerve fi- nerve cell, however, with its meter-long axon did
bers was finally established (Remak, 1838, 1841; not fit into a single section and could not be easily
Helmholtz, 1842; Hannover, 1844; Kolliker, 1844; plucked from its context or distinguished from its
Bidder, 1847; Wagner, 1847), then the nerve cell neighbors by the methods used for other cells.
appeared to have no definite boundaries and seemed New methods had to be developed. And so a true
endless. Except for the fibers attached to organs cell theory of the nervous system did not emerge
in the periphery, the processes of all nerve cells until the discovery and exploitation of special
seemed to be equivalent and to be confluent with techniques that had the merit of bringing into
one another. The nerve cells seemed to be only view entire nerve cells as if dissected or isolated
nodal points in an enormously intricate reticulum from the central nervous system.
pervading the nervous system (Gerlach, 1858, Actually, the first successful method was mi-
1872). It appeared that the cell theory did not crodissection of whole nerve cells from hardened
really apply to the nervous system; one had rather specimens of brain and spinal cord. On the basis
to speak of cell territories or spheres of influence of experience with such isolated cells, Deiters (1865)
surrounding nucleated centers. was able to distinguish between the numerous
It seems clear that one of the major obstacles branching processes that we now call dendrites
to the appreciation of the cellular nature of the and the single process that slips into a myelin
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