front matter 1

I. Introduction

A. Overview of Atlas Organization

This Atlas focuses on the development of the human

spinal cord, and is the fi rst 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 fi rst 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 magnifi cation 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 fi rst trimester (VIA) and of moto-

neuron columns in the ventral horn at cervical, thoracic,

and lumbar levels in three specimens at the end of the

fi rst trimester and the beginning of the second (VIB). In

Part VII, quantitative summaries of spinal cord develop-

ment are presented in several graphs along with estimates

of timetables for neurogenesis, dates of cell migration and

settling, and sequences of myelination in major fi ber tracts.

A Glossary gives brief defi nitions for each label in the

Plates, and defi nes other terms that are used in notes and

fi gure 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 devel-

opmental 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, Washing-

ton D.C. Five of the specimens (designated by a C prefi x)

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 fi xatives, 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 Carn-

egie Institution of Washington.

Seven of the specimens (designated by an M prefi x)

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 speci-

mens and their similar appearance, we presume that they

are preserved in the same way although there are no exist-

ing records describing fi xation procedures. The slides con-

tain information regarding the section number, the section

thickness (10 μm), and the stain (aluminum cochineal).

Most embryos are cut in the same transverse plane, prob-

ably by Minot himself. Because of the constancy in their

preparation, the Minot specimens are the source of quanti-

tative data that are used to determine developmental trends

during the early fi rst trimester. To our knowledge, the ner-

vous system has never been examined in the Minot speci-

mens, other than by Altman and Bayer (2001).

Eight of the specimens (designated by a Y prefi x)

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 fi xed in for-

malin, embedded in celloidin, and are cut in 35 μm sec-

tions. 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 modifi cation

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 Olym-

pus photomicroscope (the Carnegie and Minot specimens)

or a Wild photomakroskop (the Yakovlev specimens) using

Kodak technical pan black and white negative fi lm #TP442.

The fi lm is developed for 6 to 7 minutes in dilution F of

Kodak HC–110 developer, followed by stop bath for 30

seconds, Kodak fi xer for 5 minutes, Kodak hypo clearing

agent for 1 minute, running water rinse for 10 minutes,

and a brief rinse in Kodak photo–fl o before drying. Each

specimen is photographed at the magnifi cation that fi lled

the microscopic fi eld with the largest cross-section of the

spinal cord.