front matter 1

VI. Three-Dimensional Reconstructions of the Developing Spinal Cord

A. The cervical level of eight fi rst–trimester specimens

These fi gures demonstrate three important relation-

ships 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 pop-

ulation proliferate and expand. Those stem cells recede

and ultimately disappear when all the neurons in that popu-

lation are generated. Second, there is a ventral (fi rst) to

dorsal (last) gradient of neuroepithelial growth and decline.

Ventral horn neurons, especially the large alpha motoneu-

rons, are generated earlier than neurons in the intermediate

gray and the latest neurons to be generated are in the dorsal

horn. These morphogenetic observations confi rm the neu-

rogenetic gradient that has been well documented in the rat

spinal cord with^3 H–thymidine autoradiography (Altman

and Bayer, 1984, 2001). Third, the sequential neuroepi-

thelial gradient is followed by a sequential gradient of mor-

phogenesis. The ventral horn appears fi rst, followed by the

intermediate gray and fi nally by the dorsal horn.

Figures 2 through 9 feature computer reconstructed

three-dimensional models of the cervical part of the devel-

oping spinal cord in eight fi rst-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 neuroepi-

thelium have disappeared, neurons have been generated,

migration to the gray matter is largely completed, and most

major components of the white matter appear and fi ll with

growing axons. Table VI A–2 lists the colors used to dis-

tinguish 13 different structures in the developing spinal

cord in all the fi gures. The reconstructed models in the fi g-

ures 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.

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.

Table VI A–1: Specimens Used for

3-D Reconstructions

of the Cervical Spinal Cord

NAME CR (mm) GW

C6144 3.3

C836 4.0

M2065 8.0

C6517 10.5

C8965 19.1

C8553 22.0

M2050 36.0

Y380-62 56.0

3.5-4.0

4.0

5.25

5.5

7.0

7.2

8.5

14

Table VI A–2: Color Key for

Structures in the Spinal Cord

STRUCTURE COLOR

Undivided neuroepithelium Dark cyan

Ventral neuroepithelium Dark red

Intermediate neuroepithelium Dark yellow-green

Dorsal neuroepithelium Dark yellow

Gray matter Bright cyan

Roof plate Brown

Floor plate Brown

Ventral horn Bright red

Dorsal horn Bright yellow

Intermediate gray Bright yellow-green

Entire outside edge Transparent clear

Dorsal funiculus Pale blue

Ventral funiculus Pale violet

Lateral funiculus Pale purple

LEFT SIDE:

BOTH SIDES OR IN MIDLINE:

RIGHT SIDE: