front matter 1

B. Spinal cord of a GW8.5 embryo

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 neuroepi-

thelium, an ependymal layer is appearing as low as lumbar

levels.

The gray matter displays maturational changes indic-

ative of the adult spinal cord. In the ventral horn, moto-

neurons 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 repre-

sent 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 lat-

eral cervical nucleus, and an accumulation of large neu-

rons 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 dis-

tinctive clumps of densely packed small cells, presumably

settling neurons of laminae II and III (substantia gela-

tinosa). 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 fl oor plate, and

moves upward as the ventral neuroepithelium and ventral

spinal canal disappear. The dorsal funiculus is accumulat-

ing axons on either side of the dorsal midline along ver-

tically aligned cells that may connect the retreating roof

plate to the pia.

Plate 13 is a survey of sections from seven levels of

the spinal cord in M2050, a specimen in the Minot Col-

lection 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 sec-

tion number; and the total area of the section in square

millimeters (mm^2 ). Note that the areal measurements are

determined after fi xation, while the crown rump length is

measured before fi xation. The unfi xed 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 enlarge-

ment is still 50% smaller than the cervical enlargement,

and the smallest cross-sectional areas are at sacral and coc-

cygeal levels. These size differences refl ect 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 auto-

nomic 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 neuro-

epithelium still appears to be active at low thoracic and

lumbar levels, in contrast to cervical levels. At the coccy-

geal level, even the ventral neuroepithelium appears to be

active. Thus, neurons are still being generated in the spinal

cord on GW8.5.

Part II: The First Trimester (continued)