front matter 1

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

fi bers, but also unmyelinated fi bers. The heavily myelin-

ated axons are presumably from large sensory neurons in

the dorsal root ganglia, while the unmyelinated axons are

from small nociceptive ganglion cells. The remaining fi ber

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 fi ber

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 moto-

neurons continue to be prominent and show greater degrees

of segregation in the ventral horn. The dorsal horn has

fairly well-defi ned clusters of small neurons in the substan-

tia gelatinosa. The accumulation of lateral horn motoneu-

rons is very obvious at the thoracic level. Clarke’s column

is also prominent in this specimen.

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-fi xation)

of the section in square millimeters (mm^2 ). Full-page

normal contrast photographs of each specimen are in

Plates 54A-61A. Low contrast photographs with superim-

posed 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 rang-

ing 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 dimin-

ishing 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 match-

ing cell body stained section in all cases. Evidently, the

myelin staining procedure produces greater tissue shrink-

age 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 indica-

tive 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). Myelin-

ated fi bers from the dorsal root collateralization zone pen-

etrate the gray matter, and there is a light dusting of reactive

glia in the subgelatinosal plexus in the dorsal gray as fi ner

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

IV. The Third Trimester

in the spinal cord of a GW31 fetus -------------------------------------------- A. Matched myelin and cell body stained sections

Name

Myelination

Reactive glia

Proliferating glia

DORSAL FUNICULUS:

LATERAL and VENTRAL

FUNICULI:

dorsal root col. zone Many fibers --- Very dense

deep fas. gracilis Many fibers --- Very dense

superficial fas. gracilis Some fibers Dense Dense

lat. reticulospinal tract Some fibers Dense Sparse

intraspinal tracts Some fibers Dense Sparse

spinocephalic tracts

rubrospinal tract

Some fibers Dense Sparse

spinocerebellar tracts Some fibers Dense Dense

Some fibers Dense Sparse

vestibulospinal tract Some fibers Dense Sparse

dorsal root bif. zone Many fibers* --- Sparse

ven. commissure Many fibers --- Sparse

Lissauer's tract --- None Very sparse

lat. cortricospinal tract --- Very sparseVery sparse

ven. cortricospinal tract --- Sparse Very sparse

Table IV A-1: Glia types and concentration

in the white matter at GW31

* intermingled in a bed of nonreactive glia (associated with

Lissauer's tract fibers?)