front matter 1

join Lissauer’s tract. The reticulated area is more promi-

nently myelinated in the lateral intermediate gray and lat-

eral neck region of the dorsal gray. The reactive glia in the

subgelatinosal plexus is more concentrated, indicating that

terminal parts of the dorsal root collaterals are approach-

ing myelination. It is interesting to note that the white

matter in the cell body stained sections (except the sacral/

coccygeal level, Plate 78) show a uniformly reduced den-

sity of glia, suggesting the decline or cessation of myelina-

tion gliosis (see column 4 in Table IV B-1). But again,

the lateral and ventral corticospinal tracts generally stand

out as having slightly lower concentrations of glia; in these

tracts, myelination gliosis has not yet begun.

Plate 62 is a survey of matched myelin stained and

cell body stained sections from Y117-61, a specimen in the

Yakovlev Collection with a crown rump length of 310 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 63A-78A. Low contrast photographs with superim-

posed labels and outlines of structural details are in Plates

63B-78B. In this specimen, the myelin stained and cell

body stained sections were preserved on separate large

glass plates without any section numbers. This specimen

has the most complete set of spinal cord sections in the

Yakovlev Collection with all levels preserved. Twenty

myelin stained sections and 19 cell body stained sections

were photographed ranging from upper cervical to sacral/

coccygeal levels. The 39 photographic prints were intu-

itively arranged in order from upper cervical to sacral/

coccygeal levels, using internal features such as the size of

the corticospinal tracts and the width of the ventral horn.

Then, myelin and cell body stained sections were matched

and eight different levels were analyzed.

As in the previous specimens, the cross-sectional

area of a myelin stained section is smaller than the match-

ing cell body stained section in all cases, except at the

upper thoracic level, where the areas are the same. Using

the total areas of the myelin stained sections, the overall

size differences between levels indicate the following: the

cervical enlargement has the largest area, being larger than

the lumbar enlargement by 30%. The middle thoracic level

has the smallest area and is 6% smaller than the sacral/

coccygeal level.

Myelination continues to advance in this specimen

(see columns 2 and 3 in Table IV B-1). Dense staining

indicative of either advanced or beginning myelination is

seen throughout the ventral funiculus and the lateral funicu-

lus except the ventral and lateral corticospinal tracts, the

sacral/coccygeal level excepted. All parts of the dorsal

funiculus are either myelinated or are myelinating, except

Lissauer’s tract and the areas in the dorsal root bifurcation

zone which most likely contains incoming axons that will

in the spinal cord of a 4–week infant ----------------------------------------- B. Matched myelin and cell body stained sections

Part IV: The Third Trimester (concluded)

Name

Myelination

Reactive glia

Proliferating glia

DORSAL FUNICULUS:

DORSAL ROOT

VENTRAL ROOT

LATERAL and VENTRAL

FUNICULI:

dorsal root col. zone Myelinated --- Sparse†

Myelinated

Myelinated

deep fas. gracilis Myelinated --- Sparse

superficial fas. gracilis Many fibers --- Sparse

deep fas. cuneatus Myelinated --- Sparse

superficial fas. cuneatus Many fibers --- Sparse

intraspinal tract

---

Many fibers --- Sparse

spinocephalic tract Some fibersVery dense Sparse

spinocerebellar tracts

---

Many fibers Sparse

lat. reticulospinal tract Many fibers Sparse

med. long. fasiculus Many fibers --- Sparse

vestibulospinal tract --- Dense Sparse

dorsal root bif. zone Many fibers* Sparse

---

Dense

---

---

---

ven. commissure Myelinated Sparse

---

Lissauer's tract --- None Sparse

lat. cortricospinal tract --- Very sparseVery sparse

ven. cortricospinal tract --- Very sparseVery sparse

Table IV B-1: Glia types and concentration

in the white matter at GW37

* intermingled in a bed of nonreactive glia (associated with

Lissauer's tract fibers?)

† dense at the sacral/coccygeal level