front matter 1

ated axons from the ventral corticospinal tract (see p. 198

in Ranson and Clark, 1959). Prior to this time, axons in

the ventral corticospinal tract are present, but they may not

have grown decussating collaterals.

The gray matter in the myelin stained sections

shows heavy fascicles of myelinated fi bers penetrating the

dorsal horn from above and medially where it joins the

intermediate gray. There is a large subgelatinosal plexus

of myelinated fi bers in the myelin stained sections. Fine

fascicles of myelinated fi bers are also scattered throughout

the ventral horn, only labeled in the lumbar enlargement

where there is a high ratio of gray matter to white matter.

Many of these myelinated axons are from the large neu-

rons in the dorsal root ganglion penetrating the dorsal horn

from above and below (in the subgelatinosal plexus), arbo-

rizing in the intermediate gray (especially around Clarke’s

column), and in the ventral horn. Other myelinated axons

are from the motoneurons in the ventral horn that later

organize into fascicles to form the ventral rootlets of spinal

nerves. The intraspinal tract is one of the earliest myelin-

ating; see the dense reactive glia in this tract at GW26

(Plates 47, 49, and 51 ).

Plate 92 is a survey of matched myelin stained and

cell body stained sections from Y23-60, a specimen in the

Yakovlev Collection with a crown rump length of 410 mm.

All sections are shown at the same scale. The boxes enclos-

ing 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 93A-102A. Low contrast photographs with

superimposed labels and outlines of structural details are

in Plates 93B-102B. In this specimen, the myelin stained

and cell body stained sections were preserved on separate

large glass plates without consecutive section numbers. In

addition, some sections were fl ipped over before they were

placed on the plates. Sixteen myelin stained sections and

16 cell body stained sections were photographed ranging

from upper cervical to lumbar enlargement levels. The 32

photographic prints were intuitively arranged in order from

upper cervical to lumbar levels, using internal features such

as the reduced size of the corticospinal tracts from rostral

to caudal levels. Then, myelin and cell body stained sec-

tions were matched and 6 different levels are analyzed.

Only myelin stained sections are illustrated at the cervical

enlargement and upper lumbar levels because there are no

matching cell body stained sections.

As in the previous specimens, the cross-sectional

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

ing cell body stained section with the exception of the

upper thoracic level. At that level, the cell body and myelin

stained sections are not perfectly matched; the cell body

stained section is probably 3 to 4 sections below the myelin

stained section. Using the total areas of the myelin-stained

sections, the cervical enlargement has the largest cross-sec-

tional area, being larger than the lumbar enlargement by

20%; the two thoracic levels have smaller cross-sectional

areas, averaging 74% smaller than the cervical enlargement

and 45% smaller than the lumbar enlargement.

Myelination in this specimen has two different char-

acteristics (Table V B-1) than in the 4-day-old infant. First,

there are myelination gradients in two fi ber tracts. (1) The

lateral corticospinal tract has a gradient of medial (dense)

to lateral (sparse) reactive glia at cervical levels; myelin-

ated fi bers will fi rst appear medially and last laterally (see

Chapter 9, Figures 9-25 through 9-32 in Altman and Bayer,

2001). (2) There is some indication of a myelination gra-

dient in the spinocephalic tracts at both cervical levels,

because the outer fi bers appear to myelinate later than the

inner fi bers (those closer to the intraspinal tracts). Second,

the ventral commissure, which is either myelinating or is

myelinated at GW37 and the 4-day-old infant, appears to

be less advanced in this specimen, especially at cervical

levels. That is probably due to the crossing of unmyelin-

Part V: The Early Postnatal Period (continued)

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

infant

Name

Myelination

Reactive glia

Proliferating glia

DORSAL FUNICULUS:

DORSAL ROOT

VENTRAL ROOT

LATERAL and VENTRAL

FUNICULI:

dorsal root col. zone Myelinated --- Sparse

Myelinated

Myelinated

fas. gracilis Myelinated --- Sparse

fas. cuneatus Myelinated --- Sparse

intraspinal tract Myelinated --- Sparse

spinocephalic tract Gradient†† Gradient†† Sparse

spinocerebellar tracts Myelinated --- Sparse

med. long. fasiculus Many fibers --- Sparse

vestibulospinal tract Some fibers --- Sparse

dorsal root bif. zone Many fibers* Sparse

---

Sparse•

---

---

---

ven. commissure Myelinated** --- Sparse

Lissauer's tract --- None Sparse

lat. cortricospinal tract --- Gradient† Sparse

ven. cortricospinal tract --- Sparse Sparse

Table V B-1: Glia types and concentration

in the white matter in a 4-week-old infant

• dense band in a sparse field in Plate 102

• intermingled in a bed of nonreactive glia
  † dense to sparse at cervical and thoracic levels,
  sparse at remaining levels
  ** some unmyelinated fibers at cervical levels
  ††at cervical levels: myelinating internally, reactive
  glia externally; at other levels: myelinated or myelinating