front matter 1

V. The Early Postnatal Period:

Infants at 4 days, 4 weeks, and 4 months

Plate 79 is a survey of matched myelin stained and

cell body stained sections from Y299-62, a specimen in the

Yakovlev Collection with a crown rump length of 350 mm.

All sections are shown at the same scale. The boxes

enclosing each section list the approximate level and the

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

page normal contrast photographs of each specimen are in

Plates 80A-91A. Low contrast photographs with superim-

posed labels and outlines of structural details are in Plates

80B-91B. In this specimen, the myelin stained and cell

body stained sections were preserved on separate large

glass plates without any section numbers. Twenty-two

myelin stained sections and 22 cell body stained sections

were photographed ranging from the cervical enlargement

to sacral/coccygeal levels. The 44 photographic prints were

intuitively 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 six different levels were analyzed. Since many sections

in the middle thoracic region are damaged, that level is not

illustrated.

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. Using the total

areas of the myelin stained sections, the overall size differ-

ences between levels indicate the following comparisons:

the cervical enlargement is the level with the largest cross-

sectional area, being larger than the lumbar enlargement by

65%. The sacral/coccygeal level has the smallest cross-

sectional area and is 37% smaller than the lower thoracic

level.

Myelination in this specimen is slightly advanced

compared to the GW37 specimen (columns 2 and 3, Table

V A-1) because there is no longer a gradient between

superfi cial and deep parts of the gracile and cuneate fas-

ciculi, and more myelinated axons are in the subgelatino-

sal plexus. The ventral corticospinal tract is smaller in this

specimen than the one at GW37. For a discussion of indi-

vidual differences between specimens regarding the cor-

ticospinal tracts, see Chapter 7, Section 7.2.2 in Altman

and Bayer, 2001). The corticospinal tracts in the 4-day-

old infant indicate more advanced myelination than in the

GW37 fetus by having at least a sparse concentration of

reactive glia and a high concentration of proliferating glia

(columns 3 and 4, Table V A-1). Another indication of

maturation is the sparse (medial) to very sparse (lateral)

gradient of reactive glia in the lateral corticospinal tract.

Myelination follows a medial to lateral gradient in that

tract (Altman and Bayer, 2001). There is an overall gra-

dient of sparse reactive glia (superfi cial fi bers in the ves-

tibulospinal and spinocephalic tracts) to the beginnings of

myelination (deep fi bers in the intraspinal tracts) in the

ventral and lateral funiculi that cuts across the borders of

the fi ber tracts.

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

4-day-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 tracts Many fibers --- Dense

spinocephalic tracts Some fibers†Very sparse• Dense

lat. reticulospinal tract Some fibersVery sparse Sparse

spinocerebellar tracts Myelinated --- Sparse

med. long. fasiculus Some fibers --- Sparse

vestibulospinal tract Some fibers --- Sparse

dorsal root bif. zone Many fibers* Sparse

---

Sparse**

---

---

---

ven. commissure Myelinated --- Dense

Lissauer's tract --- None Sparse

lat. cortricospinal tract --- Very sparse Sparse

ven. cortricospinal tract --- Very sparse Sparse

Table V A-1: Glia types and concentration

in the white matter in a 4-day-old Infant

** dense band in a sparse field (part of the boundary cap?)

* intermingled in a bed of nonreactive glia

† few fibers at the cervical level

• peripheral fibers at the cervical level