14 19 26 37 NB 4 16
5
10
15
20
25
30
35
5
10
15
20
25
30
35
WHITE MATTER
GRAY MATTER
Area (mm
2
)
BIRTH
Age (Gestational Weeks) Age (Postnatal Weeks)
Area (mm
2
)
Alpha
MotoneuronsVentral Horn
InterneuronsIntermediate
GrayDorsal Horn
Layers I,IV,VDorsal Horn
Layers II,IIICentral Auto-
nomic AreaNAME GW4.0-4.5 GW4.75-5.25 GW5.5-6.8 GW7.25-8.5 GW10.5 on
Stem cells
proliferateStem cells
proliferateStem cells
proliferateStem cells
proliferateStem cells
dormant?Stem cells
dormant?Stem cells
proliferateStem cells
proliferateMost neurons
generatedNeurons
generatedNeurons
generatedNeurons
generatedNeurons
generatedMigration
and settlingMigration
and settlingMigration
and settlingMigration
and settlingMigration
and settlingMaturationMaturationMaturationMaturationMaturationMigration
First neurons and settling Segregation into columns and maturation
generatedNeurons
generatedTable VII–1: Developmental Events in Neuronal Populations during the First TrimesterFigure 33. The area of the gray matter (dark gray) and white matter (light gray) at the cervical enlargement from the beginning of the second trimester
(GW14), birth (NB-newborn), to 16 weeks after birth. From GW26 on, data of the myelin stained section area is used. The area graphs are stacked so
that the scale on the Y axis indicates the total area of the spinal cord. These areas are measured in fi xed tissue, so the actual values in the living spinal
cord will be higher by an unknown amount. The areas of the ependyma and spinal canal are so small that they do not register above the baseline. Since
neurogenesis fi nishes in the fi rst trimester (see Figure 32), the gradual increase in the area of the gray matter throughout this period is due to the growth
and differentiation of the neurons themselves. The white matter increases more rapidly, especially between GW19 and GW26 (compare Plates 40 and
47 ). There is gradual growth from GW 26 to the perinatal period (between GW37 and postnatal week 4), then more rapid growth between postnatal
weeks 4 and 16 (compare Plates 95 and 106 ). The fi rst growth spurt is probably due to the accumulation of axons in all of the major fi ber tracts, including
both parts of the corticospinal tract. The second growth spurt is probably due to the completion of myelination throughout the cervical enlargement at
the 4th postnatal month. Only the outermost crescent of the corticospinal tract is still myelinating at this time (see Plate 106). From GW26 on, the area
of the white matter exceeds the area of the gray matter.
Changes in Area from GW14 to the 4th Postnatal Month
Table VII-1. The major events that take place in neuronal populations in the cervical spinal cord during the fi rst trimester. Dark green indicates the fi rst
step in development when the stem cells rapidly proliferate just prior to neurogenesis. Light green indicates the approximate period of neurogenesis.
Pink indicates the time during which neurons migrate away from the neuroepithelium and settle in the gray matter. Orange indicates the period of
maturation, which continues throughout the second and third trimesters and well after birth. Light brown indicates the postulated time when the stem
cells of neuronal populations that are generated late are either dormant or have not yet appeared. The time periods for the various events in each popula-
tion are based on the data in Figure 32 and the qualitative changes visible in Plates 2 through 12.
TABLE VII-1 and FIGURE 33