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Neuroepithelium

Age in Gestational Weeks (GW)

Area (mm

2

)

Area (mm

2

)

Area (mm

2

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Area (mm

2

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Area (mm

2

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Area (mm

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Area (mm

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Area (mm

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Age in Gestational Weeks (GW)

Spinal Canal

Gray Matter

White Matter

A

B

C

D

1 23 4

5

6

7

8

9

(^1234)
5
6
7
8
9
1
2
3 4
5 6
7
8
9
1
2
3
4
5
6
7
8
9
Figure 32. The total area of the neuroepithelium (A), spinal canal
(B), gray matter (C), and white matter (D) at cervical levels during
the fi rst trimester. The italicized numbers in each graph indicate
data points. The fi rst six data points in each graph are from Minot
specimens only because the same fi xative was used and there is a
more constant cutting plane between specimens. Thus, quantita-
tive data more reliably indicate developmental trends.
The red line marks the peak area of the neuroepithelium (A) and
serves as a dividing line. First consider what happens before the
peak. Between GW4 and GW4.5 (data points 1 and 2, A), the
neuroepithelium expands, especially its ventral part (see Plates 2
and 3 ). Neuroepithelial area reaches a plateau (single red arrow,
A) between GW4.75 and GW5.25 (data points 3 and 4, A). That
plateau occurs at the time when the area of the ventral neuroepi-
thelium remains stable during the production of the large moto-
neurons in the ventral horn (see Plates 4 and 5 ). The most rapid
growth spurt in the neuroepithelium occurs between GW5.25 and
GW5.5 (data points 4 and 5, A) when the spinal canal reaches
its peak area (red arrow, B) as the dorsal neuroepithelium rap-
idly expands about the dorsal part of the spinal canal. There is
a simultaneous growth spurt in the area of the gray matter (C) as
neurons accumulate in the ventral horn and in the area of the white
matter (D) as axons accumulate in the ventral funiculus, ventral
commissure, and dorsal root bifurcation zone of the dorsal funicu-
lus. These growth spurts are qualitatively visible by looking at
the changing appearance of the spinal cord in Plates 5, 6, and 7.
After GW5.5, the area of the neuroepithelium grows more slowly
to reach a peak at GW6.8 (data point 6, A). That slow growth
is due to concurrent shrinking of the ventral neuroepithelium and
expansion of the dorsal neuroepithelium (see Plates 8-10).
Now consider what happens after the peak. The neuroepithelial
area gradually declines by GW7.25 (data point 7, A), then plum-
mets by GW8.5 (data point 8, A). During the rapid decline
(double red arrows, A; see Plates 11 and 12 ) the dorsal neuro-
epithelium shrinks dramatically as late-generated neurons migrate
into the dorsal horn and the dorsal and ventral parts of the spinal
canal disappear to leave only a central canal (B). During the
decline in the neuroepithelium, the gray matter (C) and white
matter (D) continue to increase. However, throughout the entire
fi rst trimester, the area of the gray matter is greater than the area
of the white matter.
FIGURE 32 Changes in Area During the First Trimester