A
B
C
D
E
F
1 mm1 mm
1 mm1 mm
1 mm1 mm
1 mm1 mm
1 mm1 mm
0.5 mm0.5 mm
CR 8.0 mm
CR 10.5 mm
CR 19.1 mm
CR 22.0 mm
CR 36.0 mm
CR 56.0 mm
VF
FP
VC
DF
RP
CCCC
DN
IN
VNVN
DHDH
IG
VH
GM
LF
*
***
*
**
*
**
* **
*Figure 9. The emergence and growth of the white matter (see Table VI A– 2 for color code) in the following specimens: CR 8 mm (A), CR 10.5 mm
(B), CR 19.1 mm (C), CR 22 mm (D), CR 36 mm (E), and CR 56 mm (F). Note scale differences between specimens. All parts of the gray matter and
neuroepithelium are visible in the models. Asterisks (A to D) indicate where the top of the roof plate (RP) is visible in the dorsal midline. The dorsal
funiculus (DF) and ventral funiculus (VF) appear simultaneously in A; the lateral funiculus (LF) is defi nite in C.
The fi rst axons in the ventral funiculus are most likely those of the early generated commissural neurons that accumulate in the space between the lateral
edges of the fl oor plate (FP), the most ventral part of the ventral neuroepithelium (VN), and the ventromedial edge of the ventral horn (VH). In some
sections of the model in A, the axons are even closer to the midline, so the ventral funiculus fi rst has a medial direction of growth (bottom arrows in
A pointing toward the midline). In B, these fi bers cross the midline in the ventral commissure (VC), and the ventral funiculus increases in size and fi lls
out laterally as well as medially. The ventral funiculus continues to enlarge throughout the remainder of spinal cord development and contains several
descending tracts (medial longitudinal fasciculus, tectospinal tract, and vestibulospinal tract) as well as some of the ascending spinocephalic tracts and
the collaterals of axons from neurons throughout the ventral and intermediate gray in the intraspinal tract (also known as the propriospinal tract).
The dorsal funiculus has a complex pattern of growth that features three different gradients. In A, the dorsal funiculus contains axons from large neurons
in the dorsal root ganglia that enter the spinal cord and bifurcate into ascending and descending branches. This is the dorsal root bifurcation zone (also
known as the oval bundle of His). That zone is more obvious at lower than upper cervical levels because the highest dorsal root ganglion is below the
highest cervical level of the spinal cord, and axons have not yet reached there. The forward-facing arrows in A and B indicate the advance of the dorsal
root bifurcation zone to upper cervical levels, which are fi lled with axons in C. Axons from the dorsal root continue to fi ll the bifurcation zone, and the
next gradient of growth is from ventrolateral to dorsomedial (arrows arching upward toward the midline in C and D). During this time, the bifurcat-
ing fi bers are growing collaterals (the dorsal root collateralization zone) that will eventually penetrate the gray matter. In E and F, the dorsal funiculus
reaches the midline and expands downward (downward facing arrows). These axons are segregated in the midline by a band of glia that extends to the
pia from the top of the roof plate. This medial part of the dorsal funiculus contains the long ascending branches of the dorsal root axons in the fasciculus
gracilis and fasciculus cuneatus (see Chapter 5, Figures 5-28 and 5-49 in Altman and Bayer, 2001).
FIGURE 9 Growth of the White Matter – GW5.25 to GW14