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Neural tube
A
Marginal
zone
Mantle
zone
Ependymal
zone
B
G1
S
G2
M
G1
FIGURE 3.1 The process of neurogenesis in the neural tube. Section through the neural tube soon after
it has formed shows a section through the chick embryo neural tube to the left and a schematic version of
the progenitor cells to the right. The progenitor cells take up BrdU in the central region of the neural tube,
indicating that they are in S-phase at this location. The cells then undergo mitosis at the ventricular surface,
and this can be visualized with an antibody against a phosphorylated form of histoneH3, which is present
only in late G2- and M-phases of the cell cycle. After the cells have withdrawn from the cell cycle and begun
their differentiation as neurons, they migrate radially from the ventricular zone to the mantle zone, and these
can now be visualized with an antibody against neurofilament protein. The movements of the progenitor
cells are diagrammed to the left. The progenitor cells span the thickness of the neural tube, and their nuclei
translocate to the mantle zone during the S-phase of the cell cycle. The nuclei return to the ventricular surface
during G2 and the M-phase of the cell cycle always occurs at the ventricular surface. (A is courtesy of Branden
Nelson)
system. The German developmental biologist Wilhelm
His (an early proponent of the theory that the nervous
system is composed of individual cells) divided the
layers of the neural tube into the ependymal zone
(near the ventricle), the mantle zone, and the marginal
zone (containing postmitotic neurons). The ependymal
zone, the innermost zone of the tube, is where the
mitotic figures are located, and His thought these were
actually different cells from the more elongate mantle
cells (see Jacobson, 1991). Through the work of Sauer
and others, we know that the nuclei of the progenitor
cells undergo a constant up-and-down migration
through the stages of the cell cycle, from the ventricu-
lar surface during M-phase to the mantle zone during
S-phase (Figure 3.1). This phenomenon is known as
interkinetic nuclear migration .
One of the clearest demonstrations of the constant
motion of the nuclei of these cells came from the use of
3H-thymidine to label cells in the S-phase of the cell
cycle during the active phase of DNA replication.
Figure 3.2 diagrams this type of experiment in a sec-
tion through the neural tube. If an injection of 3H-
thymidine is made into an embryo and the tissue is
removed within an hour, the labeled cells are all found
in the outer part of the ventricular zone, away from the
ventricle. If the embryo is allowed to survive for 4
hours after the injection of thymidine, the labeled cells
are all at the ventricular surface undergoing M-phase
and can be seen as metaphase nuclei. If the embryo is
allowed to survive for 8 hours, the labeled cells are in
the outer half of the ventricular zone, and by 12 hours
they are back at the ventricular surface. The thymidine
labeling also shows a progressive increase in the
number of labeled cells, as the cells divide. However,
because the thymidine was only available for incorpo-
ration into cells for the first hour after injection (before
it is cleared from the circulation), the cells labeled in
subsequent divisions progressively dilute their label
and appear more lightly labeled with each successive
division. Although the function of interkinetic nuclear
migration is unknown, it may be necessary for the pro-
genitor cells to receive specific signals at different times
in the cell cycle. Modern terminology now combines
the ependymal and mantle zones into the term ventric-
ular zone , the layer of the neural tube closest to the ven-
tricle, where cells are in the cell cycle (Sauer, 1935).
 
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