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Pial surface
A
Ventricular surface
B
E 10
E 13
E 17
0
4
8
12
G 1
Synthesis
G 2
Mitosis
FIGURE 3.3 The overall length of the progenitor cell cycle
increases during embryogenesis. The cell cycles of progenitor cells
from the mouse cerebral cortex are plotted as circles of increasing
size from E10 to E17. The increase in the cell-cycle period is largely
due to an increase in the G1 phase, which nearly triples in length
(shown in red).
Time (hours)
FIGURE 3.2 Thymidine labeling reveals the cell-cycle length of
neural progenitor cells. A. After an injection of 3H-thymidine into a
developing embryo, the S-phase cells incorporate the label into their
DNA (red). When the labeled cells proceed to M-phase, they are rec-
ognizable as mitotic figures. A plot of the number of labeled mitotic
figures (vertical axis) against the time after the thymidine injection
shows that their number increases as the S-phase cells reach M-
phase. As these cells complete M-phase and proceed into G1 and the
next S-phase, they are no longer counted as mitotic figures, and their
number drops. As the first cells finish their second S-phase and
reenter mitosis, the number of labeled mitotic figures once again
increases and we see a second peak. B. The length of time between
the first and second peak is the time taken for the labeled cells to go
from one mitosis to another, the cell-cycle length.
their number will continue to increase as a greater pro-
portion of the S-phase cells reach M-phase (Figure 3.2).
However, as these cells complete M-phase and proceed
into G1 and the next S-phase, they are no longer
counted as mitotic figures and their number drops. As
the first cells finish their second S-phase and reenter
mitosis, the number of labeled mitotic figures once
again increases and we see a second peak. The length
of time between the first and second peak is therefore
the time taken for the cohort of labeled cells to go from
one mitosis to another, the average cell cycle length.
In the vertebrate CNS, these types of experiments
have been carried out for many regions of the brain,
and again, some general principles emerge. First, the
overall length of the cell cycle increases progressively
during embryogenesis. Progenitor cells from the chick
optic tectum, for example, have an overall cell cycle
time of 8 hours on embryonic day 3, but this increases
to 15 hours by embryonic day 6. A similar increase
in cell-cycle period occurs in the mammal, as rat cor-
tical progenitor cells increase their cell-cycle time
from 11 hours on embryonic day 12 to 19 hours at
embryonic day 18. The second generality that can
be made is that the increase in the cell-cycle period is
largely due to increases in the G1 phase. As shown in
Figure 3.3, the M and G2 phases of the cell cycle change
little from embryonic day 10 to embryonic day 19 in
mouse cerebral cortex progenitor cells; however, the
The cells of the ventricular zone are the precursors
of the differentiated neurons and glia of the central
nervous system. These cells undergo from one to
two cell cycles per day. In the early neural tube, many
of the cells undergo symmetric cell divisions, pro-
ducing two progenitor cells as daughters; however,
some of the divisions produce asymmetric daughters:
one daughter continues to divide and the other
becomes a postmitotic neuron. In the spinal cord and
in most other areas of the developing neural tube, the
postmitotic neurons migrate from the ventricular zone
to the marginal zone, where they continue their
differentiation.
Thymidine labeling also allows one to determine
the cell cycle length of a population of mitotically
active cells. Since the thymidine is cleared from the cir-
culation of mammalian embryos within one hour after
an injection, a relatively small cohort of S-phase cells
incorporate the label into their DNA. As described
above, these cells then proceed through G2, and in M-
phase they are recognizable as mitotic figures. If one
counts the number of the mitotic figures at progres-
sively longer intervals after the thymidine injection,
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