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will depend on
in vivo
studies because purification for GFRA1 positive cells will
render a mixture of original A
s
and of A
pr
spermatogonia split into single cells during
the purification procedure. However, this information will be necessary to enable a full
understanding of the regulation stem cell renewal and spermatogonial multiplication.
4.8
Proliferative Activity of A
s
, A
pr
, and A
al
Spermatogonia
During the Epithelial Cycle
The proliferative activity of the A
s,pr,al
spermatogonia is not constant during the
cycle of the seminiferous epithelium but follows a certain pattern. For the Chinese
hamster the
3
H-thymidine labeling index has been determined for A
s,pr,al
sper-
matogonia at multiple moments during the epithelial cycle (Lok and de Rooij
1983b
). The proliferative activity of these cells is lowest, but not zero, in stages
VII-IX (Fig.
4.5
). At about stage X, A
s,pr,al
spermatogonia start to proliferate, sug-
gesting that the stimulus for this to occur is similar for all three types of sper-
matogonia. Remarkably, the labeling index of both A
s
and A
pr
spermatogonia does
not get higher than about 10%, while this parameter reaches almost 25% for A
al
spermatogonia. Knowing the cell cycle times and the
3
H-thymidine labeling index
of the cells, it is possible to calculate the growth fraction of the A
s,pr,al
spermatogo-
nia. In their period of active proliferation, during epithelial stages X to early VII,
on the average about 55% of the A
s
spermatogonia are in active cell cycle. For the
A
pr
and A
al
spermatogonia, during stage X to stage IV the growth fraction data are
66 and 80%, respectively (Lok and de Rooij
1983b
). The proliferative activity of
Fig. 4.5
3
H-thymidine labeling index of A
s
, A
pr
, and A
al
spermatogonia throughout the stages of
the cycle of the seminiferous epithelium in the Chinese hamster. Data are from Lok et al. (
1984
).
eVII, mVII - early and mid stage VII, respectively; l VII - late stage VII
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