Biomedical Engineering Reference
In-Depth Information
Recently, it became clear that there is an additional level of organization in the
seminiferous epithelium. The A
s,pr,al
spermatogonia appeared to be preferentially
localized in those parts of the basal membrane near the stretches of interstitial
tissue and or blood vessels (Chiarini-Garcia et al.
2001, 2003
; Yoshida et al.
2007
;
de Rooij
2009
). Such an area of preferred localization is called a niche and is
found in many other tissues too. The data discussed in this chapter do not directly
relate to the presence of niches and the niche will be discussed in detail elsewhere
in this topic.
4.5
Numbers of A
s,pr,al
Spermatogonia During
the Epithelial Cycle
Detailed counts of A
s,pr,al
spermatogonia throughout (most of) the epithelial cycle
have been performed in rats, mice, Chinese hamsters, and the ram (de Rooij
1973
;
Huckins
1971c
; Lok et al.
1982
; Oakberg
1971
; Tegelenbosch and de Rooij
1993
). In some strains of mice, the density of the A
s
, A
pr
, and A
al
spermatogonial
clones is too high to tell the clones apart in all instances (Tegelenbosch and de
Rooij
1993
). In such cases, the clones are close together and the differences in
cell cycle phase and, with that, the morphology of the cells in the neighboring
clones, are not always large enough to unequivocally tell the clones apart and to
determine clonal sizes.
The cell count results always show that the numbers of A
s
and of A
pr
sper-
matogonia do not fluctuate very much during the epithelial cycle while there is
a steep increase in the numbers of A
al
spermatogonia from stage X to about stage
IV, after which the increase diminishes (Fig.
4.4
). What happens is that the A
s
spermatogonia proliferate and form clones of A
pr
spermatogonia as well as
renew themselves in such a way that the A
s
spermatogonial numbers remain
more or less similar. At the same time, while new A
pr
spermatogonia are formed
by stem cells, already existing A
pr
spermatogonia divide into chains of four A
al
spermatogonia. The balance between the loss of A
pr
spermatogonia because of
the formation of A
al
spermatogonia and the replenishment of A
pr
spermatogonia
by differentiating stem cell divisions is such that the numbers of A
pr
spermatogo-
nia remain about constant. Finally, a steep increase in A
al
spermatogonial num-
bers is observed because new chains of four keep being formed by A
pr
spermatogonia while already existing chains of four and eight A
al
spermatogonia
divide on to become longer chains. However, chains of 32 A
al
spermatogonia are
very rare. Apparently, chains of 16 are less likely to proliferate any further.
A complete quantification of the spermatogonial compartment has been carried
out in C3H/101 F1 hybrid mice. In this strain of mice, there are about 35,000 stem
cells per testis and 1.3 and 10.6%, respectively, of all spermatogonia and A
s,pr,al
spermatogonia in the testis are stem cells (Tegelenbosch and de Rooij
1993
).
However, these numbers vary with species and strains. For example, in mouse
strains the total numbers of A
s,pr,al
spermatogonia in epithelial stage VIII varies from
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