Biomedical Engineering Reference
In-Depth Information
they divide and rearrange their topology to become A-paired (A
pr
) and A-aligned (A
al
)
cells, they show molecular changes and have initiated their differentiation process
(Yoshida et al.
2007
). They further divide to become A
1
, A
2
, A
3
, and A
4
spermatogo-
nia, which most certainly lack stem cell potential. In contrast, in primates, there are
two categories of A spermatogonia, A-dark (A
dark
) and A-pale (A
pale
) (Clermont
1969
). The A
pale
are believed to be self-renewing stem cells, which normally give rise
to the differentiating cells, the B spermatogonia (Clermont
1966, 1969
). The A
dark
are
believed to be reserve stem cells, which are only active after the A
pale
are depleted, but
this model has not been definitively proven (Ehmcke et al.
2005b
).
Stem cell survival can also be inferred from the numbers of colonies containing
differentiated germ cells, which must have been derived from surviving stem cells
after treatment with cytotoxic therapies. Since germ cells are absent from many
tubule cross-sections, which contain only Sertoli cells, spermatogonial stem cell
survival in the mouse and rat has been assessed by the percentages of cross-sections
of seminiferous tubules showing differentiating spermatogenic cells 5-11 weeks
after cytotoxic therapies (Delic et al.
1987
; Kramer et al.
1974
; Meistrich and van
Beek
1993
; Withers et al.
1974
). This inference has been rigorously validated in the
mouse testis (de Ruiter-Bootsma et al.
1976
), but there are instances where it is not
valid in the rat. Surviving spermatogonial stem cells are unable to produce differ-
entiating cells or repopulate the tubules after irradiation of certain inbred strains
(Kangasniemi et al.
1996b
) or even after other cytotoxic treatments of outbred
Sprague-Dawley rats (Boekelheide and Hall
1991
).
In addition, the prolonged depression of sperm count when, following gonado-
toxic exposure, surviving stem cells should be producing sperm can be used as a
surrogate measure for stem cell killing in mice (Meistrich
1982, 1986a
). This
approach has application to studies in the human where sperm counts are much
more readily obtained than is testicular material.
Recently it has become possible to assess spermatogonial stem cell survival
directly by transplanting the cells (Brinster and Zimmermann
1994
; Nagano et al.
2001
) in a suspension taken from the testis of an animal treated with the gonadotoxic
agent into a recipient testis that lacks endogenous germ cells and counting colonies
of germ cells. Mutant mice lacking endogenous germ cells (Kanatsu-Shinohara et al.
2003
) or busulfan-treated mice (Zhang et al.
2003
) have been used as recipients to
assess stem cell survival in donor mice treated with gonadotoxic agents. Busulfan-
treated nude mice have been used as recipients to quantify the spermatogonial stem
cell survival in the donor testes from other species (Hermann et al.
2007
).
9.4
Stem Spermatogonial Survival-Results
9.4.1
Counts of Type A Spermatogonia
In rats and mice, A
s
spermatogonial numbers were not affected 7-8 days after 2 Gy
irradiation, but by that time the numbers fell to about 40% of control after 6 Gy
Search WWH ::
Custom Search