Regenerative Potential of Spermatogenesis after Gonadotoxic Therapies - Male Germline Stem Cells Developmental and Regenerative Potential - page 184

(Erickson 1976, 1981 ). However, these numbers are an underestimate of killing of
A s spermatogonia because in the mice the surviving stem cells undergo about three
self-renewing divisions within this time (Van Beek et al. 1986 ; Van Beek and
Meistrich 1990 ). After 6 Gy, there is also a subsequent progressive loss of A s sper-
matogonia in both rodent species to roughly 10% of control at 8 weeks after irradia-
tion (Erickson and Hall 1983 ). The functional significance of this decline is not
known because, at least in the mouse, sperm production shows progressive recovery
during this time period (Meistrich et al. 1978 ).
In monkeys, after radiation doses in the 1-4 Gy range, there is a rapid decline in
the numbers of A pale spermatogonia to about 20% within 10 days and then to about
2% of control at 6 weeks, whereas the numbers of A dark spermatogonia are unaf-
fected for 10 days but then decline to about 3% of control at 6 weeks (van Alphen
et al. 1988a ). In human, the decline in A dark spermatogonia after irradiation is also
gradual, reaching about 7 or 1% of control at about 26 weeks after 1 or 6 Gy irradia-
tion, respectively (Clifton and Bremner 1983 ; Meistrich and van Beek 1990 ;
Rowley et al. 1974 ). The numbers of A pale spermatogonia also decline but there is
disagreement as to whether their numbers parallel the gradual A dark decline or
rapidly decline (Meistrich and van Beek 1990 ; Paulsen 1973 ; Rowley et al. 1974 ).

9.4.2 Assessing Stem Cell Survival by Recovery Potential

The reduction in the percentage of tubules containing differentiated germ cells
derived from stem cells (repopulation index) has been used to quantify the killing
of stem spermatogonia by irradiation in mice (de Ruiter-Bootsma et al. 1976 ; Lu
and Meistrich 1979 ). This assay demonstrated the sensitivity of mouse spermatogo-
nial stem cells to some chemotherapeutic drugs, such as certain alkylating agents
(busulfan and thio-TEPA), and doxorubicin, but not other drugs (Bucci et al. 1985 ;
Lu et al. 1980 ; Meistrich 1986a ; Meistrich et al. 1982 ) (Table 9.1 ). Killing of stem
spermatogonia by single doses of procarbazine and chlorambucil could not be dem-
onstrated by this assay; however, there was an indication of stem cell killing as
assessed by reduced sperm counts, and multiple doses of these drugs clearly
resulted in stem cell killing as demonstrated by reduced repopulation indices (ML
Meistrich, unpublished data). Other chemotherapeutic agents, such as some alkylating
agents (e.g., cyclophosphamide), cisplatin, antimetabolites, microtubule inhibitors,
and topoisomerase inhibitors (e.g., amsacrine, Table 9.1 ) (Da Cunha et al. 1985 ),
were not toxic to mouse stem cells.
The effects of these agents on stem cell survival have also been studied in the rat
by assessment of the ability of spermatogenesis to recover at 8-11 weeks after
treatment. Complete recovery indicates that few, if any stem cells were killed, but
the absence of recovery may indicate stem cell killing but may, in part, result from
damage to the somatic environment preventing recovery of spermatogenesis from
the surviving stem cells (Zhang et al. 2006 ). Similar to the mouse, radiation, pro-
carbazine, doxorubicin, and chlorambucil (Delic et al. 1986a, b, 1987 ; Lui et al. 1986 )