Biomedical Engineering Reference
In-Depth Information
busulfan, cyclophosphamide, chlorambucil, and melphalan), and cisplatin, also
produce prolonged azoospermia (Meistrich et al.
2005
). Others, particularly topoi-
somerase inhibitors (e.g., amsacrine), antimetabolites (e.g., methotrexate), and
microtubule inhibitors do not produce prolonged azoospermia in most combinations,
but doxorubicin, microtubule inhibitors (e.g., vincristine), and select antimetabolites
(e.g., cytosine arabinoside) can have additive effects on azoospermia induction when
given with the highly gonadotoxic agents listed above (Meistrich et al.
1989
).
9.4.3
Functional Assay of Stem Cells by Transplantation
The survival of stem cells after gonadotoxic insult has also been assessed by sper-
matogonial transplantation. In mice, busulfan at 15 mg/kg reduced functional stem
cell numbers to a minimum of about 4% of control at 3 days after treatment
(Kanatsu-Shinohara et al.
2003
). This agrees with the functional assessment of the
stem cells left in situ, based on subsequent sperm head counts, which were reduced
to 8% of control (Bucci and Meistrich
1987
).
This assay was also used to demonstrate that the spermatogonia remaining in
6-Gy-irradiated testes of rats, which did not produce differentiated cells in the irra-
diated rat testis, were functional when transplanted to a nude mouse testes depleted
of endogenous germ cells (Zhang et al.
2006
).
The killing of functional spermatogonial stem cells in busulfan-treated macaques
was assessed by the reduction of their ability to form colonies after transplantation
into depleted nude mouse testes (Hermann et al.
2007
). However, the sensitivity of
this assay may be limited since one of the donor monkeys showed endogenous
recovery of spermatogenesis in about half of the tubules, but the spermatogonia did
not form colonies after transplantation.
9.5
Recovery of Spermatogenesis from Surviving
Stem Spermatogonia
The eventual recovery of sperm production depends on the survival of the sper-
matogonial stem cells, the regeneration of their numbers, and their ability to dif-
ferentiate (Table
9.2
).
In the mouse, after irradiation stem spermatogonia (A
s
) initially divide with
about a 2-day cell cycle time and a high probability of self-renewal, thereby
increasing their numbers, but they also begin to produce differentiating A
pr
and A
al
spermatogonia within the first week after irradiation (Van Beek et al.
1986, 1990
).
Differentiation continues and progressive recovery of sperm production begins at
5 weeks (Meistrich and Samuels
1985
). It is puzzling that the numbers of sper-
matogonial stem cells, assayed either by endogenous colonization ability (Meistrich
et al.
1978
) or counts of A
s
spermatogonia (Erickson and Hall
1983
), does not begin
continuous progressive recovery until about 8 weeks after 6 Gy irradiation.
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