Spermatogonial Stem Cell Transplantation and Culture - Male Germline Stem Cells Developmental and Regenerative Potential - page 114

spermatogenesis (Shinohara et al. 1999 ; Kanatsu-Shinohara et al. 2008b ).
Additionally, because of the role of b1-integrin in homing of transplanted SSCs to
the recipient niche, it is possible that it may also have a role in the endogenous
homing of gonocytes to the basement membrane during testis development. It will
be interesting to learn if b1-integrin is important for function of stem cells in other
tissues, because of its wide distribution, or play a unique role in spermatogenesis.
The mechanism of SSC homing is of great importance and the above experiments
provide guidance in understanding this phenomenon, which has proven difficult to
study and quantify in all stem cell systems.

5.2.5.3 Recipient Preparation and Injection

Successful SSC transplantation is directly dependent on the recipient's ability to
initiate and maintain donor-derived spermatogenesis. The best recipients are those
that are able to maintain spermatogenesis and are devoid of endogenous spermato-
genesis. Thus, efficient recipient preparation is essential for successful donor-
derived spermatogenesis (Brinster et al. 2003 ). Recipients can either be naturally
sterile or generated by experimental means. The most useful natural mutant for
SSC transplantation is the W mouse (Brinster and Zimmermann 1994 ). These mice
have defects of various severities in the c-kit receptor. The interaction between the
c-kit receptor and its cognate ligand kit (stem cell factor) is essential for migration
of the PGC to the genital ridge and later, differentiation of spermatogonia in the
adult testis. Even though the testes of W animals contain a greatly reduced number
of SSCs, they serve as excellent recipients for SSC transplantation because the
endogenous germ cells cannot differentiate. Treatment with busulfan and irradia-
tion are two experimental means to destroy endogenous spermatogenesis (Brinster
and Avarbock 1994 ). Busulfan is a cytotoxic drug used for chemotherapy that
destroys SSCs. Irradiation also selectively destroys actively dividing cells and is
useful for non-rodent species in which the level of busulfan needed to eliminate
spermatogenesis is toxic (Withers et al. 1974 ; Meistrich et al. 1978 ; Van Beek et al.
1990 ; Zhang et al. 2006 ). Neither busulfan nor irradiation completely eliminates
endogenous spermatogenesis, so transplantation of labeled donor cells is still
desirable in order to readily differentiate donor-derived from endogenous
spermatogenesis.
The optimal route of cell injection varies among species, but include microinjec-
tion of cells into efferent ducts, the rete testis, or seminiferous tubules (Ogawa et al.
1997 ) (Fig. 5.3 ). The efferent ducts connect the rete testis to the caput epididymis.
In rodents the efferent ducts are outside of the testis and are easily injected using a
microinjector. However, in non-rodent species, the efferent ducts are not readily
available for injection. In addition, the rete testis in non-rodent species lies inside
the testis and consist of many interconnected tubules. Thus, the most efficient
means of cell injection in this instance is to use ultrasound guided injection of the
rete testis (Honaramooz et al. 2002 ; Schlatt et al. 1999 ). Alternatively, cells can be
directly injected into the seminiferous tubules. However, direct injection of cells