Spermatogonial Stem Cell Transplantation and Culture - Male Germline Stem Cells Developmental and Regenerative Potential - page 108

5.1 Introduction

There are no known molecular or phenotypic markers that definitively distinguish
a spermatogonial stem cell (SSC) from its daughter cells committed to differentiate
into sperm. Thus, it is impossible to state that an individual spermatogenic cell is a
SSC. Because the SSCs lack definitive markers, the study of this important cell has
been limited. Furthermore, lack of an in vitro culture system and a reliable assay to
quantify SSC activity exacerbated the inability to conduct reliable experiments
examining the SSC. The absence of these techniques was further hampered by the
extreme rarity of the SSC in the adult mouse testis, which was estimated to be present
at a concentration of 1 in 3,000 testis cells (Tegelenbosch and de Rooij 1993 ).
However, over the last 15 years our laboratory and others have developed and
continuously refined techniques to quantify SSC activity (SSC transplantation) and
to maintain the SSC in vitro (SSC culture), and when combined, SSC transplanta-
tion and culture provide a powerful tool to identify mechanisms regulating SSC
function. The development of these techniques has allowed for many important
discoveries pertaining to the male germline, and translation of these techniques to
other species will open many doors for novel methods of reproductive management
and are currently the next frontier in the study of SSC biology.

5.2 Spermatogonial Stem Cell Transplantation

5.2.1 History

The existence of a germline stem cell population that resided in the testis and that
was responsible for continued fertility throughout the life of a male was first postu-
lated by Huckins and Clermont in 1968 . However, direct study of the properties of
the spermatogonial stem cell was not feasible until a functional transplantation assay
was developed in 1994. In this work, Brinster and Zimmermann ( 1994 ) and Brinster
and Avarbock ( 1994 ) demonstrated that when placed in the seminiferous tubules of
infertile recipients, donor spermatogonial stem cells were able to migrate from the
lumen through the tight junctions to the basement membrane and initiate complete
donor cell-derived spermatogenesis. Furthermore, donor spermatozoa were fully
functional and could generate normal offspring through natural mating (Fig. 5.1 ).

5.2.2 Implications

In addition to the basic study of male reproduction and stem cell biology, the impli-
cations for SSC transplantation are far-reaching. From a clinical standpoint, the
future extension of SSC transplantation to humans would allow for the preservation