Spermatogonial stem cells (SSCs) are stem cells of the male germ line and support
spermatogenesis for a lifetime after puberty by continuously self-renewing and generating
committed progenitors. Accordingly, SSCs are defined functionally by their ability to
regenerate and maintain spermatogenesis and are detected unequivocally based on their
regenerative capacity. Here, we summarize past achievements of morphological and
functional studies of SSCs and discuss issues to be addressed in future investigations …