Defining the phenotype and function of mammalian spermatogonial stem cells

Spermatogonial stem cells (SSCs) are a subpopulation of undifferentiated spermatogonia that maintain spermatogenesis throughout adult life and are essential for male fertility. At each cell division, an SSC produces daughter cells that will either self-renew to produce more SSCs or initiate differentiation to ultimately produce spermatozoa. Consequently, fertility throughout the mammalian male lifespan depends on formation of a foundational SSC pool and then balanced SSC self-renewal and differentiation once steady-state spermatogenesis is achieved. Fundamental studies of SSCs, however, are complicated by their extraordinary rarity in the adult testis (0.01%) and lack of definitive molecular markers that have allowed their prospective identification at any stage of testis development in any species. Despite these challenges, powerful experimental strategies such as transplantation and lineage tracing, which provide retrospective stem cell assessments, have revealed considerable phenotypic information about SSCs over the past two decades. This chapter provides an overview of the key phenotypic and functional characteristics of SSCs, the relative value of differing assessment methods, and the best-substantiated markers of SSCs. Particular emphasis will be placed on emerging technologies, such as single-cell molecular profiling and the use of ID4 reporters, which are facilitating the first prospective, comprehensive molecular characterizations of SSCs that will transform our understanding of the underlying regulatory framework controlling their function.