Maintaining the male germline: regulation of spermatogonial stem cells.

Spermatogonial stem cells (SSCs) are a self-renewing population of adult stem cells capable of producing progeny cells for sperm production throughout the life of the male. Regulation of the SSC population includes establishment and maintenance of a niche microenvironment in the seminiferous tubules of the testis. Signaling from somatic cells within the niche determines the fate of SSCs by either supporting self-renewal or initiating differentiation leading to meiotic entry and production of spermatozoa. Despite the importance of these processes, little is known about the biochemical and cellular mechanisms that govern SSC fate and identity. This review discusses research findings regarding systemic, endocrine, and local cues that stimulate somatic niche cells to produce factors that contribute to the homeostasis of SSCs in mammals. In addition to their importance for male fertility, SSCs represent a model for the investigation of adult stem cells because they can be maintained in culture, and the presence, proliferation, or loss of SSCs in a cell population can be determined with the use of a transplantation assay. Defining the mechanisms that regulate the self-renewal and differentiation of SSCs will fundamentally improve the understanding of male fertility and provide information about the regulation of adult stem cells in other tissues.