In mammals, homeostasis relies on stem cells replenishing tissue lineages with differentiating cells that are continually lost due to cytotoxic injury and terminal differentiation. In embryonic and neonatal development these stem cells are also tasked with establishing tissue lineages while setting aside a self-renewing population that will remain undifferentiated and be sustained throughout life. To carry out their functions, stem cells possess the capacity for both self-renewal to maintain a pool of stem cells and generation of progenitor cells that are set on a pathway of differentiation. These fate decisions are tightly regulated by influences from microenvironments referred to as ‘niches’. In general, stem cell niches are themselves tissue-specific being composed of a growth factor milieu and architectural support that are dictated by resident support cells. The possible immortal nature of stem cells provides a potential avenue for regenerative medicine to treat a variety of degenerative diseases caused by loss of tissue homeostasis. Achieving this goal relies on the deciphering of molecular mechanisms within stem cells that control fate decisions and defining the components that constitute niche microenvironments.
Spermatogenesis is a classic model of tissue-specific stem cell biology relying on the activity of spermatogonial stem cells and support from their cognate niche that is provided by contributions from testis somatic cell populations. Also, spermatogenesis is essential for the continuity of a species, contributes to genetic diversity, and determines sex ratios in most mammalian populations. Reduction in or loss of spermatogonial stem cell function disrupts spermatogenesis leading to sub-fertility or infertility in males. In addition, because spermatogonial stem cells are the only cells in the body that self-renew and contribute genes to the next generation, they provide an avenue to alter genes within a male’s germline. Aside from medical implications in humans, preservation of genetic lines of endangered species and expanded use of gametes from valuable food or companion animals represents a potential application of spermatogonial stem cell populations utilizing their capacity for regeneration of male germlines upon transplantation. Furthermore, information gained from studying spermatogonial stem cells may be applicable to other tissue-specific stem cell populations.
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