Investigation of factors regulating spermatogonial stem cell homeostasis in the mammalian testis

Spermatogonial stem cells (SSCs) are the only adult stem cell population capable of transmitting genetics to offspring and accomplish this feat through the production of spermatozoa. Indeed, spermatogenesis yields a virtually unlimited supply of gametes throughout the lifespan of the male as a result of coordinated SSC differentiation and self-renewal. In essence, SSCs are the keepers of the male germ-line and this population originates from primitive germ cell lineages during neonatal life in a species-specific manner.;SSCs, like other adult stem cells, are thought to reside in a niche microenvironment that must be established and maintained in the seminiferous tubules of the neonatal and adult testis. Extrinsic signals produced by somatic cells within the niche act to direct the cell 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 homeostasis and identity due to the lack of SSCs specific biomarkers and complex cell biology of the testis. These challenges underscore the importance of functional stem cell transplantation for investigating the role of endocrine and locally produced factors that regulate the biological activity of SSCs. Moreover, donor-derived spermatogenesis after transplantation is the only reliable and accurate means to identify a bonafide SSC in a cell population and assay SSC self-renewal in response in vivo treatments.;This dissertation provides insight regarding the in vivo regulation of SSC homeostasis in the neonatal and adult testis, highlighted by functional studies to determine how a complex network of growth factors and environmental exposure to toxic substances may govern the cell fate of SSCs. Additionally, the advent and implementation of a less abrasive SSC isolation protocol (Ch. 3,4) increases the power of the transplantation assay for use in basic and applied studies. This methodology, combined with our ability to obtain stage-specific, seminiferous tubule microenvironments (Ch. 3) provide a novel means to investigate the mechanisms of SSC proliferation, differentiation, survival and self-renewal in mammals. Collectively, the applications of our findings have tremendous potential for increasing agricultural productivity and biomedical progress.