ABSTRACT
In healthy men, millions of sperm are produced daily in the testis through a process known as spermatogenesis, owing to the tremendous replenishing power of spermatogonia, a heterogeneous population consisting of both spermatogonial stem cells and committed progenitors. The spermatogonia need to undergo three phases—the mitotic phase, the meiotic phase, and the postmeiotic differentiation phase—to give rise to spermatozoa. In the mitotic phase, spermatogonia undergo a series of mitosis to expand the number of daughter cells. The cultured spermatogonia remain heterogeneous and the stem cell activity is maintained in a subpopulation of cells. The spermatogonial niche comprises of various cell types and extracellular matrix proteins. In contrast with the paracrine growth factors, little is known about the autocrine regulation in spermatogonia. Studies suggest that the metabolism of spermatogonia also affects the fate decision. The mammalian target of rapamycin, an atypical serine/threonine kinase, is a master regulator of cell growth and metabolism.
