THE TAO OF STEM-CELLS IN THE GERMLINE

Germline stem cells (GSCs) are the self-renewing population of germ ce lls that serve as the source for gametogenesis. GSCs exist in diverse forms, from those that undergo strict self-renewing asymmetric divisio ns in Drosophila to those that maintain their population by balancing between mitosis and differentiation in Caenorhabditis elegans. Most ve rtebrate spermatogonial GSCs appear to adopt an intermediate strategy. In most animals, GSCs are established during preadult gonadogenesis f ollowing the proliferation and migration of embryonic primordial germ cells. GSCs produce numerous gametes throughout the sexually active pe riod of adult life. The establishment and self-renewing division of GS Cs are controlled by extracellular signals such as hormones from the h ypothalamic-pituitary axis and local interactions between GSCs and the ir neighboring cells. These extracellular signals may then influence d ifferential gene expression, cell cycle machinery, and cytoskeletal or ganization of GSCs for their formation and/or divisional asymmetry. In addition, the GSC mechanism is related to that for germline and sex d etermination. Current knowledge has provided a solid framework for fur ther study of GSCs and stem cells in general.