Germline Stem Cells

Abstract

Germline stem cells (GSCs) serve as the self-renewing source for the continuous production of gametes in diverse organisms. These stem cells can be classified into two types—stereotypic and populational-according to how they self-renew. Stereotypic GSCs self-renew by asymmetric division, producing a daughter stem cell and a differentiated daughter cell. An example is the Drosophila ovary. Each ovariole contains two to three GSCs at its apical region. These cells divide to produce one daughter cell that remains a stem cell and another that begins differentiation as it is displaced from the cap cell. Populational stem cells divide symmetrically and self-perpetuate only at the populational level. In the gonads of Caenorhabditis elegans, the mitotically active germline nuclei share a common cytoplasm at the distal end of the gonad and are the self-renewing population of stem cells for gametogenesis. The mitotic nuclei show a gradient of mitotic ability, with those more than 20 nuclei away from the distal tip cell eventually entering meiosis and differentiating into haploid gametes. In mammalian testes, GSCs exist as single cells among type A spermatogonia (type As). An As spermatogonium will either divide completely to form two daughter As spermatogonia or undergo incomplete cytokinesis to produce a pair of connected daughter cells (Apr) that are both committed to differentiation. In most animals, GSCs are established during preadult gonadogenesis following the proliferation and migration of embryonic primordial germ cells. However, it is not known how the stem cell fate is established in the preadult gonad.