The stem-cell niche theory: lessons from flies

A stem-cell niche is a restricted locale in an organ that supports the self-renewing division of stem cells and so prevents them from differentiating. Cap and terminal filament cells and hub cells constitute the stem-cell niche in the fly ovary and testis, respectively. Both ovarian and testicular stem-cell niches are dual-functional niches that emanate signals that are essential for the maintenance of both germline and somatic stem cells. The ovarian niche generates several signalling pathways, including the Decapentaplegic (Dpp) and female sterile (1) Yb (Yb)/Piwi/Hedgehog (Hh)-mediated pathways, which act in parallel to maintain germline stem cells. The Yb/Piwi/Hh-mediated pathway bifurcates downstream of Yb, with the Piwi and Hh branches mainly responsible for the maintenance of germline and somatic stem cells, respectively. Cell adhesions through adherens junctions anchor both germline and somatic stem cells to the niche. The testicular niche generates the Unpaired ligand that is essential for the self-renewal of both germline and somatic stem cells. In contrast to its oogenic function, the Dpp pathway functions to restrict the proliferation of gonialblasts and spermatogonia in males. Epidermal growth factor receptor and Raf also have this function. The organization and signalling pathways of the fly niches provide guiding principles for studying stem-cell niches in mammalian systems.