Fig. 1. GSC niches in the Drosophila ovary and testis. (A) Schematic of the early steps in Drosophila spermatogenesis. The GSCs (light green) surround and are in contact with a cluster of post-mitotic somatic cells known as the apical hub (red). The hub cells are a primary component of the male GSC niche. Each GSC is surrounded by two somatic stem cells known as the cyst progenitor cells (light gray). The GSC undergoes asymmetric cell division, giving rise to one daughter cell that will retain stem cell identity and one daughter cell, a gonialblast (dark green), which will undergo four rounds of cell division with incomplete cytokinesis to give rise to 16 spermatogonia. The gonialblast is surrounded by cyst cells (dark gray), which ensure spermatogonial differentiation. (B) Schematic of a Drosophila germarium, which houses the GSCs. Anterior is to the left and posterior is to the right. The terminal filament, cap and inner sheath cells (red) express molecules important for the maintenance and self-renewal of female GSCs and make up the stem cell niche. GSCs (light green) undergo asymmetric cell division, giving rise to one daughter cell that will retain stem cell identity and one daughter cell, a cystoblast, which will initiate differentiation (dark green). As these divisions are taking place, the more mature cysts are displaced towards the posterior of the germarium. Cyst encapsulation by the somatic stem cell derivatives (gray) occurs in region 2a/b. Mature encapsulated cysts budding off from the germarium make up region 3.