Germ cell development in the Drosophila ovary. In the adult ovary, two to three germ-line stem cells (GSCs) give rise to cytoblasts (CBs), then divide four times to form 16-cell cysts. One cell within the 16-cell cyst undergoes meiosis and differentiates into an oocyte (not shown). The level of key regulatory proteins (illustrated as high or low) changes rapidly as the germ cell passes through each stage. Bam, Bag of marbles; pMad, phosphorylated Mothers against Decapentaplegic; Sxl, Sex-lethal.