Figure 1. Drawing of the ovarian niche with one GSC cell. The daughter CB lies just outside of the niche. The ovary is composed of about 20 ovarioles each of which contains an linear array of germ cells at progressive stages of development. The somatic niche, the microenvironment that maintains GSC fate by a BMP signaling cascade is located at the tip of each ovariole. (A) In wild type the GSC to CB cell fate switch occurs as one of the daughter cells moves out of this microenvironment permitting the initiation of the differentiation program that includes significant accumulation of the Bam protein and rapid downregulation of a set of GSC specific markers including Nanos protein. Note that Nanos and Bam proteins are expressed in non-overlapping domains. In contrast, Sxl protein (not shown) is expressed in both Nanos- and Bam-expressing cells. (B) Germ cells that lack Sxl protein fail to exit the stem cell stage, continue to proliferate, and form a tumor. GSC markers, including Nanos protein, are co-expressed with Bam in the majority of the tumor cells, except for the presumptive GSCs located at the tip of the ovariole.