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. 2010 Jan;2(1):a001313. doi: 10.1101/cshperspect.a001313

Figure 2.

Figure 2.

The anatomy of the Drosophila germ line stem cell (GSC) niche and asymmetric stem cell division. (A) Male GSC niche: GSCs and cyst stem cells (CySCs) are attached to hub cells via adherens junctions (AJs). GSCs divide asymmetrically to self-renew and produce a gonialblast (GB) that initiates the differentiation program. The GB further undergoes four synchronous, transit-amplifying divisions to yield 16 spermatogonia interconnected by the fusome. The spectrosome is a spherical version of the fusome observed in GSCs. A pair of CySCs encapsulates the GSCs and provides signals required for GSC identity. Similar to GSCs, CySCs divide asymmetrically to self-renew and produce cyst cells. Cyst cells exit the cell cycle, a pair of which encapsulates the GB and spermatogonia to promote differentiation. (B) Asymmetric stem cell division of male GSC by centrosome orientation: Upd ligand is secreted from hub cells to activate the JAK-STAT pathway in GSCs and CySCs to specify their stem cell identity. The Zfh-1 transcription factor controls CySC identity. Together with hub cells, CySCs dictate GSC identity. The mitotic spindle is oriented toward the hub cells via positioning of the centrosome. Spectrosomes in male GSCs are not oriented with respect to the hub cells during interphase. EGFR signaling ensures the encapsulation of germ cells by cyst cells. (C) Female GSC niche: GSCs are attached to the cap cells via adherens junctions. GSCs divide asymmetrically to self-renew and produce a cystoblast (CB) that initiates differentiation. The CB divides four times to give rise to 16 germ cells interconnected by the fusome, only one of which becomes an oocyte, whereas the remaining 15 cells become nurse cells. Escort stem cells (ESCs) encapsulate the GSC, while their daughters, escort cells, encapsulate the developing germ cells. Escort cells are later replaced by follicle cells, which are daughters of follicle stem cells (FSCs). (D) Asymmetric stem cell division of female GSC by the spectrosome: BMP signaling and Piwi controls GSC identity via niche-GSC interaction. The mitotic spindle is oriented toward the cap cells via anchoring of one spindle pole to the spectrosome, which localizes consistently to the apical side of GSCs. (Figures are adapted and modified from Fuller and Spradling 2007.)