Figure 1: Schematic of the adult Drosophila testis.
The testis is enclosed by a smooth muscle sheath and an underlying basal lamina (gray). Approximately 12 hub cells (yellow) form the stem cell niche, some of which are anchored to the basal lamina of the overlying muscle sheath. Germline stem cells (GSCs; magenta) contact hub cells and divide with oriented mitosis to produce a self-renewing GSC and a differentiating gonialblast (Gb). The Gb undergoes transit-amplifying divisions to generate 16-cell spermatogonial cysts, which then enter meiosis and differentiate into sperm. Spermatogonial stages (light purple) can be distinguished by the morphology of the germ cell–specific organelle, the fusome: dot-shaped in GSCs and Gbs, dumbbell-shaped in 2-cell spermatogonia, and branched in 4-, 8-, and 16-cell spermatogonia. Cyst stem cells (CySCs; dark blue) contact hub cells via thin cytoplasmic extensions and divide to produce early cyst cells (light blue) that exit the niche. Two early cyst cells encapsulate each Gb, and as the enclosed germ cells undergo transit-amplifying divisions, the surrounding somatic cells differentiate into late cyst cells (light blue cells that are further from the hub).