Fig. 6. The stem cell niche at the tip of the Drosophila testis. (A) A small group of somatic hub cells (blue) are located at the tip of the Drosophila testis and express the UPD ligand and so maintain the fate of adjacent stem cells (yellow). As the germline stem cells divide (top), daughter cells are born (middle), with one remaining adjacent to the hub and maintaining stem cell fate, and the other beginning to differentiate towards the gonialblast and spermatogonial fate (dark yellow). Somatic stem cells (light green) are also present that give rise to the cyst cells (dark green), which encase the differentiating spermatogonia. (B) An enhancer trap in the upd locus mirrors the hub cell-specific expression of upd at the testis tip. Scale bar: 10 µm. Reproduced, with permission, from Tulina and Matunis (Tulina and Matunis, 2001).