Tissue maintenance relies on adult stem cells that both self-renew and produce differentiating progeny in specialised niches. But stem cells are not immortal, so how are lost stem cells replaced? On , Rebecca Sheng and Erika Matunis use extended live imaging of the Drosophila testis niche to investigate this question. Germline stem cells (GSCs) in the Drosophila testis are attached to somatic hub cells and divide asymmetrically to produce a stem cell that remains attached to the hub and a daughter cell that is displaced away from the hub. Unexpectedly, Sheng and Matunis show that `symmetric renewal', a process in which GSC daughter cell pairs swivel so that both cells contact the hub, generates new GSCs in the testis niche. Moreover, after severe genetically induced GSC loss, the rate of symmetric renewal increases and, in addition, spermatogonia de-differentiate. Thus, asymmetric stem cell divisions do not always lead to an asymmetric cell fate, and lost stem cells can be regenerated by multiple mechanisms.
