During Drosophila egg development, differentiated germ cells, which are produced by germline stem cells (GSCs), are accompanied by escort cells (ECs) to the centre of the germarium, where the germ cells form egg chambers. It has been proposed that these ECs are generated by a population of escort stem cells, but Ting Xie and co-workers now overturn this idea (). The researchers show that ECs undergo slow turnover and that lost cells are replaced by self-duplication rather than by stem cell division. Using fluorescent markers, they show that ECs extend elaborate cellular processes that interact with differentiated germ cells and that these processes are missing when GSC differentiation is blocked. Conversely, disruption of Rho function in ECs, which disrupts the formation of EC processes, leads to the accumulation of ill-differentiated single germ cells and the gradual loss of ECs. These findings reveal a mutual dependence between ECs and differentiated GSC progeny, and suggest that self-maintained ECs form a niche that controls GSC lineage differentiation.
