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Sperm Replace the Germ
The physical location of many stem cell niches is difficult to identify, but in the Drosophila testis, the niche is easy to locate. At the tip of each testis is a cluster of nondividing cells called the hub (blue nuclei on left-hand side) with two populations of stem cells surrounding it: the male germline stem cells (green) give rise to interconnected spermatogonia, whereas the somatic cyst stem cells (bright red nuclei) produce cyst cells (faint red nuclei). The cyst cells envelope the germline cells throughout their differentiation and are analogous to Sertoli cells in mammals. Images such as this one have shown that clusters of interconnected spermatogonia can replace missing germline stem cells.
Image: A whole Drosophila testis is imaged with a laser scanning confocal microscope. A nuclear marker stains somatic cyst stem cells bright red, whereas it diminishes in their daughters. Germline stem cells and their progeny are marked green with a cortically localized GFP. DNA is blue. (Erika Matunis and Becca Sheng, Johns Hopkins University