A Sertoli valve niche at the terminal end of seminiferous tubules for mammalian spermatogenic stem cells.

In mammalian testes, GFRα1-positive spermatogonia, including spermatogonial stem cells (SSCs), are maintained in the basal compartment of seminiferous tubules. These cells actively migrate along the tubules within an 'open' niche, under the regulation of temporally fluctuating GDNF and retinoic acid (RA) signals, as well as constitutive FGF. In contrast, a newly discovered SSC niche exists in the Sertoli valve (SV) region at the terminal end of the seminiferous tubules, influenced by adjacent SOX17-positive rete testis. The SV epithelium, composed of specialized Sertoli cells, constitutively expresses GDNF and CYP26A1, an RA-degrading enzyme, supporting stable maintenance of GFRα1-positive spermatogonia in a region-specific manner. Unlike those in convoluted seminiferous tubules, SV Sertoli cells in rats and hamsters retain proliferative capacity in adulthood. In hamsters, these Sertoli cells relocate distally to support spermatogenesis, suggesting the presence of Sertoli cell progenitors in the SV region. This review discusses SV formation and its role as a 'closed' niche in mammalian spermatogenesis.