Age-Dependent Clonal Expansion of Non-Sperm-Forming Spermatogonial Stem Cells in Mouse Testes

The continuous formation of sperm from spermatogonial stem cells (SSCs) throughout life ensures the efficient transmission of male genetic information. However, the effects of aging on SSC function in the testicular open niche are poorly understood. In this study, we investigated the clonal fate behavior of SSCs in the testes of aged mice with reduced but ongoing spermatogenesis. As aging progresses, undifferentiated spermatogonia maintained heterogeneity in gene expression. Among these, GFRa1+ cells, which exhibit state heterogeneity, accelerated proliferation, and persistent motility, functioned as SSCs in aged testes. SSCs that do not form spermatids were characterized by reduced expression of Egr4 and Cops5. In old mice, the proportion of non-sperm-forming SSC clones increased and expanded along the seminiferous tubules, a phenomenon that was not observed in young mice. These age-specific clonal fate characteristics serve as hallmarks of stem cell aging in the testicular open niche.