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

In male mammals, spermatogonial stem cells (SSCs) are essential for sustaining lifelong spermatogenesis within the testicular open niche, a unique environment that allows SSC migration over an extended niche area. As SSCs undergo continuous mitotic division, mutations accumulate and are transmitted to the descendant SSC clones. Therefore, SSC clonal fate behaviors, in terms of their efficiencies in completing spermatogenesis and undergoing expansion within the niche, influence sperm genomic diversity. We aimed to elucidate the effects of physiological aging on SSC clonal fate behavior within the testicular open niche. We used single-cell RNA sequencing, lineage tracing, and intravital live imaging to investigate SSC behavior in aged mouse testes, where spermatogenesis, although reduced, persists. We found that undifferentiated spermatogonia maintained gene expression heterogeneity during aging. Among these, GFRα1⁺ cells, which exhibited state heterogeneity, showed accelerated proliferation and persistent motility, continuing to function as SSCs in older mice. In contrast, a subset of SSCs characterized by low Egr4 and Cops5 expression did not contribute to spermatid formation. These non-sperm-forming SSC clones increased in proportion among the total SSC clones and expanded spatially within the testicular open niche in old mice, a phenomenon not observed in young mice. The expansion of non-sperm-forming SSC clones in aged testes suggests that they occupy a niche space, limiting the availability of functional SSCs and potentially reducing sperm production and genetic diversity. These findings highlight age-specific clonal characteristics as hallmarks of stem cell aging within the testicular open niche and provide novel insights into the mechanisms governing reproductive aging.