Paracrine Wnt/β-catenin signaling mediates proliferation of undifferentiated spermatogonia in the adult mouse testis

Spermatogonial stem cells (SSCs) fuel the production of male germ cells but the mechanisms behind SSC self-renewal, proliferation, and differentiation are still poorly understood. Using the Wnt target gene Axin2 and genetic lineage-tracing experiments, we found that undifferentiated spermatogonia, comprising SSCs and transit amplifying progenitor cells, respond to Wnt/beta-catenin signals. Genetic elimination of beta-catenin indicates that Wnt/beta-catenin signaling promotes the proliferation of these cells. Signaling is likely initiated by Wnt6, which is uniquely expressed by neighboring Sertoli cells, the only somatic cells in the seminiferous tubule that support germ cells and act as a niche for SSCs. Therefore, unlike other stem cell systems where Wnt/beta-catenin signaling is implicated in self-renewal, the Wnt pathway in the testis specifically contributes to the proliferation of SSCs and progenitor cells.