FOXC2 marks and maintains the primitive spermatogonial stem cells subpopulation in the adult testis

In adult mammals, spermatogenesis embodies the complex transition from spermatogonial stem cells (SSCs) to spermatozoa. This process is initiated by the dynamic transition among a series of SSCs subpopulations. However, it remains elusive and controversial for the identity of the primitive adult SSCs at the top of this developmental hierarchy. Using single-cell analysis and lineage tracing, we identified forkhead box protein C2 (FOXC2) as a specific marker for the primitive SSCs subpopulation in adult mice and humans. During homeostasis, FOXC2+-SSCs can initiate spermatogenesis, and through which give rise to all sets of spermatogenic progenies. Specific ablation of the FOXC2+-SSC results in depletion of the undifferentiated spermatogonia pool. During germline regeneration, spermatogenesis can be completely restored by FOXC2+-SSCs. Germ cell specific Foxc2 knockout resulted in accelerated exhaustion of SSCs and eventually led to male infertility. Mechanistically, FOXC2 is required for maintaining the quiescent state of the primitive SSCs by promoting the expression of negative regulators of cell cycle phase transition. Overall, this work proposed FOXC2+-SSCs as an indispensable and primitive subgroup during homeostasis and regeneration in the adult testis. ### Competing Interest Statement The authors have declared no competing interest.