Molecular dissection of the male germ cell lineage identifies putative spermatogonial stem cells in rhesus macaques

The spermatogonial stem cell (SSC) pool in the testes of non-human primates is poorly defined. To begin characterizing SSCs in rhesus macaque testes, we employed fluorescence-activated cell sorting (FACS), a xenotransplant bioassay and immunohistochemical methods and correlated our findings with classical descriptions of germ cell nuclear morphology (i.e. A(dark) and A(pale) spermatogonia). FACS analysis identified a THY-1(+) fraction of rhesus testis cells that was enriched for consensus SSC markers (i.e. PLZF, GFR alpha 1) and exhibited enhanced colonizing activity upon transplantation to nude mouse testes. We observed a substantial conservation of spermatogonial markers from mice to monkeys [PLZF, GFR alpha 1, Neurogenin 3 (NGN3), cKIT]. Assuming that molecular characteristics correlate with function, the pool of putative SSCs (THY-1(+), PLZF(+), GFR alpha 1(+), NGN3(+/-), cKIT(-)) comprises most A(dark) and A(pale) and is considerably larger in primates than in rodents. It is noteworthy that the majority of A(dark) and A(pale) share a common molecular phenotype, considering their distinct functional classifications as reserve and renewing stem cells, respectively. NGN3 is absent from A(dark), but is expressed by some A(pale) and may mark the transition from undifferentiated (cKIT(-)) to differentiating (cKIT(+)) spermatogonia. Finally, the pool of transit-amplifying progenitor spermatogonia (PLZF(+), GFR alpha 1(+), NGN3(+), cKIT(+/-)) is smaller in primates than in rodents. These results provide an in-depth analysis of molecular characteristics of primate spermatogonia, including SSCs, and lay a foundation for future studies investigating the kinetics of spermatogonial renewal, clonal expansion and differentiation during primate spermatogenesis.