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, GFRα1) and exhibited enhanced colonizing activity upon transplantation to nude mouse testes. We observed a substantial conservation of spermatogonial markers from mice to monkeys [PLZF, GFRα1, Neurogenin 3 (NGN3), cKIT]. Assuming that molecular characteristics correlate with function, the pool of putative SSCs (THY-1 +, PLZF +, GFRα1 +, 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 +, GFRα1 +, NGN3 +, cKIT +/−) is smaller in primates than in rodents.