Spermatogonial expansion in man and non-human primates has been studied for decades. Controversy persists about the cell type representing the testicular stem cell and the exact kinetics of spermatogonial proliferation. We recently determined the starting point of spermatogenesis and proposed a model for clonal expansion of spermatogonia in adult macaques. Here we want to confirm the initiation event, study and compare the details of the kinetics of spermatogonial expansion in vivo and in vitro, and characterize a population of A spermatogonia acting as testicular stem cells.