Spermatogenesis in higher primates have is characterized by distinct differences from the process in other genera. At birth, type Ad and Ap spermatogonia, indistinguishable from those cells in the adult testis, are the only germ cells present. Two distinct phases of Sertoli cell proliferation occur in higher primates: one during infancy and the second during puberty. In addition, LH and FSH are equally effective in stimulating Sertoli cell proliferation in the physioligically hypogonadotropic juvenile monkey. The mechanism of stem cell proliferation in primates is less complex than in rodents. The undifferentiated spermatogonia of primates have distinctively different roles. The Ad spermatogonia are usually mitotically quiescent and only become active when a depletion of all more mature cells occurs. These cells are the reserve stem cells of the seminiferous epithelium of the primate. The Ap spermatogonia are mitotically active in the adult seminiferous epithelium. They are the renewing stem cells that divide periodically and produce either Ap spermatogonia or differentiated, type B, spermatogonia. This period, or cell cycle time, of Ap cell division sets the duration of the cycle of the seminiferous epithelium.
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