Driving asynchronous spermatogenesis: is retinoic acid the answer?

Spermatogenesis in adult mammals is highly organized, with the goal being the uninterrupted production of sperm. How continual, asynchronous sperm production is initiated and maintained in the mammalian testis is still under investigation but retinoic acid (RA) likely plays a key role. Investigations utilizing vitamin A deficient rodents, transgenic mouse models, whole testis and isolated germ cell culture and inhibitors of vitamin A metabolism indicate that RA is required to drive spermatogonial differentiation and the release of spermatids from the seminiferous epithelium. These studies, and the fact that spermatogonial differentiation and spermatid release happen simultaneously in the adult testis, have led to the hypothesis that RA generates both the cycle of the seminiferous epithelium and the spermatogenic wave.