Cycles, waves, and pulses: Retinoic acid and the organization of spermatogenesis.

Both the Sertoli cells and the germ cells have the capacity to synthesize retinoic acid from retinol and in the mouse the entry into the cycle of the seminiferous epithelium, and the subsequent conversion of undifferentiated spermatogonia into differentiating spermatogonia is governed by a peak of RA synthesis occurring at stages VIII-IX of the cycle. Normal asynchronous spermatogenesis can be modified by altering RA levels, and as a result the entire testis will consist of a few closely related stages of the cycle.