Fig. 8.

A proposed model by which RA coordinates spermatogenesis. (A) Periodic RA signaling coordinates four transitions. Blue, high RA concentration; purple, spermatogonial differentiation; green, meiotic initiation; brown, initiation of spermatid elongation; gray, release of spermatozoa. (B) Sertoli cells express Aldh1a1 to produce RA required for spermatogonial differentiation and meiotic initiation. Pachytene spermatocytes express Aldh1a2 to produce RA required for initiation of spermatid elongation and release of spermatozoa.