RETINOIC ACID SYNTHESIS IN THE SOMATIC-CELLS OF RAT SEMINIFEROUS TUBULES

At physiological plasma concentrations, retinoic acid (RA) cannot cros s the blood-testis barrier formed by Sertoli and peritubular cells, an d it is thought to be mainly synthesized in situ through the oxidation of retinol. We have thus examined the in vitro RA biosynthetic capaci ty of cultured Sertoli and peritubular cells isolated from the seminif erous tubules of prepubertal rats, using hole-cellular retinol binding protein (CRBP) as a substrate. Although both somatic cell types conta in CRBP and retinoic acid nuclear receptors, RA synthesis was only det ected with Sertoli cell subcellular fractions. Most of the RA synthesi zing activity of these cells is contributed by a microsomal-cytosolic system that shares many functional similarities with a RA biosynthetic pathway originally identified in rat liver. RA synthesis is maximal a t a time of postnatal life (20 days) preceding meiotic cell accumulati on and remains nearly constant thereafter. The unique ability of Serto li cell subcellular fractions to support RA formation from holoCRBP, a long with the observed age-dependent modulation of this activity, indi cate that Sertoli cells represent the main site of intratubular RA pro duction and that they may play a key role in controlling RA-dependent processes within the seminiferous tubule.