Testosterone Synthesis in Rhesus Fetal Testes: Comparison Between Middle and Late Gestation1

Several aspects of testosterone synthesis were examined in fetal testes of rhesus monkeys at middle (Days 79 to 82) and late gestation (Days 140 to 149). Histological examination of fetal testes revealed that Leydig cells are smaller at late gestation and that there are approximately one-third as many Leydig cells per unit area compared with midgestation. The concentration of testosterone in umbilical arterial serum, however, was not significantly lower at late gestation. In vitro synthesis of testosterone and cyclic AMP (cAMP) was stimulated by human chorionic gonadotropin (hCG) in tissues collected at both stages of gestation. The testosterone concentration of testicular tissue and testosterone synthesis in the absence of hCG were ∿2-fold greater at midgestation (P<0.05), but the rate of testosterone synthesis in the presence of hCG did not differ between the two stages. In contrast, cAMP synthesis was 4-fold greater in the absence of hCG and 9-fold greater in the presence of hCG in tissues collected at midgestation (P<0.01). Concentrations of biologically active luteinizing hormone (LH) in the fetal circulation were found to be 2-fold greater at late gestation than at midgestation (P<0.01).The data indicate that fetal testes of the rhesus monkey are steroidogenically active and capable of responding to gonadotropin in vitro at both middle and late gestation. The testicular adenylate cyclase system appears to be more active and more responsive to exogenous gonadotropin at midgestation, but Leydig cells at late gestation secrete testosterone at a rate that is as great as or greater than at midgestation. The greater concentrations of biologically active LH in the fetal circulation at late gestation may be the reason fewer Leydig cells are able to maintain high serum testosterone levels.