Sex steroids in the umbilical circulation of fetal rhesus monkeys from the time of gonadal differentiation.

Male rhesus monkey fetuses have significantly more testosterone (T) in their circulation than females on days 35--50 of gestation (P less than 0.01; n = 6 males and 6 females). However, we found no sex differences for androstenedione (delta 4). T concentrations remained significantly higher in male fetuses than in females later in gestation, e.g. days 79--84, 100--133, and 140--160. Levels of delta 4 differed between the sexes only on days 79--84, and dihydrotestosterone concentrations were significantly higher in male fetuses than in females on days 100--133 and 140--163. The fact that delta 4 concentrations were not different between the sexes at the earliest period studied (days 35--50) indicates that systemic concentrations of this hormone in the fetus probably are not important for sexual differentiation, especially of the central nervous system. Quantification of three steroids (T, delta 4, and dihydrotestosterone) in umbilical arterial and venous plasma from five male and nine female fetuses (days 35--100) revealed significant arterial/venous differences only for T in males (arterial greater than venous). These data, which suggest that fetal testes secrete T during morphological differentiation, lend credence to the hypothesis that endogenous T partially regulates sexual differentiation.