Two aspects of the differentiation of the fetal rabbit gonad have been studied. First, the ability of the fetal ovary to synthesize 17β-estradiol from [7-3H]pregnenolone was measured. By day 18 of gestation, the fetal ovary was capable of synthesizing 17β-estradiol from radioactive pregnenolone (5 mol 2 h-1 mg protein-1), thus establishing that from the onset of estrogen synthesis the fetal ovary possesses all of the enzymes necessary to form estrogens from pregnenolone, including sufficient 3β-hydroxysteroid dehydrogenase-Δ4, 5-isomerase activity. Second, a method was developed to estimate the rate of cholesterol side chain cleavage activity by measuring the total amount of six steroids [progesterone, pregnenolone, dehydroepiandrosterone, androstenedione, Δ5-androstenediol (5-androstene-3β, 17β-diol), and testosterone] produced in 2-h incubations of fetal rabbit gonads. Cholesterol side chain cleavage activity in fetal testes was linear with time up to 2 h and with increasing concentrations of hCG up to a concentration of 10 IU/ml. A developmental study of cholesterol side chain cleavage activity in fetal gonads revealed that both fetal ovaries and testes synthesized steroids de novo from cholesterol at the time of phenotypic sexual differentiation (days 18-21). Thus, from the onset of steroidogenesis in the fetal ovary and testis, both tissues contain the full capacity for the de novo synthesis of steroid hormones from cholesterol. In addition, the effect of hCG on cholesterol side chain cleavage was assessed, and a stimulatory response was found to correlate with the appearance of LH/hCG receptors in fetal testes but was absent in fetal ovaries which lack gonadotropin receptors.
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