The capacity of fetal testes from rats aged 14.5-20.5 days to synthesize testosterone and respond to LH was studied in vitro. Synthetic capacity was measured as the rate of appearance of radioimmunoassayable testosterone in the incubation medium with or without added LH. In the absence of LH the testes retained 20-25% of the total testosterone synthesized. Testosterone in the medium increased with tissue weight and time of incubation and was not metabolized to other steroids. Testes from 14.5-day-old fetuses did not secrete testosterone. The unstimulated rates (ng T/testis/h) of testosterone secretion were: 0.25 ± 0.12 at 15.5 days; 0.68 ± 0.35 at 16.5 days; 1.02 ± 0.06 at 18.5 days; and 1.4 ± 0.48 at 20.5 days. Responsiveness and sensitivity to LH increased with development. At 15.5 days, 160 ng LH/ml increased testosterone secretion 4-fold and at 16.5 days the same dose increased secretion 8-fold. At 18.5 days, 64 ng LH/ml increased testosterone secretion maximally, 15-fold. At 20.5 days, testosterone secretion was increased 4-fold at an LH concentration of 1.0 ng LH/ml and 20-fold at a dose of 16 ng LH/ml. Sensitivity to LH increased from 1.6 ng/ml at 18.5 days to 0.8 ng/ml at 20.5 days. In conclusion, testicular development from 14.5-20.5 days is characterized by an increased capacity to synthesize testosterone and to respond to LH. Both parameters appear on day 15.5 of gestation. These results indicate that testosterone synthesis could be under gonadotropic regulation. (Endocrinology102: 999, 1978)
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Feldman et al. (1978) studied this question.