Gonado-pituitary relationships in the fetal mouse at various times during sexual differentiation.

Levels of testosterone in plasma and concentrations of LH in both plasma and pituitary glands of fetal mice aged 14, 16 and 18 days were measured by radioimmunoassays in a representative number of fetuses. During this period levels of testosterone in the plasma of male mice were significantly higher than those in the females. Levels of testosterone in plasma of male mice increased from day 14 to day 16 of gestation and decreased on day 1 before parturition. Plasma concentrations of LH remained undetectable in male and female fetuses until day 16 of gestation. levels of LH rose slightly in both sexes in later gestation, but still remained significantly lower in the plasma of male fetuses on days 17-18. In contrast, higher but not significantly different concentrations of LH were observed in pituitary glands from days 14 to 18 in male compared with female mice. These observations suggest that the high levels of testosterone in the plasma of male fetal mice might be responsible for feedback inhibition of LH secretion during the last days of gestation.