Sex Differences in Biologically Active and Immunoreactive Gonadotropins in the Fetal Circulation of Rhesus Monkeys*

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Abstract
To study the ontogenesis of fetal pituitary gonadotropin secretion in the rhesus monkey, LH was measured in fetal serum (n = 95) from days 47–163 of gestation with a mouse Leydig cell bioassay. In addition, FSH was measured in some samples (n = 46) by RIA. Concentrations of LH determined by bioassay were compared with concentrations determined by two different RIAs for rhesus LH. Values obtained by bioassay were highly correlated with values obtained with the rhesus:anti-hCG RIA, but not with values obtained with the ovine:antiovine RIA. Levels of biologically active LH (LER-1909-2) in female fetuses reached peak values of 15–20 μg/ml between 80‐120 days of gestation, and then declined near term. Levels of biologically active LH in fetal males remained relatively low (2–4 μg/lml) throughout gestation. From 79-163 days of gestation, concentrations of FSH and LH in fetal sera were significantly greater in females than in males. The level of biologically active LH in the maternal circulation remained low (< 1.0 μg/ml) throughout gestation, and there were no differences in LH concentrations between matched samples from umbilical artery and vein (n = 24). The data demonstrate an unequivocal sex difference in concentrations of LH and FSH in the circulation of fetal rhesus monkeys and suggest the presence of a gonadal-hypothalamicpituitary- negative feedback loop that is operative in fetal males but not in females.

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