The fetal gonads as a potential source of immunoreactive inhibin in pregnant mares

The source of immunoreactive (ir)-inhibin in the maternal circulation during the second half of pregnancy in mares were investigated by 1) morphometric study of interstitial cells of fetal gonads, 2)immunocytochemical localization of inhibin alpha-subunit in fetal gonads, and 3)measurement of circulating ir-inhibin in fetuses and dams. In the fetal ovary, inhibin alpha-subunit was detected in interstitial cells. Localization of inhibin alpha-subunit in the fetal testis was first detected in the interstitial cells, then positive reaction was also detected in Sertoli cells within the seminiferous tubules. Concentrations of ir-inhibin in the fetal plasma of both sexes (120-270 days of gestation) were much higher than in the maternal plasma. These results show that the developing fetal gonads of both sexes have the capacity to produce ir-inhibin. The present study, therefore, strongly suggest that the enlarged fetal gonad is a potential source of ir-inhibin during pregnancy in mares.