Sources of inhibin in early pregnancy

Inhibin is a glycoprotein which suppresses the release of follicle stimulating hormone (FSH) from the pituitary. Plasma immunoreactive inhibin levels during pregnancy have been reported to be higher than those during the normal menstrual cycle. In the present study, we investigated the cellular localization of inhibin alpha-, beta A-, and beta B-subunits in the human corpus luteum and placenta during early pregnancy. Luteal cells and theca luteal cells in the corpus luteum of pregnancy exhibited positive staining for all three inhibin subunits. We also observed positive staining with antisera against each inhibin subunit in the syncytiotrophoblast, but not in the cytotrophoblast, in the villi during early pregnancy. The staining with antisera against inhibin alpha- and beta A-subunits was clearly observed, whereas the staining for the beta B-subunit was faint. These results suggest that the gestational corpus luteum and the villous tissue may be major sources of inhibin during early pregnancy.