Immunohistochemical localization of inhibin/activin subunits in human placenta.

OBJECTIVE: To examine the cellular localization of each inhibin subunit in human placenta throughout pregnancy. METHOD: Placental tissues were collected and fixed in Bouin's solution, and studied with the immunohistochemical technique avidin-biotin-peroxidase complex. RESULTS: There was immunohistochemical staining with antisera against each inhibin subunit in the syncytiotrophoblast, but not in the cytotrophoblast. In the first-trimester placenta, positive immunostaining for alpha- and beta A-subunits was clearly observed in the syncytial layer of villi, whereas staining for beta B-subunit was faint. In the second-trimester placenta, the relative intensities of staining for alpha- and beta A-subunits were similar to those in the first-trimester placenta, and enhanced positive immunostaining with beta B-subunit antiserum was observed. The relative amount of immunostainable alpha-subunit declined within the syncytiotrophoblast of the third-trimester placenta, whereas levels of immunostainable beta-subunits were unchanged. CONCLUSIONS: Inhibin subunits may be produced in the syncytiotrophoblast throughout pregnancy, and activin as well as inhibin may be synthesized in the syncytiotrophoblast of the term placenta.