The distribution of activin and activin receptors in gestational tissues across human pregnancy and during labour

The aim of this study was to investigate localization and content of activin beta A-subunit and activin receptors in gestational tissues throughout pregnancy and in association with term labour. Placenta and fetal membranes were collected from women undergoing first and second trimester terminations and from women before and after term labour. Activin beta A-subunit and activin receptors IA, IB, IIA and IIB were studied by immunohistochemistry. Term tissues were analysed for activin A and follistatin content by ELISA and the presence of receptor proteins was assessed by Western hybridization. Activin beta A-subunit was localized to the syncytiotrophoblast and cytotrophoblast in placentae from all gestational ages, and to the amniotic epithelial and chorionic trophoblast layer at term. In placentae of first and second trimester, receptor proteins were localized to the syncytium, whereas at term, the distribution was confined predominantly to vascular endothelial cells of villous blood vessels. Receptor proteins in amnion were localized to some epithelial cells, mesenchyme and chorionic trophoblast. These findings suggest that activin A is secreted by and targets the placental syncytium and amniotic epithelium in early pregnancy, but at term targets the vascular endothelium of placenta and the fetal membranes. There were no differences with labour onset.