Insulin receptors in syncytitrophoblast and fetal endothelium of human placenta. Immunohistochemical evidence for developmental changes in distribution pattern

The localisation of insulin receptors (IR) was investigated on cryosections of human non-pathologic first trimester and full term placentae by indirect immunohistochemistry with three different monoclonal antibodies (MABS). In placentae from 6 to 10 weeks post-menstruation (p-m.), only syncytiotrophoblast was stained, predominantly that of mesenchymal villi and syncytial sprouts, which are areas of high proliferative activity. In placentae from 11 to 14 weeks p-m., endothelial cells commenced to react with the IR MABS and the syncytiotrophoblast was less intensely labelled than at weeks 6 to 10 p-m. In term placentae, the microvillous membrane of the syncytiotrophoblast showed only patches of weak immunoreactivity. In contrast, the endothelial cells in the placenta but not in the umbilical cord were strongly stained. The amniotic epithelium in the chorionic plate and fibroblasts in the stroma were conspicuously labelled. The data indicate: (1) the receptor density on villous syncytiotrophoblast decreases and that of fetal endothelium increases throughout gestation; (2) syncytiotrophoblast of human term placentae expresses a low level per unit area of surface IR; and (3) the majority of IR in human term placentae is located in fetal endothelium. Apart from yet unknown functional effects of maternal and fetal insulin at the placental barrier, the results suggest a growth promoting effect on the trophoblast of maternal insulin in first trimester as well as developmental effects of fetal insulin on the feto-placental vessels at term.