Abstract
Since one-way transport of iron via the human placenta from maternal serum to the fetus has been noted, the localization of transferrin and the dynamics of the iron transport mechanism in the human placental tissue were investigated.The results were as follows. The peroxidase conjugated antibody method was used for localizing transferrin at the microvillous surface of the syncytiotrophoblast, and the specific and reversible attachment of transferrin at this site was recognized. The villous tissue in vitro revealed autoradiographically that radioactive iron is taken up vigorously from 59Fe-diferric transferrin and accumulated into hemosiderin particles neighboring the basement membrane of trophoblast cells, and the colloidal gold technique revealed endocytosis of diferric transferrin attaching on the surface of the syncytiotrophoblast into the cytoplasm and final uptake of transferrin into the lysosomes. Apotransferrin, however, was found to be neither attaching itself to the surface nor incorporaring itself into the cytoplasm. These results suggest that the first phase of the placental iron transport is a specific attachment of maternal transferrin at the site of transferrin receptors of the syncytiotrophoblast plasma membrane, and the second phase is so-called receptor-mediated endocytosis, incorporation of transferrin-bound receptors into the cytoplasm.
