Membrane dynamics during migration of placental cells through trophectodermal tight junctions in sheep and goats

Binucleate cells in ruminant trophectodermal epithelium are unique in that they form part of the tight junction as they migrate across it, maintaining the ionic barrier seal to the internal milieu of the fetus. Such participation imposes considerable constraints on the cell migration because membrane cannot flow through a tight junction. We report quantitative ultrastructural immunocytochemical evidence for vesicle membrane insertion into the binucleate cell plasmalemma which allows the cells to form a pseudopodium past the tight junction. This pseudopodium increases continuously in area by vesicle insertion and develops a close apposition to the plasmalemma of the fetomaternal syncytium which constitutes the fetomaternal boundary in the placenta of the sheep and goat. Eventually the apposed membranes of the binucleate cell pseudopodium and the syncytium fuse by vesiculation and the cytoplasm and nuclei of the binucleate cell merge into the fetomaternal syncytium. The binucleate cell plasmalemma remaining on the trophectodermal side of the tight junction is blebbed off into, and phagocytosed by, the uninucleate trophectodermal cells between which the binucleate cell passed. This process permits the delivery of the binucleate cell granules to the maternal side of the placenta but none of the fetal molecules expressed on the plasma membrane of the binucleate cells are exposed to potential maternal immunological rejection.