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, m aintaining the ionic barrier seal to the internal milieu of the fetus. Such participation imposes considerable constraints on the cell migra tion because membrane cannot flow through a tight junction. We report quantitative ultrastructural immunocytochemical evidence for vesicle m embrane insertion into the binucleate cell plasmalemma which allows th e cells to form a pseudopodium past the tight junction. This pseudopod ium increases continuously in area by vesicle insertion and develops a close apposition to the plasmalemma of the fetomaternal syncytium whi ch constitutes the fetomaternal boundary in the placenta of the sheep and goat. Eventually the apposed membranes of the binucleate cell pseu dopodium and the syncytium fuse by vesiculation and the cytoplasm and nuclei of the binucleate cell merge into the fetomaternal syncytium. T he binucleate cell plasmalemma remaining on the trophectodermal side o f the tight junction is blebbed off into, and phagocytosed by, the uni nucleate trophectodermal cells between which the binucleate cell passe d. This process permits the delivery of the binucleate cell granules t o the maternal side of the placenta but none of the fetal molecules ex pressed on the plasma membrane of the binucleate cells are exposed to potential maternal immunological rejection.