The role of the binucleate cell in ruminant placental structure

Binucleate cells are found in the fetal trophectodermal epithelium of all ruminant placentas so far examined. They are present in fairly constant proportions from implantation to parturition. Light microscope investigations initially indicated that binucleate cells migrated across the feto-maternal junction, but subsequent electron microscope studies could find no evidence for this. The demonstration that placental lactogen in the sheep placenta is restricted to the binucleate cells prompted a re-examination of their function. The development of a selective stain for binucleate cells on electron microscope sections facilitated the demonstration that migration does occur in cows, deer, goats and sheep at all stages of pregnancy. Counts of the frequency of binucleate cells and their migration indicated that most, if not all, binucleate cells migrated, and supporting evidence was obtained using autoradiographic and immunocytochemical techniques. The result of the migration is fusion of a binucleate cell with a uterine epithelial cell or a syncytial layer. This fusion delivers the characteristic binucleate cell granules close to the maternal circulation while maintaining the trophectodermal barrier to other feto-maternal exchange. The ruminant binucleate cell therefore seems to play a central role in forming the structures and secretions at the feto-maternal interface which may be crucial in establishing and maintaining pregnancy.