Cellular dynamics of growth in sheep and goat synepitheliochorial placentomes: an autoradiographic study.

This paper demonstrates that in sheep and goats the two definitive fetomaternal interface layers are developmentally related. The fetal trophectoderm consists of binucleate and uninucleate cells. The apical microvilli of the trophectoderm interdigitate with a layer consisting of syncytial plaques of limited area bounding the maternal connective tissue. Our previous histological ultrastructural and immunocytochemical work has indicated that throughout pregnancy the fetal binucleate cells migrate to and fuse with the uterine epithelium or its derivatives to form these syncytial plaques which constitute a persistent fetomaternal tissue unique to ruminants. This quantitative autoradiographic study of thymidine incorporation into sheep and goat placentas confirms the central role of the binucleate cell in placental growth, demonstrates that throughout pregnancy all binucleate cells migrate and indicates that most of the nuclei of the syncytial plaques, which appear to have a limited lifespan, derive from binucleate cell fusion.