Migration of binucleate cells in the sheep placenta during normal pregnancy.

The maternofetal interface in the placentomes of the sheep placenta consists of a fetal cellular chorion layer whose apical microvilli interdigitate with those of a syncytial layer which borders the maternal connective tissue. Most of the granulated binucleate cells found in the chorion epithelium reach neither its basement membrane nor its microvillar junction apex. However, up to one fifth of the binucleate cells can be shown to be part of or push pseudopodia across the microvillar junction or are located within the syncytium. The syncytium and chorionic binucleate cells both have a nuclear chromatin structure and distinctive cytoplasmic granules which are not found in any other placental cells. This suggests that most, if not all, of the syncytium is derived by fusion of fetal binucleate cells which have migrated across the microvillar junction from the chorion, and that the immunological maternofetal junction does not lie between chorion and syncytial layers.