Studies on the distribution of binucleate cells in the placenta of the sheep with a monoclonal antibody SBU-3.

The distribution of cells recognised by the monoclonal antibody SBU-3 raised against trophoblast microvilli during development of the sheep placenta was investigated using the indirect immunoperoxidase technique. At 21 days of gestation, the placental antigen recognised by the monoclonal antibody SBU-3 was observed in the binucleate cells in the trophoblast located in close apposition to the caruncular epithelium. From 30-100 days there was a dramatic increase in the number of SBU-3-positive cells in the placentomal trophoblast and the syncytial layer in the placentome. An insignificant number of SBU-3-positive cells was observed in the interplacentomal trophoblast. By 120-145 days, the syncytial layer became less intensely stained, but strongly SBU-3-positive binucleate cells were still present in the placentomal trophoblast. It is concluded that the antigen recognised by the monoclonal antibody SBU-3 is a secretory product of the binucleate cells of the trophoblast, whose function is at present unknown. The findings in this study are consistent with the theory that the syncytium is formed by fusion of migrating fetal binucleate cells.