An immunological cryo-ultrastructural study of a sequential appearance of proteins in placental binucleate cells in early pregnancy in the cow

Using the most sensitive immunocytochemical method available, on ultrathin frozen sections, the results in this paper demonstrate that bovine placental lactogen (bPL) is present in the earliest fetal binucleate cells found at 21 days post coitum in the trophectoderm. A second protein, the SBU-3 antigen, which is absent in the early stages of pregnancy appears abruptly in the binucleate cell granules at 30 days post coitum coincident with the start of villus development. Subsequently, the granules contain both bPL and the SBU-3 antigen. This sequential production of unlike proteins indicates that the binucleate cell has different functions depending on the stage of pregnancy and has important roles to play both at implantation and in villus development.