A trophoblast specific antigen, recognized by monoclonal antibody MA21, locates a unique trophoblast cell population in the murine placenta

Monoclonal antibody MA21 recognized a 44kDa plasma membrane protein on F9 teratocarcinoma cells, trophectoderm of mouse peri-implantation-stage blastocyst and ectoplacental cone cells of 5 day postcoitum implanted blastocyst (Vernon, Linnemeyer and Hamilton, 1989). We show here that this antigen is expressed by trophoblast cells of the maturing placenta. Immunohistochemical assays of early and mature placental tissue sections, indirect immunofluorescence labelling of placental cultures and blastocyst outgrowths in vitro, and immunoprecipitation of 35S-labelled NP-40 extracts of placental cultures indicate the presence of a plasma membrane-associated antigen with the same characteristics as MA21 antigen of peri-implantation embryos and F9 teratocarcinoma cells. In sections of placentae, antigen-positive cells are always situated in a thin layer between trophoblastic giant cells and maternal tissue. In cultures of postimplantation stage embryos, attached trophoblast cells express MA21 antigen initially, but following transformation to the giant cell state, antigen is no longer expressed. These results indicate the presence of a plasma membrane protein antigen associated with a distinct population of cells believed to be trophoblast. We believe that these cells are the foremost trophoblast cells opposing maternal decidua and that they may give rise to secondary trophoblastic giant cells.