The blood group I antigen defined by monoclonal antibody C6 is a marker of early mesoderm during murine embryogenesis

The distribution of a blood group I-associated antigen in the postimplantation mouse embryo was examined using immunohistochemical, immunofluorescence, and immunoblotting techniques. The following results were obtained1. The I antigen defined by monoclonal antibody C6 was largely, but not entirely, restricted to mesodermal cells and their derivatives in day-6.5 to day-10 embryos. A gradual increase in the surface expression of the I antigen was observed as mesodermal cells moved away from the primitive streak in posterior and lateral directions between the embryonic ectoderm and visceral endoderm. I expression declined in hindlimb-bud-stage embryos in a posterior-to-anterior progression, and was not observed in fetal tissues after day 11.2. The appearance of mesodermal cells was associated with a switch in surface carbohydrate expression. Two-color labeling experiments using SSEA-1 and C6 monoclonal antibodies suggested that mesodermal cells leaving the primitive streak lose surface Le^x and acquire I. This suggestion was supported by flow-cytofluorimetric analyses of single cell preparations of day-8 and day-10 embryos: cell-surface Le^x expression declined markedly between days 8 and 10, while I expression increased.3. The glycoproteins carrying Le^x and I antigens were observed to change in molecular weight during early development. Le^x was identified on a broad glycoprotein band with an apparent molecular weight > 200 000, whereas the I antigen was identified on glycoprotein bands with apparent molecular weights of 155 000 (day 8), and 140 000, 155 000, 210 000, and 250 000 (day 10). The results indicate that the I antigen is a useful marker of early mesoderm in the mouse embryo.