Characterization of a developmentally regulated mouse embryonic antigen

A mammalian embryonic cell surface glycoprotein (ESGp), whose expression and biochemical structure seem to be developmentally regulated, has been isolated and characterized. The molecule expressed in two cell through morula stage mouse embryos has a molecular weight, by electrophoretic analyses, of 90 kDa. At the blastocyst stage, however, the molecule migrates as a broad, heterogeneous band ranging from 90 to 110 kDa. Evidence obtained from studies of embryonal carcinoma (EC) cells indicates that this band is actually a composite of three distinct molecules (molecular weight 90, 95, and 105 to 110 kDa), each of which is synthesized uniquely by one of the different cell types of the blastocyst: the embryonic ectoderm and visceral and parietal endoderms, respectively. A survey of various mouse tissues and cell lines has revealed that undifferentiated cells express the low molecular weight form (90 kDa) characteristic of embryonic ectoderm, whereas differentiated cells and adult tissues express the high molecular weight form (110 kDa) characteristic of parietal endoderm. Only the EC visceral endoderm cell analogues have been shown to express the intermediate molecule (95 kDa). In embryos, the antigen is uniformly distributed over the cell surface during early cleavage stages (two to eight cell); just before compaction, however, it seems to redistribute and becomes polarized at the outside exposed edges of blastomeres. In cultured EC cells, ESGp is found only in areas of cell-to-cell contact; free-standing surfaces of cells are negative for expression. It is possible, therefore, that ESGp may be involved in the intercellular adhesion of both EC cells and compacting embryos.