Expression of the cell surface-associated glycoprotein, fibronectin, in the early mouse embryo.

The expression of the cell surface-associated glycoprotein fibronectin was studied by indirect immunofluorescence in the early stages of mouse embryogenesis. Fibronectin was not detectable in early preimplantation embryos. Trace amounts of the protein were first found between the cells of the inner cell mass of late blastocysts. In implanted early egg cylinders, fibronectin was deposited between the ectoderm and endoderm of the inner cell mass and in the nascent Reichert's membrane. With development, the visceral and the parietal endoderm cells became positive for the protein, but no fibronectin was detected in ectoderm cells. During segregation of mesoderm from ectoderm, fibronectin appeared in mesoderm cells and as a band between the two germ layers. In the developing amnion and chorion, the protein was localized between the ectodermal and mesodermal cell layers. The results indicate that fibronectin is an early differentiation market for the stage of endoderm formation in the inner cell mass of the mouse blastocyst. It is also a marker of mesoderm appearance and seems to be associated with the accumulating extracellular matrix material in the developing embryo.