Parietal and visceral endoderm differ in their expression of intermediate filaments

Two layers of extra-embryonic endoderm, viz. the parietal endoderm (PE) and the visceral endoderm (VE), arise in the mouse embryo shortly after implantation. Both cell populations apparently originate from the primitive endoderm of the blastocyst. While the endoderm differentiation has been studied both in the embryo and in the embryonal carcinoma model system, the investigation has been hampered by the paucity of unequivocal markers of differentiation, especially in the case of the PE. Here we show that the PE and VE of mouse conceptuses differ in their expression of intermediate filaments: while both cell types contain cytokeratin, expression of vimentin was only revealed in the cells of the PE. The association between the differentiation of PE and the appearance of vimentin filaments is discussed.