Visceral endoderm induces specification of cardiomyocytes in mice

The endoderm plays an inductive role in the formation of cardiomyocytes in many vertebrates. Here, we provide further evidence for this in the mouse and demonstrate enhanced cardiomyogenesis in mouse embryonic stem cells cultured in the presence of native visceral endoderm. Isolated mesoderm from late-primitive streak stage mouse embryos that still have an open proamniotic canal had a reduced capacity to form cardiomyocytes after 4 days in culture compared with mesoderm isolated from later stages but prior to cardiomyogenesis. Moreover, removal of the visceral endoderm but not the primitive streak reduced the formation of beating areas in embryo explants in culture. Coculture with the END2 cell line, which has visceral endoderm-like properties, restored the formation of beating areas. Immunohistochemical analysis showed that the expected candidate signaling pathways downstream of Wnts and bone morphogenetic proteins (BMPs) were active in the embryo at the appropriate time and place to be involved. Overall, the results show that, as in other vertebrates, the (visceral) endoderm plays an important role in the early events of mouse cardiomyogenesis.