Anterior endoderm is a specific effector of terminal cardiac myocyte differentiation of cells from the embryonic heart forming region

The ability of anterior lateral plate mesoderm cells in the heart-forming region (HFR) of stage 6 chicken embryos to respond to cardiogenic stimuli from cells in adjacent germ layers has been investigated using explants cultured under defined conditions. Two types of explantation were evaluated: those in which two germ layers were explanted in contiguity, and those in which germ layers were isolated and co-cultured. Two parameters--contractility and expression of sarcomeric alpha-actin--were monitored to evaluate the terminal differentiation of cardiac myocytes. Contiguously explanted anterior endoderm/mesoderm became multilayered and underwent terminal differentiation within 2 days. By contrast, although contiguous anterior ectoderm/mesoderm or posterior endoderm/mesoderm co-explants also became multilayered, these explants did not differentiate, up to 5 days. To ascertain the cardiogenic potential of cells from different regions of the embryo, individual germ layers were isolated and co-cultured by placing the explants in separate areas of the culture chamber. These determinations demonstrated that anterior, but not posterior, endoderm effected differentiation of anterior mesoderm. As before, mesoderm in both types of co-culture survived and became multilayered; by contrast, mesoderm did not survive when cultured in isolation. These experiments provide evidence that anterior endoderm regulates the terminal differentiation, as opposed to growth, of presumptive cardiac myocytes in mesoderm cells from the anterior lateral plate. Finally, anterior endoderm was co-cultured with mesoderm from the posterior half of the embryo, which does not contain an HFR. The failure of these co-cultured explants to differentiate infers that pre-cardiac myoblasts in stage 6 anterior mesoderm are previously specified to respond to the terminal cardiogenic effects of endoderm.