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-form ing 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 cont iguity, and those in which germ layers were isolated and cocultured. T wo parameters-contractility and expression of sarcomeric alpha-actin-w ere monitored to evaluate the terminal differentiation of cardiac myoc ytes. Contiguously explanted anterior endoderm/mesoderm became multila yered and underwent terminal differentiation within 2 days. By contras t, although contiguous anterior ectoderm/mesoderm or posterior endoder m/mesoderm co-explants also became multilayered, these explants did no t 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 area s of the culture chamber. These determinations demonstrated that anter ior, 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 cultu red in isolation. These experiments provide evidence that anterior end oderm regulates the terminal differentiation, as opposed to growth, of presumptive cardiac myocytes in mesoderm cells from the anterior late ral plate. Finally, anterior endoderm was co-cultured with mesoderm fr om the posterior half of the embryo, which does not contain an HFR. Th e failure of these co-cultured explants to differentiate infers that p re-cardiac myoblasts in stage 6 anterior mesoderm are previously speci fied to respond to the terminal cardiogenic effects of endoderm. (C) 1 994 Wiley-Liss, Inc.