The molecular cues that control patterning of the heart tube during early cardiogenesis are largely unknown. The present study has explored the embryonic stem (ES) cell differentiation system to determine if this in vitro model could be useful in studying the process of regional specification of cardiac muscle cells at the earliest possible stages. As assessed by polymerase chain reaction, ribonuclease protection, in situ hybridization, and immunohistochemical analyses, ES cell differentiation

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... nto embryoid bodies is characterized by the transcriptional and translational activation of the ventricular regulatory (phosphorylatable) myosin light chain gene, demonstrating that ventricular specification occurs during ES cell cardiogenesis. The finding of a ventricular-specific marker in an in vitro system in the absence of an intact heart tube provides evidence for cardiac regional specification independent of positional cues or physiologic stimuli. The temporal expression of the myogenic regulatory factors, myogenin and MyoD, suggests activation of the skeletal muscle program following cardiac myogenesis in vitro, indicating temporal fidelity to the progression of in vivo myogenesis. These data establish the mouse embryonic stem cell system as a model for cardiac chamber specification and suggest a promising approach in the study of regional specification in genetically engineered cardiac muscle cells.