During cardiac development, interactions between the endocardium – the endothelial lining of the heart – and the overlying myocardium are important for myocardial development, whereas BMP signals from the myocardium have been found to regulate late stages of endocardial morphogenesis. However, relatively little is known about early endocardial specification, or the potential role of the myocardium in this. Hedgehog signalling is known to be necessary but not sufficient for endocardial differentiation, but what other mechanisms are involved? Saulius Sumanas and colleagues now show (p. ) that myocardium-derived BMP plays a role in this early stage of endocardial development. When the myocardium is disrupted – either in hand2 mutants or upon genetic ablation of myocardial cells – endocardial fate fails to be specified or is lost in the endothelial progenitors. Expression of endocardial markers can be rescued by ectopic provision of BMP signals, while endocardial differentiation fails upon disruption of the BMP pathway. These data add a further layer of interaction between endocardium and myocardium, and underscore the importance of BMP signalling at multiple stages of heart development.
