The role of the epicardium and neural crest as extracardiac contributors to coronary vascular development.

At species-specific times in embryonic development, the pro-epicardial organ appears as an outcropping of the mesothelial body wall, near the sinus venosus-liver region. The pro-epicardial vesicles attach to the myocardium, flatten, and join to form the epicardium. The epicardium shows epithelial-mesenchymal transformation: cells detach from the epithelium, fill the subepicardial space, and invade the heart tube. Epicardium-derived cells migrate as far as the core of the endocardial cushions, which differentiate into the atrioventricular valve leaflets. In the cardiac wall, other epicardium-derived cells differentiate into interstitial fibroblasts and adventitial and smooth muscle cells of the coronary arteries. Using neural crest tracings in mouse embryos (Wnt1-Cre-lacZ), we studied the patterning of cardiac neural crest cells during development. Participation of neural crest cells in the formation of the vascular media could not be excluded, although epicardium-derived cells have hitherto been considered responsible for formation of the coronary arterial smooth muscle cells. The endothelial cells of the coronary network derive mostly from the endothelium of the sinus venosus-liver region by vasculogenesis and angiogenesis. However, an epicardium-derived cell origin of some endothelial cells cannot be ruled out. The coronary vasculature is closely related to the differentiating Purkinje network, but isolated epicardium-derived cells are also associated with Purkinje cells. After ablating the pro-epicardial organ in quail embryos, we found severe malformations in the myocardial architecture, leading to the hypothesis that epicardium-derived cells give instructive signals to the myocardium for proper differentiation of the compact and the trabeculated compartments.