Early development of quail heart epicardium and associated vascular and glandular structures.

As in the other vertebrates the epicardium of the quail embryo develops from proepicardial tissue located between the sinus horns and the liver primordium. The cuboidal cells of the coelomic lining above the proepicardium are transformed into mesothelial cells which in cooperation with the underlying mesenchymal cells elaborate a large quantity of extracellular matrix, so producing the villous outgrowths of the proepicardium. The mesenchymal cells of this area are attached to each other with typical desmosomes and have anti-alpha cytokeratin-stained tonofilament bundles. These cells resemble keratinocytes and are designated as proepicardial matrix keratinocytes. The proepicardium proliferates first in the sulci of the U-shaped tubular heart, and within 2 days (between stages 15-25) establishes the visceral layer of the epicardium. The proliferating proepicardium consists of gland-like tubular strands, formed by the invaginations of the surface mesothelial cells, mesenchymal cells, fibroblasts, angioblasts, blood cells and capillaries. Because of its heterogeneous structure and multiple functions, the proepicardium is considered a transitory organ of the developing heart. In the quail embryo the forerunners of the coronary vessels grow from the perihepatic area into the proepicardial organ, and when the epicardial covering is completed, but before the coronary artery orifices open, these primordial vessels form a subepicardial and intramural vascular network in the ventricular myocardium. After the completion of the epicardial covering the proepicardium involutes and is not seem from stage 26 onward.