Abstract-We used a genetic lineage-labeling system to establish the material contributions of the progeny of 3 specific cell types to the cardiac valves. Thus, we labeled irreversibly the myocardial (␣MHC-Creϩ), endocardial (Tie2-Creϩ), and neural crest (Wnt1-Creϩ) cells during development and assessed their eventual contribution to the definitive valvar complexes. The leaflets and tendinous cords of the mitral and tricuspid valves, the atrioventricular fibrous continuity, and the leaflets of the outflow tract valves were all found to be generated from mesenchyme derived from the endocardium, with no substantial contribution from cells of the myocardial and neural crest lineages. Analysis of chicken-quail chimeras revealed absence of any substantial contribution from proepicardially derived cells. Molecular and morphogenetic analysis revealed several new aspects of atrioventricular valvar formation. Marked similarities are seen during the formation of the mural leaflets of the mitral and tricuspid valves. These leaflets form by protrusion and growth of a sheet of atrioventricular myocardium into the ventricular lumen, with subsequent formation of valvar mesenchyme on its surface rather than by delamination of lateral cushions from the ventricular myocardial wall. The myocardial layer is subsequently removed by the process of apoptosis. In contrast, the aortic leaflet of the mitral valve, the septal leaflet of the tricuspid valve, and the atrioventricular fibrous continuity between these valves develop from the mesenchyme of the inferior and superior atrioventricular cushions. The tricuspid septal leaflet then delaminates from the muscular ventricular septum late in development.

Questions on the embryonic origin and developmental significance of the epicardium did not receive much recognition for more than a century. It was generally thought that the epicardium was derived from the outermost layer of the primitive myocardium of the early embryonic heart tube. During the past few years, however, there has been an increasing interest in the development of the epicardium. This was caused by a series of new embryological data. The first data showed that the epicardium did not derive from the primitive myocardium but from a primarily extracardiac primordium, called the proepicardial serosa. Subsequent data then suggested that the proepicardial serosa and the newly formed epicardium provided nearly all cellular elements of the subepicardial and intermyocardial connective tissue, and of the coronary vasculature. Recent data even suggest important modulatory roles of the epicardium and of other proepicardium-derived cells in the differentiation of the embryonic myocardium and cardiac conduction system. The present paper reviews our current knowledge on the origin and embryonic development of the epicardium.