Formation of the epicardium studied with the scanning electron microscope

The topographical characteristics of epicardial and myocardial cells of the embryonic chick heart are sufficiently distinct to enable reproducible identification by scanning electron microscopy. This made it possible to observe the development of the epicardial investment on the myocardial wall. The epicardial cells migrate from the mesothelium of the sinus venosus, cover first circularly the ventricular wall, and then extend cranially and caudally to ensheath the entire heart surface. Our observation argues against the generally accepted concept that the epicardium is a derivative of the outer myocardial layer. We strongly support the suggestion that the term “epimyocardium” is a misnomer [Manasek, F. J. (1969). J. Embryol. Exp. Morphol.22, 333–348].