Embryonic development of the heart. II. Formation of the epicardium.

The mature heart may be thought of as consisting of three layers, endocardium, myocardium, and an outer investing tissue called the epicardium. During early formation of the tubular heart of chick embryos, at about the 8-somite stage, two tissue layers become clearly discernible with the light microscope: the endocardium and the developing myocardial wall. The outer epicardial layer does not appear until later in development. It is generally accepted that embryonic heart wall or 'epimyocardium' is composed of muscle and undifferentiated cells. As its name implies, the epimyocardium is thought to give rise to myocardium and epicardium. Kurkiewicz (1909) suggested that the epicardium was not an epimyocardial derivative but rather is formed from cells originating in the sinus venosus region, which migrate over the surface of the heart. Nevertheless, it has become generally accepted that the outer cell layer of the embryonic heart wall differentiates in situ to give rise to the definitive visceral epicardium (Patten, 1953). For reviews, see Romanoff (1960) and DeHaan (1965). An earlier light and electron microscopic study of cardiogenesis in the chick demonstrated (Manasek, 1968) that by stage 1 2 (15-somites) the heart wall was composed only of cells that contained myofibrils, and that no epicardial cells were present. It was suggested that the heart wall could not give rise to epicardium unless dedifferentiation occurred, thus supporting Kurkiewicz's (1909) observations. These findings also contradict Bruno's (1918) observations which suggested that epicardial differentiation began at the 13-somite stage. In the present study, the early epicardium was examined in greater detail, and its relationship to the developing myocardial wall was investigated. Techniques of light and electron microscopy were utilized to elucidate both the cytology of the developing epicardial cells and the histogenesis of this tissue.