Early differentiation of the heart in mouse embryos

The differentiation of the heart was examined in mouse embryos, isolated between the afternoons of the eighth and ninth day of gestation, by means of scanning electron microscopy as well as by light microscopy of standard histological material and thin plastic sections. Cavitation within the presumptive pericardial mesoderm commences during the late presomite stage (afternoon of eighth day) and myocardial rudiments are established by thickening of the pericardial lining adjacent to the endoderm near the base of the foregut pocket. At first the thickening lies on the ventral aspect of the pericardial cavity but, as the foregut pocket deepens and the trunk region of the embryo moves en masse caudally and ventrally, it comes to lie on the dorsal wall. A separate layer of cells comprising the primitive endocardium differentiates deep to the myocardial thickening, between the latter and the endoderm. Cavitation of the endocardial tissue occurs bilaterally about the 1-2 somite stage so as to establish bilateral endocardial tubes. These endocardial tubes become enclosed within the myocardial mantle which bulges into the pericardial cavity. A layer of acellular material (the cardiac jelly) appears between the endocardium and myocardium at about this time. At about the 4-somite stage, the endocardial tubes coalesce across the mid-line to establish a single median cardiac tube, and the entire heart rudiment is suspended in the mid-line by a wide dorsal mesocardium. Our observations indicate that, contrary to previous views, at no time during cardiogenesis in the mouse are two distinct heart primordia present which subsequently fuse to form a single median entity. The principal similarities and differences between cardiogenesis in the mouse, and mammals in general, and avians are discussed.