Rush Center for Congenital
and Structural Heart Disease




Pericardial sac and coronary arteries

The right and left intracelomic cavities approach mid-line as the two heart tubes are fusing into a single heart tube at about day 21 of development.  The two cavities approach each other and surround the heart tube.  The ventral mesoderm is immediately absorbed and the two cavities communicate.  The dorsal mesoderm persists till day 25.  After the mesoderm is absorbed, the heart becomes suspended from the cranial and caudal ends.

The proepicardial organ is a villous structure located at the dorsal aspect of the atrioventricular sulcus.  Cells from this organ envelop the heart to form the epicardium.  This is followed by elaboration of extracellular matrix into the subepicardial space.  This space then becomes populated by mesenchymal cells from the proepicardial organ and epicardial cells.  These mesenchymal cells eventually differentiate into coronary endothelial and smooth muscle cells as well as myocardial fibroblasts.  The coronary arterial plexus forming in the subepicardial space invades the myocardium to form the coronary vascular tree which eventually connects to the aorta through the periaortic plexus of coronary vessels.  It has been shown that the coronary arterial vasculature grows into the aorta and not the other way round, such as through aortic outgrowth as previously thought.  Manipulations interfering with normal epicardial development results in abnormal coronary plexus development. [373]

A band of connective tissue grows from the epicardium into the atrioventricular junction when the heart is four chambered, resulting in separation of atrial and ventricular myocardium, and the bundle of his remains the only mean of electrical conduction from atria to ventricles.

The SA node, AV node and the bundle of His receive sympathetic and parasympathetic nervous supply throughout the rest of gestation and even after birth to complete the development of the cardiac conduction system.