Embryonic development of the proepicardium and coronary vessels.

In the last few years, an increasing interest in progenitor cells has been noted. These cells are a source of undifferentiated elements from which cellular components of tissues and organs develop. Such progenitor tissue delivering stem cells for cardiac development is the proepicardium. The proepicardium is a transient organ which occurs near the venous pole of the embryonic heart and protrudes to the pericardial cavity. The proepicardium is a source of the epicardial epithelium delivering cellular components of vascular wall and interstitial tissue fibroblasts. It contributes partially to a fibrous tissue skeleton of the heart. Epicardial derived cells play also an inductive role in differentiation of cardiac myocytes into conductive tissue of the heart. Coronary vessel formation proceeds by vasculogenesis and angiogenesis. The first tubules are formed from blood islands which subsequently coalesce forming the primitive vascular plexus. Coronary arteries are formed by directional growth of vascular protrusions towards the aorta and establishing contact with the aortic wall. The coronary vascular wall matures by attaching smooth muscle cell precursors and fibroblast precursors to the endothelial cell wall. The cells of tunica media differentiate subsequently into vascular smooth muscle by acquiring specific contractile and cytoskeletal markers of smooth muscle cells in a proximal - distal direction. The coronary artery wall matures first before cardiac veins. Maturity of the vessel wall is demonstrated by the specific shape of the internal surface of the vascular wall.