Development of myocardial vascularisation in the rat.

In rat, the development of the vascularisation of the myocardium starts in an embryo of 13 days, as a primarily solitary vascular plexus situated in the subepicardial mesenchyme. This plexus spreads out between the cells of the marginal layer of the myocardium of the ventricles. Thereafter it comes into contact with the intertrabecular spaces and with some endothelial offshoots of the left sinus horn. At day 15, just before the entire septation of the left and right arterial parts of the heart, this plexus contacts offshoots of the aorta. The myocardial trabeculae continue to grow longer from an enlarging base. In conjunction with this occurrence, there is a shift into the direction of the cells, resulting in a three-layered myocardium; from insided outwards a radial (trabecular) layer, a radio-tangential layer and a marginal layer can be found. A narrowing down of the intertrabecular spaces is another consequence of the enlarging base. The spaces deflect parallel to the radio-tangential layer. Thus a second plexus, deeply embedded in the myocardium, comes into existence. At day 19 a histological differentiation of the wall of the interconnected subepicardial-myocardial plexus to either arteries or veins will start. In all stages, the vascular system runs parallel to the myocardial structures of the ventricles of the heart.