Structure of the coronary arteries during the prenatal period in man.

The coronary arteries of human foetuses from the 4th to 10th month of gestation were examined in the light and the electron microscope. Marked changes were demonstrated in the course of prenatal development, particularly in the media and adventitia (thickening, an increase in the number of collagen fibrils). In regions of coronary artery branching, here and there we found intimal pads or diffuse thickening of the intima, which were separated from the media by a mostly thin or discontinuous inner elastic membrane with an accumulation of smooth muscle cells and a smaller quantity of elastic fibrils. The intimal thickenings, which occurred mostly in the region of bifurcation of the left coronary artery and at sites where branches sprang from the r. ventricularis anterior of the left coronary artery, were already discernible at the beginning of the period in question (i.e. in the 4th month). In some cases, oedematous changes were found in the pads, where they were characterized by dilatation of the elastic membranes and smooth muscle cells by proliferating structureless matter.