Scanning electron microscopy substantiates histology in showing the inadequacy of the existing theories on the development of the proximal coronary arteries and their connections with the arterial trunks.

Development of proximal coronary arterial segments and coronary arterial orifices was studied by scanning electron microscopy in 20 rat embryos and by light microscopy in serial sections of 20 human and another 18 rat embryos. Neither by scanning electron microscopy nor by light microscopy did we observe more than two coronary arterial orifices. These coronary orifices were always situated in the sinuses of the aorta that faced the pulmonary artery. In the human embryos the coronary orifices emerged between 37-39 days of gestation (16-19 mm crown-rump length, Streeter horizon XVIII-XIX) and were invariably present beyond 39 days (19 mm crown-rump length, Streeter horizon XIX). In rat embryos, the coronary orifices emerged in both scanning electron microscopy and light microscopy at 15-17 days of gestation (13-17 mm crown-rump length) and were invariably present beyond 17 days (17 mm crown-rump length). In both human and rat embryos, either by scanning electron microscopy and light microscopy, the left coronary orifice was observed significantly earlier. In all the investigated embryos, human as well as rat, septation at arterial orifice level was complete, including the earliest stages studied. Light microscopy showed that at the emerging stages of the coronary orifices, the proximal epicardial segments of the left and right coronary arteries could already be identified in a peritruncal ring of epicardial vasculature, before the coronary orifice was observed. This was the case in human as well as in rat embryos. Thus, a coronary orifice was never seen in the absence of a proximal coronary artery. The present theories on development of the proximal coronary arteries and coronary orifices do not offer an adequate explanation for either these data or the known possible congenital abnormalities of the coronary arteries. Our study supports dual proximal coronary arterial development. These two proximal coronary arteries develop out of a peritruncal ring of vascular structures on to the aorta. The process by which the coronary orifices actually develop remains to be explained.