The inadequacy of existing theories on development of the proximal coronary arteries and their connexions with the arterial trunks.

Coronary arterial development was studied in complete microseries of 20 human embryos and microseries of the hearts from 18 rat embryos. We never observed more than two coronary arterial orifices; these always originated from the facing aortic sinuses. In the human embryos these coronary orifices were variably identified between 16-19 mm crown-rump length, but were invariably present above 19 mm crown-rump length. In rat embryos, the orifices were variably identified at 13-17 mm and invariably present above 17 mm crown-rump length. In both human and rat embryos the left coronary orifice was observed significantly earlier. In all the embryos septation at arterial orifice level was complete. At the stages in which identification of the coronary orifices was variable, the proximal epicardial segments of the left and right coronary arteries could usually already be identified, in human as well as in rat embryos. On the other hand, a coronary orifice was never seen in the absence of a proximal coronary artery. At all stages studied (in human embryos from 10 mm crown-rump length and in rat embryos from 11 mm crown-rump length) vascular structures could be identified in the epicardial covering of the heart. The present theories on proximal coronary artery development are inadequate to explain either these data or the known possible congenital abnormalities of the coronary arteries. Our study offers a detailed chronology of development of these proximal coronary arteries and mostly supports dual coronary arterial development. The process by which the coronary orifices are brought into contact with the main coronary arteries still remains to be explained.