Anatomy, histology, and development of the cardiac valvular system in elasmobranchs.

We report here on the anatomy, histology, and development of the three sets of cardiac valves in embryonic and adult elasmobranch fishes. The sinus venosus is the first segment of the heart to receive blood, and a pair of sinoatrial (SA) valves prevent backward flow of blood into the sinus venosus. The SA valves derive from two dorsolateral infoldings of the cardiac wall and consist of a simple endocardium covering transverse sheets rich in collagen.