CARDIAC DEVELOPMENT IN THE DOGFISH (SCYLIORHINUS-CANICULA) - A MODEL FOR THE STUDY OF VERTEBRATE CARDIOGENESIS

We have studied the cardiac development of the dogfish (Scyliorhinus c anicula) in six serially sectioned embryos ranging from 14 to 40 mm in total length. Our preliminary results show some significant similarit ies with the cardiac development of higher vertebrates, in spite of ab out 400 millions years of divergent evolution. The dogfish cardiac tub e is composed of endocardium and myocardium separated by a thick layer of cardiac jelly. Large clefts form in the atrial and ventricular myo cardium before the cardiac jelly disappears. These clefts seem to be r elated to the origin of the intertrabecular sinusoids. Myocardial pore s in the sinus venosus and atrium might allow the flow of some cardiac jelly to the subepicardial space. Two atrioventricular and three cona l endocardial cushions are formed by epithelial-mesenchymal transforma tion. The atrioventricular and conal valves seem to develop from these cushions, while the sinoatrial valve seems to derive from two transve rsed in-foldings of the cardial wall. The epicardium forms from mesoth elial cells processing first from the liver and sinus venosus lining, and then from the developing septum transversum. A subepicardial space appears early and it is populated by mesenchymal cells which seem to proceed at least partly from the epicardium. These subepicardial cells apparently form capillary-like structures some of which coalesce in l arge annular veins around the atrioventricular and conoventricular gro oves. The veins connect with ventricular sinusoids and the sinus venos us lumen.