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 canicula) in six serially sectioned embryos ranging from 14 to 40 mm in total length. Our preliminary results show some significant similarities with the cardiac development of higher vertebrates, in spite of about 400 millions years of divergent evolution. The dogfish cardiac tube is composed of endocardium and myocardium separated by a thick layer of cardiac jelly. Large clefts form in the atrial and ventricular myocardium before the cardiac jelly disappears. These clefts seem to be related to the origin of the intertrabecular sinusoids. Myocardial pores in the sinus venosus and atrium might allow the flow of some cardiac jelly to the subepicardial space. Two atrioventricular and three conal endocardial cushions are formed by epithelial-mesenchymal transformation. The atrioventricular and conal valves seem to develop from these cushions, while the sinoatrial valve seems to derive from two transversal infoldings of the cardiac wall. The epicardium forms from mesothelial cells proceeding 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 large annular veins around the atrioventricular and conoventricular grooves. The veins connect with ventricular sinusoids and the sinus venosus lumen.