Chondrichthyans have a bulbus arteriosus at the arterial pole of the heart

Abstract

The aim was to test our hypothesis that the bulbus arteriosus is a genuine arterial pole component of the primitive heart of gnathostomates. We examined adult and embryonic hearts of a number of chondrichthyan species by means of histochemical and immunohistochemical techniques. In all cases, a conspicuous intrapericardial, nonmyocardial, tubular segment connected the myocardial conus arteriosus with the ventral aorta. This segment could be well distinguished from the aorta because it 1) showed a different arrangement of the tissue layers; 2) was covered by epicardium; and 3) was crossed by the coronary arteries. Experimental in vivo labelling with the fluorescent nitric oxide marker 4,5‐diaminofluorescein diacetate indicated that this segment is homologous to the bulbus arteriosus of teleosts. Therefore, we conclude that the bulbus arteriosus is not an apomorphy of the actinopterygians. It appeared in an earlier period of the vertebrate story. In addition, our findings, together with data from the literature, suggest that, from the evolutionary view, the bulbus arteriosus of the chondrichthyans is the precursor of the basal portions of the aorta and pulmonary artery of tetrapods, a fact which raises the question whether there is a secondary heart field in phylogenetically basal vertebrates. This study was supported by grant CGL06‐00693 (Ministerio de Educación y Ciencia, Spain).