Embryonic development of the bulbus arteriosus of the primitive heart of jawed vertebrates

Recent work has shown that the outflow tract (OFT) of the chondrichthyan heart does not consist of a single component, the conus arteriosus, as has been classically accepted, but two, the myocardial conus arteriosus and the non-myocardial bulbus arteriosus. The aim of this study was to describe the embryonic development of the bulbus, under the assumption that the heart of extant chondrichthyans represents the primitive condition of this organ in jawed vertebrates. The study was carried out in Atlantic sawtail catshark embryos belonging to developmental stages 20–34 of Ballard et al. (1993). They were examined using histochemical and immunohistochemical techniques for light and fluorescence microscopy. The entire OFT displays a myocardial character during early development. At stage 28, a new, non-myocardial segment forms at the distal end of the OFT. This segment, which is the precursor of the bulbus arteriosus, shows positive reactivity for 4,5-diaminofluorescein 2-diacetate throughout development from stage 31. Smooth muscle α-actin-positive cells appear in the prospective bulbus at stage 33; they markedly increase in number during stage 34. Subsequently, they extend backwards to overlap with the conal myocardium, giving rise to a myocardium and smooth muscle interface that has been described elsewhere as the middle component of the vertebrate OFT. Our embryological findings in a elasmobranch species are consistent with the notion that a second heart field has existed since the beginning of the jawed vertebrate radiation and that the bulbus arteriosus of chondrichthyans is a second heart field-derived structure.