Evolution of the building plan of the heart.

In the hearts of mammals and birds, a system of nodes and fast-conducting tissues govern the activation pattern that induces myocardial contractions. We have previously shown that the atrioventricular conduction axis, consisting of the atrioventricular junction and the ventricles, is evolutionary conserved. From fish to man, the embryonic heart is patterned into compartments by the same genes. It has a muscular and slow conducting atrioventricular canal (Tbx2/3 positive) and a spongy ventricle (Cx40 and Anf positive) that serves the dual purpose of fast conduction and contraction. This design is maintained in fishes, amphibians and reptiles, whereas mammals and birds develop a fibrous insulating plane, thick compact ventricular walls and septum and, hence, form a discrete ventricular (His-Purkinje) conduction system from the embryonic spongy ventricle. Our understanding of the cardiac pacemaker, seen as a sinus node in mammals, is less clear. Most vertebrates have myocardium upstream of the right atrium (atrium in fishes) called the sinus venosus. It remains disputed if the dominant pacemaker of the heart is a property of the sinus venosus in entirety or of a specialised subset of sinus venosus myocardium. In the anole lizard the sinus venosus consists of three sinus horns that together equal the size of the right atrium. We investigated the electrical activation of the sinus venosus with optical mapping. Activation was earliest in the posterior sinus horn and then spread to the anterior sinus horns both of which are guarded by valves at the sinus-venous border. Later, the atria were activated from the dorsal sinu-atrial border, where the sinus node of mammals and birds is located. This region expressed the conserved sinus node markers Isl1 and Tbx3. We conclude that chambers and border regions of the vertebrate heart have highly similar patterns of gene expression and phenotypes, and that the cardiac conduction system simply follows from this building plan. Consequently, modifications rather than novelties characterise cardiac evolution. [ABSTRACT FROM AUTHOR]