The epicardium has several essential functions in development of cardiac architecture and differentiation of the myocardium in vertebrates. We uncovered a novel function of the epicardium in species with partial or complete ventricular septation including reptiles, birds and mammals. Most reptiles have a complex ventricle with three cava, partially separated by the horizontal and vertical septa. Crocodilians, birds and mammals, however, have completely separated left and right ventricles, a clear example of convergent evolution. We have investigated the mechanisms underlying epicardial involvement in septum formation in embryos. We find that the primitive ventricle of early embryos becomes septated by folding and fusion of the anterior ventricular wall, trapping epicardium in its core. This ‘folding septum’, as we propose to call it, develops in lizards, snakes and turtles into the horizontal septum and, in the other taxa studied, into the folding part of the interventricular septum. The vertical septum, indistinct in most reptiles, arises in crocodilians and pythonids at the posterior ventricular wall. It is homologous to the inlet septum in mammals and birds. Eventually, the various septal components merge to form the completely septated heart. In our attempt to discover homologies between the various septum components, we draw perspectives to the development of ventricular septal defects in humans.
Copyright 2016, The Author(s).