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In the first part of our review of cardiac development,1 we discussed the initial changes involved in transformation of the heart forming regions of the embryo into the great veins, the atrial and ventricular chambers, and the arterial trunks. We showed that this first part of cardiac development could be divided into phases of formation of the primary myocardial tube, looping of the tube, during which additional parts are added that give the future compartments their definitive topography, and the assembly of these components into the cardiac chambers and arterial trunks. In this second review, we discuss the processes that complete the separation of the two sides of the definitive heart, for the most part involving septation of the parts of the primary tube not themselves directly involved in formation of the chamber-specific compartments of the atriums and ventricles. In this respect, when concluding our first review, we explained how the arterial trunks were also formed by septation within the solitary outflow tract of the primary heart tube. We also showed, however, that subsequent to formation of the two arterial trunks, there was disappearance of the cushions that initially divided them. Thus, in the definitive heart, the proximal parts of the aorta and pulmonary trunk, along with the sinuses of the arterial roots and the subpulmonary infundibulum, possess their own discrete walls, separated by extra-cardiac space. Although septation by fusion of endocardial cushions will be a topic included in this second review, septation and separation of the outflow tracts is sufficiently complicated to require special treatment. Because of this, we will devote a third review specifically to the mechanisms underscoring the remodelling of the outflow tracts. In this review, therefore, we will confine our considerations to the formation of the atrial, atrioventricular, and ventricular septal structures.
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