The venous components have disparate origins The systemic venous component is initially formed from primary myocardium The pulmonary venous component, along with the atrial septum, in contrast, is derived from the mediastinal myocardium Atrioventricular Canal In early stages, the junction between the developing atrial component and the inlet of the ventricular loop is a canal with finite length (see Fig 3.13), which is then septated by fusion of the superior and inferior atrioventricular endocardial cushions (Fig 3.17) These develop within the canal by the process of epithelialto-mesenchymal transformation The cushions will eventually provide the foundations for formation of the aortic leaflet of the mitral valve, and the septal leaflet of the tricuspid valve They also contribute to closure of the interventricular foramen, forming in the process the membranous part of the septum Only subsequent to delamination of the septal leaflet of the tricuspid valve, a relatively late event, does this fibrous part become separated into its atrioventricular and interventricular portions.10 Much has been learned in recent years concerning the epithelial-to-mesenchymal transformations that take place during formation of both the atrioventricular and the outflow cushions,11 but as indicated we are confining our attention to the morphologic changes In this regard, it had been thought that failure of fusion of the atrioventricular cushions underscored the development of hearts with atrioventricular septal defect and common atrioventricular junction Hearts can develop with all the features of atrioventricular septal defect, however, when the cushions themselves have fused The problem underscoring the abnormality, therefore, is one that permits the retention of the common junction, usually involving failure of growth of the vestibular spine.12 Normally, nonetheless, the atrioventricular cushions do fuse so as to divide the atrioventricular canal into right-sided and left-sided channels (see Fig 3.17) With ongoing development, the initial musculature of the atrioventricular canal becomes sequestrated on the atrial aspect of the plane of atrioventricular insulation to form the right and left atrial vestibules (Fig 3.18).13 FIG 3.17 Scanning electron micrograph showing the atrioventricular junctions of the developing human heart at Carnegie stage 16 The image shows the ventricular aspect of the atrioventricular canal just prior to fusion of the atrioventricular (AV) endocardial cushions