Remodeling of the Embryonic Interventricular Communication in Regard to the Description and Classification of Ventricular Septal Defects.

Ventricular septal defects are the commonest congenital cardiac malformations. Appropriate knowledge of the steps involved in completion of ventricular septation should provide clues as to the morphology of the different phenotypes. Currently, however, consensus is lacking regarding the components of the developing ventricular septum, and how best to describe the different phenotypes seen in postnatal life. We have reassessed the previous investigations devoted to closure of the embryonic interventricular communication. On this basis, we discuss how studies in the early part of the 20th century correctly identified the steps involved in the remodeling of the embryonic interventricular foramen subsequent to the stage at which the outflow tract arises entirely above the cavity of the developing right ventricle. There has, however, already been remodeling of the foramen from the stage at which the atrioventricular canal is supported exclusively by the developing left ventricle. We show how these temporal changes in morphology can provide explanations for the different ventricular septal defects seen in the clinical setting. Thus, muscular defects represent inappropriate coalescence of muscular ventricular septum. The channels that are perimembranous are due to failure of closure of the persisting embryonic interventricular foramen. Those that are doubly committed and juxta-arterial reflect failure of formation of the free-standing subpulmonary muscular infundibular sleeve. The findings also point to the importance of appropriate alignment, during development, between the developing atrial and ventricular septums, and between the apical component of the ventricular septum and the ventricular outlet components. Anat Rec, 302:19-31, 2019.