An anatomical study of 54 specimens with endocardial cushion defect is described. The material was divided into two groups, according to the morphology and attachments of the anterior component of the anteroseptal mitral leaflet. We suggest that there is no divided or undivided anterior common leaflet. There are always two anterior leaflets, a mitral and a tricuspid one, separated by a commissure; depending on the degree of straddling of the mitral component over the interventricular septum, this will protrude into the interventricular septum or into the anterior papillary muscle of the right ventricle. Thus, the commissure separating the mitral and the tricuspid components will lie over the interventricular septum or will be quite separate in the right ventricle. In the latter case the straddling mitral component gives the false appearance of an undivided anterior common leaflet. The septal tricuspid leaflet was found to be underdeveloped or absent but we were not able to find any clefts or divisions in it. We suggest that this leaflet derives solely from the division of the posteroinferior endocardial cushion into two halves, right and left. The left half later becomes the posterior component of the normal anteroseptal mitral leaflet. Associated anamalies are also reviewed.
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