Contribution to the development of the heart, Part II: Morphogenesis of congenital heart diseases.

A concept for the explanation of the formal genesis of congenital cardiac malformations is introduced, based on the observation that 1. the septa in the outflow-tract of the heart are formed in spatial relation to changes in form of the external cardiac wall and always along the longer sides of the ovally deformed pathway of flow, 2. the muscular septa (ventricular and atrial septa) arise as protrusions due to different directions of ventricular growth. Accordingly, no septa develop when normal deformation fails to occur (e.g. truncus communis); septa are incompletely developed when the deformation is insufficient (e.g. aortico-pulmonary window); septa are formed in wrong positions where the direction of deformation deviates more or less from the normal direction (as in stenosis due to slight deviation, or transposition due to large deviation from the normal). The so-called ventricular inversions and atrio-ventricular discordance are regarded as consequences of dystopic development of the ventricular septum. This concept for the explanation of the formal genesis of cardiac malformations is exemplified by variants of transpositions (anatomically corrected and uncorrected), and by variants of double-outlet-right-ventricle and double-outlet-left-ventricle. Using a common anomaly (tetralogy of Fallot) and 2 rare ones (double-outlet-right-ventricle with complete AV canal, and criss-cross-heart) it is shown that the interpretation of the development of cardiac malformations as a consequence of dystopic development of the septa allows an understanding of the formal genesis of complex malformations without involving hitherto unproven developmental processes.