The fibrous skeleton in the human heart: embryological and pathogenetic considerations.

The fibrous skeleton of the human heart is composed of several parts which are formed from different, mainly extracardiac sources. The atrioventricular valve rings are formed by invagination of sulcus tissue at the atrioventricular and bulboventricular transitions. The atrial part of the central fibrous body is formed from an ingrowth of tissue from the dorsal mesocardium, the ventricular part from ingrowth of sulcus tissue towards an endocardial structure, the left bulbar ridge. During valve development the atrioventricular endocardial cushions, initially situated between the developing atrial and ventricular parts of the central fibrous body, are almost completely removed downwards into the ventricular cavities. However, a small portion of these cushions remains on top of the ventricular septum, is "trapped" by the surrounding sulcus tissues and becomes incorporated in the central fibrous body. These embryological findings on this centrally located area of the heart have implications for certain types of cardiac abnormalities. In this respect the atrioventricular conduction system, cor triatriatum sinistrum and atrioventricular defect are discussed.

Transposition of the great arteries with ventricular septal defect and pulmonary stenosis.

A postmortem investigation was carried out of 19 heart specimens with transposition of the great arteries, ventricular septal defect, and congenital subvalvular pulmonary stenosis. Certain types of obstruction appeared to be closely related to other features of the hearts. In cases with malalignment of the infundibular septum, the obstruction was caused by this septum and the anterolateral muscle bundle of the left ventricle. If the infundibular septum was deviated considerably to the left, the pulmonary stenosis was usually severe because the infundibular septum and anterolateral muscle bundle were joined. This junction resulted in a relatively posterior position of the pulmonary orifice in the left ventricle. A less extreme deviation of the infundibular septum resulted in an obstruction by this septum and by the anterolateral muscle bundle, situated at the right and the left sides of the pulmonary orifice respectively. In some cases of paratricuspid ventricular septal defect an anomalously attached and cleft anterior leaflet of the mitral valve was found. This, together with a leftward deviation of the anterior left ventricular part of the ventricular septum, caused the obstruction.