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.