Endocardial cushion fusion

Endocardial cushions are areas of mesenchymal proliferation. As described earlier, these represent areas of the fibrous skeleton forming between the atrium and ventricle. Endocardial cushions serve two important functions:

• They form a partition in the heart tube between the atrium (PA) and ventricle (PV), known as the AV canal. The resulting two channels represent sites for the future tricuspid and bicuspid valves.
• provide a "scaffold" to which the interatrial septae and the interventricular septum will grow towards and fuse with.

SV - sinus venosus; PA - primitive atrium; PV - primitive ventricle;
BC - bulbus cordis; TA - truncus arteriosus

Defects in endocardial cushion fusion are associated with trisomies 18 and 21 (Down's syndrome). Since the scaffold for future growth is absent, ventricular septal defects are also common. The resulting condition is referred to as atrioventricular communis. This anomaly is seen with varying degrees of severity - the most severe being characterized by 4-chamber communication.

The proliferation of the fibrous skeleton, combined with the fusion of the endocardial cushions will form the biscuspid (mitral) and tricuspid valves.

Valvular defects can arise if endocardial cushion fusion does not partition the AV canal evenly.

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