Semilunar valves (Valvulae semilunares)

The semilunar cusps are believed to arise at the transition between the conus cordis and the truncus arteriosus. The aorta and the pulmonary trunk become separated due to the fusion of the ectomesenchymal cushions of the two conotruncal ridges; a third ectomesenchymal cushion develops in both the aorta and the pulmonary trunk. At S17, the three cushions of aorta and pulmonary trunk become hollowed out from distal. Their material is transformed in month 4 into collagenous connective tissue. By the time that the conus cordis becomes incorporated into the ventricles, the semilunar cusps come to lie in approximately the same plane as the atrioventricular valves (valve plane).