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The semilunar valves are produced with division of the truncus arteriosus during development. From the fifth week of gestation, the truncus arteriosus is divided up by the fusion of two truncal ridges across the lumen to form separate channels which become the roots of both the aortic and pulmonary trunks. The truncal ridges grow from the left and right walls of the chamber.
As fusion occurs, small swellings appear on the inferior margin of each of the original truncal ridges where they border each of the newly-created channels. These are the primordia of the valve leaflets. In each channel, a third swelling appears opposite those derived from the truncal ridges; this will produce the third leaflet of each of the valves.
By convention, the origin of the swelling within the truncus arteriosus determines the name of the valve leaflet in the adult heart:
Note, this terminology does not co-incide with the anatomical position of the leaflets within the mature heart; hence, there is more than one nomenclature - see submenu.
The superiorly-concave, balloon-like structure of the adult valves are produced by loss of cells from the upper surface of each leaflet.