The
endocardial cushions are thought to arise from a subset of
endothelial cells that undergo
epithelial-mesenchymal transition, a process whereby these cells break cell-to-cell contacts and migrate into the
cardiac jelly (towards the interior of the heart tube). These migrated cells form the "swellings" called the endocardial cushions seen in the heart tube. Upon sectioning of the heart the atrioventricular endocardial cushions can be observed in the lumen of the
atrial canal as two thickenings, one on its dorsal and another on its ventral wall. These thickenings will go on to fuse and remodel to eventually form the valves and septa of the mature adult heart. ==Clinical significance==