Advanced - Cardiac Septation
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All of the partitioning of the primitive heart occurs between the middle of the fourth week and the end of the fifth week. Division of the atrioventricular canal is described below while septation of the atria and ventricles is described here.
Division of the AV Canal
Two endocardial cushions form on the dorsal and ventral surfaces of the AV canal. Following expansion of the cardiac jelly, epithelial to mesenchymal transformation (EMT) of the endocardial cells in the canal occurs forming the cushions. Synergistic signalling between BMP and TGFβ facilitates EMT. The cushions grow as they are invaded by mesenchymal cells from the endocardium during the fifth week, eventually fusing to create the right and left AV canals, hence partially separating the primitive atrium and ventricle (Click image to play on current page or Play video on new page).
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