From the fourth week of gestation onwards, the atrioventricular canal enlarges and moves relatively to the right within the heart. It lies approximately within the coronal plane relative to the rest of the body. At this stage, it is inferior, ventral and distal to the primitive atrium; it is dorsal and proximal to the primitive ventricle. At the margins of the aperture there is overgrowth of mesenchyme to form ridges which narrow the groove - the endocardial cushions:
By the start of the sixth week, the superior and inferior endocardial cushions fuse together centrally to divide the atrioventricular canal into right and left atrioventricular orifices. The lateral endocardial cushions do not fuse - fusion would obliterate the newly-formed atrioventricular orifices - but instead contribute to the atrioventricular valves - see submenu.
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