Ventricular Septation & Outflow Tract Division

Here we will learn how the outflow tract and ventricle become partitioned from weeks 5 and 9. The outflow tract is divided by the spiral aorticopulmonary septum. This partition begins to form when second heart field cells contribute to the elongating outflow tract and cardiac neural crest cells migrate to the conotruncal region and populate ridges there. The primitive ventricle is divided by a septum comprised of muscular and membranous portions. The muscular interventricular septum forms as the ventricle itself expands. The membranous septum comprises the lower portion of the spiral aorticopulmonary septum and the tissues derived from ventricular side of the atrioventricular endocardial cushions. Let's begin with the heart in anterior view, with a slight rotation, in week 5. At this point, we can see the right and left atria, primitive ventricle with a developing muscular ventricular septum, and the outflow tract, which comprises the bulbus cordis and truncus arteriosus. The atrioventricular septum and canals are developing. Right and left conotruncal ridges are arising from the walls of the outflow tract and form a spiral configuration. Their helical form establishes the crossover relationship between the ascending aorta and pulmonary trunk. To see the changes occurring in the truncus arteriosus, we draw the truncal and bulbar swellings in superior view; these swellings give rise to the conotruncal ridges as they grow towards each other and pinch the single lumen. We omit the atria for simplicity. The muscular ventricular septum has elongated and now meets the membranous contribution from the atrioventricular endocardial cushions. The conotruncal ridges fuse in a cranial-to-caudal direction, towards the ventricle, and blood flow spirals around the ridges. The interventricular foramen is narrowing, but not quite closed. We re-draw the truncus arteriosus in superior view to show that the fusing septum divided a singular lumen into two channels. The aorticopulmonary septum is fully fused, and the blood exits the heart via the partitioned outflow tracts. In a superior view drawing, we show that the lumen has been fully divided to form separate vessels, the aorta and pulmonary trunk. Within the ventricle, the interventricular foramen is closed, which completes the separation of the primitive ventricle into left and right chambers. The interventricular septum now comprises a membranous portion, which was formed by the spiral septum and endocardial cushions, and a muscular portion that arose from the ventricular tissue. By this point, the two sides of the heart are established, and blood follows distinct systemic and pulmonary pathways through the heart (but recall that blood bypasses the pulmonary circulation during gestation) Failure of normal spiral septation can produce conotruncal defects such as persistent truncus arteriosus, transposition of the great arteries, tetralogy of Fallot, and double-outlet right ventricle.