And Outflow Tract And Atrial And Ventricular Septation

Once the chambers are in their correct positions after looping, extensive remodeling of the primitive vasculature and septation of the heart can occur. The cardiac neural crest is an extracardiac (from outside the primary or secondary heart fields) population of cells that arises from the neural tube in the region of the first three somites up to the midotic placode level (rhombomeres 6-8) (Fig. 5). Cardiac neural crest cells leave the neural tube during weeks 3-4 of human development and migrate through aortic arches 3, 4, and 6 (Fig. 1B) and then eventually move into the developing outflow tract of the heart during weeks 5 and 6. These cells are necessary for complete septation of the outflow tract and ventricles (which is completed by week 9 of human development), as well as for the formation of the anterior parasympathetic plexis, which contributes to cardiac innervation and the regulation of heart rate (8,16-18).

The primitive vasculature of the heart is bilaterally symmetrical, but during weeks 4 to 8 of human development, there is remodeling of the inflow end of the heart so that all systemic blood will flow into the future right atrium (8). In addition, there is extensive remodeling of the initially bilaterally symmetrical aortic arch arteries into the great arteries (septation of the aortic and pulmonary vessels) that is dependent on the presence of the cardiac neural crest (14,19). The distal outflow tract (truncus) septates into the aorta and pulmonary trunk via the fusion of two streams or prongs of cardiac neural crest that migrate into the distal outflow tract. In contrast, the proximal outflow tract septates by fusion of the endocardial cushions and eventually joins proximally with the atrioventricular endocardial cushion tissue and the ventricular septum (20,21). The endocardial cushions are formed by both atrioventricular canal and outflow tract endocardial cells that migrate into the cardiac jelly, forming bulges or cushions.

Despite its clinical importance, to date almost nothing is known about the molecular pathways that determine cell lineages in the cardiac neural crest or that regulate outflow tract septation (14). However, it is known that if the cardiac neural crest is removed before it begins to migrate, the conotruncal septa completely fails to develop, and blood leaves both the ventricles through what is termed a persistent truncus arteriosus, a rare congenital heart anomaly in humans. Failure of outflow tract septation may also be responsible for other forms of congenital heart disease, including transposition of the great vessels, high ventricular septal defects, and tetralogy of Fallot (8,16,18).

The septation of the outflow tract (conotruncus) is tightly coordinated with the septation of both the ventricles and atria to produce a functional heart. All of these septa eventually

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