Steps of heart morphogenesis. Illustrations depict cardiac development with conserved shading of morphologically related regions, seen from a ventral view. Cardiogenic precursors form a crescent (far-left panel) that is specified to form specific segments of the linear heart tube, which is patterned along the anteroposterior axis to form the various regions and chambers of the looped and mature heart. Each cardiac chamber balloons out from the outer curvature of the looped heart tube in a segmental manner. Neural crest cells populate the bilaterally symmetric aortic arch arteries (III, IV, and VI) and aortic sac (AS) that together contribute to specific segments of the mature aortic arch. Mesenchymal cells form the cardiac valves from the contruncal (CT) and atrioventricular valve (AVV) segments. Corresponding days of human embryonic development are indicated. A, atrium; V, ventricle; RV, right ventricle; LV, left ventricle; RA, right atrium; LA, left atrium; PA, pulmonary artery; Ao, aorta; DA, ductus arteriosus; RSCA, right subclavian artery; LSCA, left subclavian artery; RCC, right common carotid; LCC, left common carotid. Used, with permission, from Nature Publishing Group (Srivastava and Olson, [2000]).