Chapter 13: Development of Heart & Circulation

The development of the cardiac and circulatory systems begins with a symmetric embryonic vascular network that undergoes extensive reorganization to support placental exchange in utero and transition rapidly to pulmonary respiration at birth. Core heart structures originate from two distinct mesenchymal populations: the primary heart-forming field (cardiac crescent) and the secondary heart-forming field. The midline splanchnopleuric coelomic epithelium serves as the source for the myocardium, endocardial lining, and later the epicardium, coronary arteries, and interstitial fibroblasts. The endocardial tubes form from an endocardial plexus by vasculogenesis and fuse, while subsequent head folding positions the cardiogenic tissue ventrally, defining the future pericardial cavity. The early linear heart tube undergoes complex morphological change, specifically looping, involving rightward ventral bending, which correctly positions the developing left ventricle and the right ventricle. Crucial septation processes divide the common atrium, ventricles, and the outflow tract. Atrial division involves the crescent-shaped septum primum, which grows toward the atrioventricular (AV) cushions, and later perforates to create the secondary atrial foramen, overlapped by the developing septum secundum to form the foramen ovale shunt, essential for bypassing the fetal lungs. Endocardial cushions, resulting from epithelial to mesenchymal transition, fuse to divide the AV canal, forming the central fibrous body and giving rise to the AV and arterial valves. Neural crest cells also migrate to the arterial pole, aiding in outflow tract septation and valve formation. Cardiac myocytes differentiate into four types: primary myocardium, which retains high automaticity and slow conduction (giving rise to nodal structures), and working myocardium, characterized by fast conduction and high contractility, forming the chamber bulk. The peripheral arterial system develops from the bilateral primitive aortae and aortic arch arteries, giving rise to segmental somatic and splanchnic vessels. Concurrently, the complex venous drainage shifts from a symmetric pattern of cardinal and umbilical veins to an asymmetric arrangement, forming the superior and composite inferior vena cavae. Fetal circulation relies on placental oxygenation and is defined by the shunts: the ductus venosus, the foramen ovale, and the ductus arteriosus, which divert blood away from the inactive pulmonary circuit. Following birth, pressure shifts and increased oxygen tension cause the functional closure of the foramen ovale (as the septum primum flap closes against the septum secundum) and induce smooth muscle contraction leading to the structural obliteration of the ductus arteriosus, establishing independent systemic and pulmonary circulations.