Embryology

Heart development begins with the formation of the cardiac crescent and the linear heart tube, which is formed primary from cells that differentiated in the lateral splanchnic mesoderm, called the first heart field (FHF). As the linear heart tube extends and loops, another wave of cells from the so-called second heart field (SHF), located within the pharyngeal mesoderm medial and dorsal to the developing heart, are added to the arterial and venous pole. Eventually, the left ventricle (LV) is derived almost entirely from cells that originated in the FHF. In contrast, SHF-derived cells generate the right ventricle (RV) and the common outflow tract (OFT). As the OFT matures, it elongates and aligns itself across both the right and left ventricles. A third set of progenitor cells, the cardiac neural crest cells, then migrate into the OFT and septate it into the right and left ventricular outflows. As this septation progresses, cardiomyocytes from the SHF cells also migrate to complete the cranial (outflow) portion of the inter-ventricular septum. Hence, unlike on the right side, the LV develops primarily from the FHF, but the aortic valve and ascending aorta develop from the SHF. Following OFT septation, small mesenchymal cushions are formed from the SHF-derived endothelium in the caudal end of the OFT. These cushions grow in size and are hollowed out in their upper surface to generate the aortic valve. Given this close relationship between aortic valve and the OFT, developmental defects in the OFT (also called conotruncal defects) are frequently associated with aortic valve pathology, as is frequently seen in truncus arteriosus.