The heart is the first organ that is formed and becomes functional in the human embryo. It bears the circulatory system from the early embryonic period and alters its morphology to each stage of an embryonic growth according to its demand. The early morphogenesis of the heart and great vessels is conserved across species of living things. In the mammals, the complicated process of morphogenesis results in the separable systemic and pulmonary circulation after birth by which oxygen is efficiently supplied to different internal organs in order to maintain their function. The morphogenesis of the cardiovascular system is started around 3 weeks of gestation and mostly completed by 8 weeks of gestation in human. First, immediately after gastrulation, the cardiac-fated cells of anterolateral plate mesoderm origin form a crescent-shaped cardiac precursor in front of a notochord (Fig. 2.1a). The cardiac crescent constructs bilateral endocardial tubes that fuse to form the primitive heart tube in the midline of embryo (Fig. 2.1b). The primitive heart tube is a beating tubular structure that maintains blood flow from caudal to rostral direction of the embryo. Along with rightward looping of the primitive heart tube, the right and left ventricles and atria become morphologically apparent (Fig. 2.1c). Endocardial cushions are formed in extracellular matrix (also known as cardiac jelly) of the atrioventricular canal region from the mesenchymal cells which were separated from the endocardium and transformed. An atrioventricular canal is divided by the coalescence of the superior and inferior endocardial cushions.
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