Figure 1: Embryogenesis of the Sinoatrial Node.

The early heart tube is depicted (left) at day 10.5–12.5 of embryogenesis. The outer curvature of primary myocardium (black) differentiates and expands to form the heart chambers (blue). However, central primary myocardium remains and goes on to form the cardiac conduction system in the adult heart (right). The SAN forms from the sinus venosus (sv). Oft = outflow tract; pr = primary ring; lv = left ventricle; avc = atrioventricular canal; san = sinoatrial node; avn = atrioventricular node; avb = atrioventricular bundle; avj = atrioventricular junction; scv = superior caval vein; ra = right atrium; la = left atrium; rv = right ventricle; lbb = left bundle branch; rbb = right bundle branch; pvcs = peripheral ventricular conduction system. Adapted from Christoffels et al.⁷ Reproduced with permission from the American Heart Association, Inc.