The atrioventricular node: origin, development, and genetic program.

The sinus node generates the electrical impulse, which spreads rapidly over both atria, causing them to contract simultaneously. In the normal heart, a layer of connective tissue electrically insulates the atria and ventricles. The only pathway that crosses this plane is the atrioventricular conduction axis, through which the impulse reaches the ventricles. Within the axis, the atrioventricular node delays the impulse, allowing the ventricles to be filled before their contraction is initiated. Moreover, the atrioventricular node protects the ventricles from rapid atrial arrhythmias and may take over pacemaker function when the sinus node fails. In pathological conditions, these complex physiological properties contribute to several types of arrhythmias that originate from the atrioventricular conduction system. One example is atrioventricular block, which requires electronic pacemaker implantation because there is currently no cure for this arrhythmia. Because conduction system defects may arise during embryonic development, the mechanisms of conduction system development have been intensively studied. Nevertheless, its developmental origin, molecular composition, and phenotype have remained fertile subjects of research and debate. Lineage and expressional analyses have indicated that the atrioventricular node develops from a subpopulation of precursor cells in the dorsal part of the embryonic atrioventricular canal. These cells become distinct early in development, are less well differentiated compared to the developing working myocardium, and, in addition to their cardiogenic gene program, activate and maintain a neurogenic gene program.