The so-called specialized tissues within the heart are the sinus node, the atrioventricular conduction system, and the Purkinje network. Further structures with the characteristics of specialized tissue are also found within the atrioventricular junction, although they are less well described.

The purpose of this study was to demonstrate the location and extent of these atrioventricular ring specialized tissues, showing their relationship with the normal atrioventricular conduction system.

We identified the tissues using histology combined with immunohistochemical labeling with connexin43 (Cx43), the major gap junction in heart, and HCN4, the major isoform of the funny channel.

We observed rings of specialized tissue mainly in hearts from rats, mice, and guinea pigs, negative for Cx43 but positive for HCN4. Each ring takes its origin from an inferior extension of the atrioventricular node. The rightward ring runs around the vestibule of the tricuspid valve, whereas the leftward ring encircles the mitral valve. On returning toward the atrial septum, the tricuspid ring crosses over the penetrating part of the atrioventricular conduction system, reuniting with the mitral ring to form a superiorly located retroaortic node. The atrioventricular conduction system itself continues beyond the origin of the right and left bundle branches, forming an aortic ring that ascends toward the retroaortic node but fails to make contact because of the intervening area of aortic-to-mitral valvar fibrous continuity.

Rings of conduction tissue take their origin from inferior extensions of the atrioventricular node, passing rightward and leftward to encircle the orifices of the tricuspid and mitral valves and reuniting to form an extensive retroaortic node. Thus, a ring with morphologic features justifying a definition of specialized conduction tissue surrounds the atrioventricular junctions, although its function has yet to be established.