Discussion
Our study is the first to describe the morphological characteristics of the RN and its relationship with the CN in detail. Serial histological and immunohistochemical labeling combined with 3D reconstruction provided an advantage in identifying structural features in this region. The results show a direct connection between the CN and RN. Cardiac myocytes located in the INE were first described by Anderson and colleagues as part of a continuous structure around the tricuspid valve.5 A separate study reported that the right nodal extension continues around the tricuspid valve annulus as the right AV ring forms, whereas the left nodal extension continues around the mitral valve annulus as the left AV ring forms. The right and left AV rings loop around the two valves and meet again to form the RN.8 Previous studies have described the anatomical location of this region in little detail, and its relationship within the CN is even less clear. Due to the high-density continuous sections, multiple-staining method and immunolabelling, our study reveals that the CN and the RN connect at the start of the CN and seem to constitute the only pathway connecting the atrium to the conduction tissue in the anterior septum. Previous studies have neglected this connection, which may have been caused by the insufficient density of continuous sections in this area and the subtle, narrow connection pattern. The proven connection between the CN and the RN provides a new substrate that may further explain the complex electrophysiological phenomena that occur in the AVN area.