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.