Characteristics of RN and CN connection
The location of the main body of the RN was the same in our as in previous reports.3,8 Additional information we intended to acquire was the relationship between the RN and CN. Continuous sections at the level of the start of the CN and the end of the RN from another sample were chosen to show the structural features in this region. Part of the node-like tissue was separated from the RN and penetrated into the fibrous rings between the atrium and ventricle (Figure 3A1-3, dotted box). The following continuous sections (Figure 3B and C, dotted box) showed that the node tissue descended into the CN. HCN4 labeling further suggested that the penetrated tissue was node-like tissue. As proven repeatedly by other samples in our study, the level of the start of the CN is a more complex region than previously described. First, it stretches downward forming the transitional area between the His bundle and AVN; second, it extends backward, forming the main body of the CN; third, it accepts the insertion of the end of the RN.
Some of the samples suggested that the region connecting the RN and CN was also near the start of the tendon of Todaro (TT) (Figure 3D-F). Two narrow connections between the RN and CN at different levels near the TT region are shown in Figure 3D and E (dotted box), which suggests that there may be more than one connection between the RN and CN.
Fibrous tissue surrounding the CN at the level of almost the entire CN seems to have no direct connection with the atrium, while most of the RN was directly connected to the atrium, without surrounding fibrous tissue (Masson’s staining, Figures 2A and 3F).