3D reconstruction of the AV junction
A more direct view of the relationships among different structures could be achieved following the 3D reconstruction of the AV junction. In summary, atrial and ventricular tissues (Cx43-positive and HCN4-negative tissues), nodal tissue (Cx43-negative; HCN4-positive; Cx40-negative and strongly Cx45-positive tissue), PB tissue (Cx43-negative; HCN4-positive; Cx40-positive and strongly Cx45-positive tissue) and connective tissue (Masson’s trichrome-stained tissue, blue) were present at the AV junction. Figure 4A summarizes the distribution of cell types in all 80 levels that were studied (images are prior to alignment), wherein only the tract of the nodal tissue and nearby tissue is shown. The nodal tissue includes the CN, inferior extension and RN (bright yellow), Cx40-positive tissue of the PB (red), atrial and ventricular muscle (sandy), and connective tissue (light blue), as shown in Figure 4 (left). These images of serial sections showed changing trends in the distribution, location and volume of cell types at the AV junction. The sections shown in Figure 4 (left) were used to generate a 3D anatomical model of the AVN.
Various views of the model are shown in Figure 4 (right) (atria and ventricles were removed to highlight junctional tissues). Based on the patterns of connexin and HCN4 expression, we propose that the anterior extension, CN and inferior extension contain the same cell types. The positional and volumetric relationships among them are shown in Figure 4 (right). The inferior extension gradually decreased in volume moving away from the CN, with no obvious boundary with the CN. However, the shape of the RN was similar to a shuttle. Its connection with the CN was narrow. It expanded, with an increasing volume moving away from the CN, which then decreased again at the anterior septum. The volume of the PB decreased as the level increased and disappeared into the CN (red part of the model).