Atrioventricular node and the bundle of His in the adult - continued . The atrioventricular node is located in the subendocardium of the right side of the atrial septum. Three regions can be recognized in the AV node: 1) the proximal region, which is subendocardial; 2) the central region where the AV nodal myocytes begin to form small bundles separated by fine strands of connective tissue, and 3) the distal region, in the which the AV nodal cells are wider {10}. A. This panel shows the coronal slices prepared from the material illustrated in figure 25. They are arranged with the most dorsal sample on the left and the most ventral on the right. The anterior surface of the slices are shown in the top row (1 – 4) and their posterior surfaces in the bottom row (5 - 8). The artery to the AV node is shown in slice 2. B. Histologic section of slice no. 2 (illustrated in A) show the proximal-central junction of the AV node as it penetrates the right fibrous trigone or central fibrous body and courses along the base of the membranous interventricular septum. The site of attachment of the mitral valve to the central fibrous body is labeled MV (H&E X4). C. The trichrome stain demonstrates the compact or central AV nodal tissue as it approaches the fibrous trigone (Masson trichrome X4). D. Section of slice no. 3 (illustrated in A) shows the fibrous portion of the interventricular septum encasing a discrete, somewhat triangular fascicle that represents the bundle of His (arrowheads) (H&E 4X). E. The trichrome stain clearly shows how the bundle of His is well insulated as it perforates through the fibrous portion of the interventricular septum (Masson trichrome 4X). F. Histologic section of the AV node of an adult individual, shows that the myocytes are thinner and paler than the working myocytes. Their organization resembles that of the SA nodal myocytes shown in Figure 22.E G. Transmission electron micrograph of a Purkinje fiber, showing few myofibrils and mitochondria and abundant glycogen (5000 X).