Proximal atrioventricular bundle, atrioventricular node, and distal atrioventricular bundle are distinct anatomic structures with unique histologicalcharacteristics and innervation

Background-Direct 3D analysis (ie, stereotaxic analysis of 3 planes) has sh own that the atrioventricular (AV) node (AVN) is continuous with only speci alized myocardium of the proximal AV bundle (PAVB) and distal AV bundle (DA VB) or His bundle. The purpose of the present study was to determine whethe r the PAVB, AVN, and DAVB possess histological features distinct from each other and from the ordinary myocardium. Methods and Results-A protocol that preserves the cytoplasmic and interstit ial integrity of the tissue and permits serial sections of the AV junction region to be made in 3 orthogonal planes showed that the PAVE, AVN, and DAV B are characterized by myocardium aggregated into fascicles containing appr oximate to 8 myofibers. Myofibers within the fascicles are coiled or spiral ed about each other; and spiraling is most compact in the PAVE. Collagen en cases individual fascicles and segregates primary fascicles into secondary fascicles. Fascicles, and not myofibers. are in parallel array in the PAVE, interwoven in the AVN, and parallel in the DAVB. Narrow junctions of paral lel fascicles separate the AVN from the PAVE and DAVB. Myocytes, which are largest in DAVB, possess clear perinuclear regions; thin finger-like end pr ocesses, which are most numerous in the AVN; uniform, delicate cross-striat ions; and intercalated disks, which are broader in the PAVE and form short stacks in the AVN. Sheaves of nerve terminals, including boutons, are as fo und in skeletal muscle. Conclusions-The PAVE, AVN, and DAVB have distinct histological features. Co llagen septation of primary and secondary fascicles presents natural barrie rs within the tissues and to surrounding myocardium and structures. These f indings confirm that the AV junction region contains a specialized conducti on system that is anatomically isolated from ordinary myocardium.