Proximal Atrioventricular Bundle, Atrioventricular Node, and Distal Atrioventricular Bundle Are Distinct Anatomic Structures With Unique Histological Characteristics and Innervation

Abstract

Background-Direct 3D analysis (ie, stereotaxic analysis of 3 planes) has shown that the atrioventricular (AV) node (AVN) is continuous with only specialized myocardium of the proximal AV bundle (PAVB) and distal AV bundle (DAVB) or His bundle. The purpose of the present study was to determine whether 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 interstitial integrity of the tissue and permits serial sections of the AV junction region to be made in 3 orthogonal planes showed that the PAVB, AVN, and DAVB are characterized by myocardium aggregated into fascicles containing Ϸ8 myofibers. Myofibers within the fascicles are coiled or spiraled about each other; and spiraling is most compact in the PAVB. Collagen encases individual fascicles and segregates primary fascicles into secondary fascicles. Fascicles, and not myofibers, are in parallel array in the PAVB, interwoven in the AVN, and parallel in the DAVB. Narrow junctions of parallel fascicles separate the AVN from the PAVB and DAVB. Myocytes, which are largest in DAVB, possess clear perinuclear regions; thin finger-like end processes, which are most numerous in the AVN; uniform, delicate cross-striations; and intercalated disks, which are broader in the PAVB and form short stacks in the AVN. Sheaves of nerve terminals, including boutons, are as found in skeletal muscle. Conclusions-The PAVB, AVN, and DAVB have distinct histological features. Collagen septation of primary and secondary fascicles presents natural barriers within the tissues and to surrounding myocardium and structures. These findings confirm that the AV junction region contains a specialized conduction system that is anatomically isolated from ordinary myocardium.

The AV junction region of the heart: a comprehensive study correlating gross anatomy and direct three-dimensional analysis. Part II. Morphology and cytoarchitecture

“…TranumJensen and Bojsen-Moller (72) showed by electron microscopy of thin sections made from the stained tissue that the SA node extensions are different from the adjacent myocardium in displaying small branching myocytes containing glycogen deposits and grouped in clusters. These attributes are also similar to those for the proximal AV bundle, AV node, and distal AV bundle (56,58). The fact that the atrionodal bundles are associated with epicardium and terminate in the atrial-interventricular septal groove likely accounts for failure to detect connections of the extensions with the proximal AV bundle, the specialized AV ring tissues of Tranum-Jensen and BojsenMoller (72).…”

“…Elastic fibers are restricted to the walls of large blood vessels. The myocytes are ϳ6 m in diameter, possess uniform cross striations, and are joined by intercalated disks that appear to be more prominent and more numerous compared with those in the proximal AV bundle, AV node, and distal AV bundle (58). No cell-to-cell connections were found with the ordinary atrial myocardium.…”

“…3 B-D and 6, B and D, inset in Ref. 58). The collagen encasement of fascicles comprising the tissues most probably function as insulation, and that of the central fibrous location of the AV node and distal AV bundle as protection from compression, as adult-like ECG patterns are recorded in the bent single primordial heart tube before the appearance of collagen or the central fibrous body by Patten (47).…”

His Electrogram Alternans Reveal Dual-Wavefront Inputs Into and Longitudinal Dissociation Within the Bundle of His

“…1 Although arguments still continue about their number 15 and whether they constitute functional or anatomic substrates, 14,16,17 the dual pathways are commonly considered to be atrionodal entities. The penetrating bundle is usually considered a cable-like structure surrounded by the CFB [12][13][14] and distanced from the supraventricular reentrant loop by an interposed common final pathway.…”

Mechanistic Insights Into Very Slow Conduction in Branching Cardiac Tissue

“…In the AV node, so-called "dead-end" pathways could play this role by shunting a fraction of electrotonic current. 16,17 Furthermore, in parts of the AV node consisting of myocardial fascicles interspersed with collagen, 18,19 branchings of these fascicles could form sites of source-to-load mismatch. In the myocardium, structural complexities can develop during chronic infarction: the infarct border zone consists of interconnected islands and strands of surviving myocytes intermingled with scar tissue and is known to support very slow conduction.…”