(Circulation. 2000;101:1049.)
© 2000 American Heart Association, Inc.
Basic Science Reports |
Proximal Atrioventricular Bundle, Atrioventricular Node, and Distal Atrioventricular Bundle Are Distinct Anatomic Structures With Unique Histological Characteristics and Innervation
From the Department of Medicine, Cardiology, The Feinberg Cardiovascular Research Institute, Northwestern University, Chicago, Ill.
Correspondence to Darlene K. Racker, PhD, Northwestern University Medical School, 303 E Chicago Ave, CH-T 233, Chicago, IL 60611. E-mail darkrac{at}nwu.edu
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 
8
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.
Key Words: atrium • atrioventricular node • conduction • collagen • myocytes • nervous system
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