This article reports the morphological characteristics of the atrioventricular (AV) region of the teleost heart. A total of 29 teleost species belonging to 19 families and 10 orders were analyzed. The study includes hearts with entirely trabeculated ventricles and hearts possessing a ventricular compacta. In all cases, the AV region is formed by a ring of compact myocardium surrounded by a connective tissue ring. The myocardium contains vessels in most species having entirely trabeculated ventricles, and in all species possessing a compacta. The ring of connective tissue is rich in collagen and elastin, partially isolating the AV muscle from the surrounding musculature. Continuity between the AV muscle and the atrial and ventricular musculature is always observed. The AV muscle supports the AV valves, which are formed by two leaflets, lack papillary muscles, show a collagenous fibrosa on the atrial side, and present a variable number of cells and a variable amount of extracellular material. Ventricular trabeculae attach to the AV muscle. These trabeculae may help to bear the stress generated by ventricular contraction and may constitute a preferential pathway for the conduction of electrical impulses. In all teleosts studied, the AV region can be recognized as a distinct morphological segment interposed between the atrium and the ventricle. The morphological characteristics of the AV segment appear to be species specific.
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