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The intercellular organization of the two muscular systems in the adult salmonid heart, the compact and the spongy myocardium

The ventricle of the salmonid heart consists of an outer compact layer of circumferentially arranged cardiomyocytes encasing a spongy myocardium that spans the lumen of the ventricle with a fine arrangement of muscular trabeculae. While many studies have detailed the anatomical structure of fish hearts, few have considered how these two cardiac muscle architectures are attached to form a functional working unit. The present study considers how the spindle‐like cardiomyocytes, unlike the more rectangular structure of adult mammalian cardiomyocytes, form perpendicular connections between the two muscle layers that withstand the mechanical forces generated during cardiac systole and permit a simultaneous, coordinated contraction of both ventricular components. Therefore, hearts of rainbow trout (Oncorhynchus mykiss) and sockeye salmon (Oncorhynchus nerka) were investigated in detail using scanning electron microscopy (SEM) and various light microscopic techniques. In contrast to earlier suggestions, we found no evidence for a distinct connective tissue layer between the two muscle architectures that might ‘glue’ together the compact and the spongy myocardium. Instead, the contact layer between the compact and the spongy myocardium was characterized by a significantly higher amount of desmosome‐like (D) and fascia adhaerens‐like (FA) adhering junctions compared with either region alone. In addition, we observed that the trabeculae form muscular sheets of fairly uniform thickness and variable width rather than thick cylinders of variable diameter. This sheet‐like trabecular anatomy would minimize diffusion distance while maximizing the area of contact between the trabecular muscle and the venous blood as well as the muscle tension generated by a single trabecular sheet.