Ultrastructure of the myocardium of the atrial appendage.

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Abstract
The ultrastructure of the myocardium of the right and left atrial appendages is described. This is similar to ventricular myocardium and resembles skeletal muscle in many respects. Deep infoldings of sarcolemma and basement membranes was noted, and these are not always related to cell branching. Because of these infoldings, changes occurring extracellularly may appear on the light microscope to be within the boundaries of cartliac muscle cells. The constant relationship of intermyocardial cells and their processes to the basement membrane is described.