Ultrastructural study on the extrusion of multivesicular bodies from cardiac cells.

Cardiac conducting fibers taken from 12 dogs were studied in the electron microscope. The majority of samples contained a moderate number of multivesicular bodies (MVB's) and an extremely high number occurred in one dog. Single MVB's were found in the perinuclear region and in the cytoplasm separating myofibrils. A high accumulation of MVB's occurred at the cell periphery within cytoplasmic stalks extending into the extracellular space, in the intercalated disc, and also outside of the cell. The latter appeared to be extruded from the cell in the following way: 1) peripheral cytoplasm formed stalk-like extensions which contained MVB's; 2) the stalks detached from the cell; 3) vacuolar and sarcolemmal membranes surrounding the MVB disintegrated; 4) vesicles released from MVB adhered to the cellular surface and the coat of their membrane blended with the coat of the sarcolemma. The above pattern seems to be the same in those parts of sarcolemma which face the interstitial tissue and in the intercalated disc.