Spherical microparticles in human myocardium: an ultrastructural study.

Clusters of spherical microparticles (SMP) that averaged 500 A in diameter and were composed of dense cores surrounded by single trilaminar membranes were found in operatively obtained myocardial biopsies from 29 of 70 patients with various types of heart diseases including: left atrial myocardium (14 patients) and right atrial myocardium (four patients) of 14 patients with mitral valvular disease; left ventricular myocardium of three of 16 patients with aortic valvular disease, three of 16 patients with hypertrophic cardiomyopathy, and two of four patients with combined mitral and aortic valvular disease; and crista supraventricularis muscle of seven of 20 patients with congenital heart diseases associated with muscular obstruction to right ventricular outflow. SMP were consistently associated with interstitial fibrosis and with degeneration of the muscle cells. SMP occurred along the outer surfaces on the sides and free ends of muscle cells in areas of fibrosis, in the widened spaces between membranes of partially dissociated intercellular junctions, and within cytoplasmic vesicles considered to be phagocytic. SMP frequently were joined together by minute nexuses that were structurally identical with those forming parts of intercellular junctions of muscle cells. Evidence is presented to show that SMP occur commonly in tissues other than myocardium. It is concluded that SMP form in the heart as part of a process that mediates the remodeling of cellular surfaces, especially those of intercellular junctions undergoing dissociation.