Ultrastructural features of diseased human atrial muscle cells.

Left atrial myocardial tissues, excised at the time of surgery from 30 Japanese patients, were studied using light and electron microscopy and the freeze-fracture technique. All had mitral valvular disease, and, in 22 (74%) patients, atrial fibrillation also was present. In all patients, most of the left atrial cardiocytes were hypertrophied and surrounded by various amounts of fibrous tissue. Degenerative alterations included large masses of lipofuscin granules and lamellar bodies, disorganized myofibrils, widened Z-line material, selective loss of thick filaments, and prominent tubules of sarcoplasmic reticulum. A high incidence of tubular aggregates was observed in 70% of the cases. Such alterations were seen more often in the mitral regurgitation group as compared to the mitral stenosis group. Atrial cardiocytes were found to be more susceptible to disease processes, such as rheumatic fever and subsequent pressure and/or volume overloading, than ventricular cardiocytes. Human atrial cardiocytes may be utilized in future cytopathological studies to study the effect of processes leading to cardiocyte hypertrophy and degeneration.