Relationship between mitochondria and the development of specific lipid droplets in capillary endothelial cells of the respiratory tract in patients with sarcoidosis

The aim of this study was to morphologically evaluate damage in the capillary endothelial cells of the respiratory tract in patients with sarcoidosis. We examined tissues of the bronchus and lung obtained from 16 patients with sarcoidosis consisting of 2 stage 0, 10 stage I and 4 stage II patients, and 11 controls. The morphology of capillary endothelial cells was studied using electron microscopy. In the samples from patients with sarcoidosis, lipid droplets exhibiting dark monophasic density (unsaturated fatty acids) were surrounded by abundant lysosomes in the capillary endothelial cells, and the double-membrane structure of the mitochondria attached to these lipid droplets was lost in three cases. Biphasic lipid droplets with dark and lucent (saturated fatty acids) densities were also observed, accompanied by a few lysosomes containing the residual bodies of undigested lipid-containing substances. Lucent monophasic droplets were also detected in the tissues from patients with sarcoidosis. The plasma membrane was more often damaged in capillary endothelial cells containing biphasic lipid droplets, lucent monophasic droplets as well as in those with dark monophasic droplets. However, no lipid droplets were detected in capillary endothelial cells obtained from the control subjects, except in a single case. This study demonstrated that a large number of mitochondria were mobilized and showed notable morphological changes including swelling in the capillary endothelial cells in patients with sarcoidosis. A close relationship between mitochondria and lipid droplets was observed in capillary endothelial cells of the respiratory tract, and this relation may be involved in the pathogenesis of sarcoidosis.