Electron microscopy of lipid deposits in human atherosclerosis

The filipin probe associated with tannic acid stain was used to study intra- and extracellular lipids in surgically removed human atherosclerotic lesions (n = 20). In particular, intimal thickenings, fatty streaks and fibrolipidic plaques have been investigated by using mainly transmission and scanning electron microscopy. In the intimal thickenings, the lipid deposits were mainly localized in the subendothelial space as homogeneously sized particles (40-140 nm) and more heterogeneous uni-multilamellar vesicles (35-700 nm). Intermediate lipid forms were also observed. In the fatty streaks, the lipid deposits were intracellular and mainly observed in cells with a monocyte/macrophagic phenotype. Lipid inclusions, lipid lysosomal bodies and intracellular cholesterol crystals very similar to those observed in experimentally induced atherosclerosis were documented. In the fibrolipidic plaque the lipid deposits were found both in the intracellular and in the extracellular compartments. Lipids accumulated within arterial macrophages and smooth muscle cells, usually as lipid droplets. Clusters of lipoprotein-like particles (50 nm in diameter) as well as larger uni-multilamellar lipids (700 nm) with an occasional compound appearance were particularly observed bound to elastic tissue and collagen fibers. These morphological observations outline the complexity of lipid metabolism in the various histological aspects of human atherosclerosis.