Cytomorphology of atherosclerosis: smooth muscle cells in formation and regression of atherosclerotic lesions.

Human fibrous atherosclerotic plaques from the superficial femoral artery, obtained at surgery and autopsy, were examined by transmission electron microscopy and immunofluorescence microscopy for antibodies to intermediate filament proteins (IF) vimentin, and desmin. The results showed that smooth muscle cells (SMC) of human fibrous plaques accounted for the predominant cell type, demonstrating a spectrum of phenotypes, with prevalence of cells with prominent rough endoplasmic reticulum, implying a synthetic phenotype. In regressing atherosclerotic plaques, contractile SMC were predominant and, along with them, few SMC with abundant cellular organelles were observed, displaying ultrastructural manifestation of collagen phagocytosis. SMC of both progressing and regressing atherosclerotic lesions, regardless of the morphological phenotype of the cell, contained vimentin IF and lacked desmin IF, the marker of differentiated SMC. These observations suggest the existence of a distinctive subpopulation of SMC in the human arterial intima that differs substantially from normal medial SMC by ultrastructure, biochemistry, and function.