Quantitative ultrastructural analysis of perifibrous lipid and its association with elastin in nonatherosclerotic human aorta.

Nonatherosclerotic areas in human arteries display an age-related accumulation of cholesteryl ester in the form of small, perifibrous lipid droplets in the deeper intimal layers. We treated human aortic specimens with an osmium-thiocarbohydrazide-osmium sequence en bloc after glutaraldehyde fixation in order to provide electron dense staining of neutral lipid for ultrastructural study. Neutral lipid was quantified in terms of area fractions on thin sections. Extracellular lipid, primarily in the form of small (less than 300 nm) droplets, accounted for 91% of the lipid found in the deep intimal region. Seventy-four percent of extracellular lipid appeared in droplets or aggregates that were demonstrated as adjacent to or within elastic fibers in the plane of section. The fraction of lipid adjacent to elastin in three dimensions is likely to be considerably higher than 74%. The results support the concept that an interaction between elastin or its associated components and lipids or lipoproteins may be important in extracellular lipid deposition in human arteries.