Ultrastructure of the intima in WHHL and cholesterol-fed rabbit aortas prepared by ultra-rapid freezing and freeze-etching

Intima from aortas of normal Watanabe Heritable Hyperlipidemic (WHHL) and cholesterol-fed (10 days - 3 months) rabbits were examined by ultra-rapid freezing without chemical fixation followed by rotary shadow freeze-etching. The extracellular matrix in areas devoid of cells was seen in extraordinary detail and consisted of a reticulum of thick filaments, finer branching filaments, collagen fibrils, and granules of varying sizes. No lipid deposits were seen in normal intima. However, the subendothelial region of WHHL intima was filled with collagen fibrils surrounding and entwined between clusters of discrete lipid vesicles that ranged in size from 23 to 169 nm. Approximately 80% of the lipid vesicles in the WHHL rabbit intima measured between 70 and 169 nm. The lipid particles in the WHHL intima always appeared in clusters, many of which appeared to be fusing into larger size vesicles. These aggregates were clearly linked to the matrix filaments. A similar deposition of lipid particles was seen in the extracellular matrix of cholesterol-fed rabbits but in contrast to the particle size distribution of the WHHL intima, more than 75% of the particles in the cholesterol-fed intima had a diameter between 23 and 68 nm and 51% were between 23 and 45 nm. We conclude that in cell-free areas of WHHL and after only 10 days of cholesterol feeding, lipoprotein-derived lipid is present in the intima as clusters of vesicles enmeshed in the complex extracellular matrix.