DISTRIBUTION OF CELL REPLICATION AND APOPTOSIS IN ATHEROSCLEROTIC PLAQUES OF CHOLESTEROL-FED RABBITS

In human atherosclerosis the development of a cell-poor lipid-rich cor e is an important feature of atheromatous plaque formation. The core i s characterized by extracellular lipid deposition, cholesterol crystal s and cell death and is situated in the deep layer of the plaque. The aim of the present study was to localize apoptotic cell death and cell replication in atherosclerotic plaques of cholesterol-fed rabbits in order to examine the hypothesis that core formation is a consequence o f an imbalance between cell replication and apoptosis. New Zealand Whi te male rabbits were fed a diet supplemented with 0.3%, cholesterol fo r 16 (n = 5) and 27 weeks (n = 9). Cell replication and cell types wer e demonstrated by immunohistochemistry and apoptotic cell death was de monstrated by DNA in situ end-labeling (ISEL) and transmission electro n microscopy. Quantification was done using a colour image analysis sy stem. The plaques showed a clear distinction between a luminal layer c omposed of numerous lipid-rich foam cells of macrophage origin and a d eep layer which was fibrous, containing extracellular lipid deposits a nd few smooth muscle cells. Cell replication (expressed as percentage of total number of nuclei) in the superficial layer was higher then in the deep layer at both 16 (5.1 +/- 1.8% vs. 1.2 +/- 0.8%) and 27 week s (11.3 +/- 2.1% vs, 4.4 +/- 1.0%). This was also the case for the tot al number of nuclei per 50 000 mu m(2) cross-sectional intimal area (n umerical density): 235 +/- 13 vs. 147 +/- 7 at 16 weeks and 130 +/- 10 vs. 89 +/- 11 at 27 weeks. Apoptotic cell death (expressed as percent age of total number of nuclei) was low and there was no difference bet ween the superficial and the deep layers of the plaques (0.8% +/- 0.2% vs. 0.4%, +/- 0.2%, at 16 weeks and 0.6 +/- 0.2% vs. 1.7% +/- 0.6%, a t 27 weeks). Our results indicate that the control of cell number in s uperficial vs. deep regions of the plaque is mainly a consequence of d ifferences in cell replication. This may be due to a gradient of endot helial and plasma-derived growth factors. Cells can disappear by apopt osis, albeit at a relatively low level, throughout the lesion. This pr ocess may contribute to the pronounced cell loss in more advanced huma n atherosclerotic plaques, setting the base for plaque rupture.