Population dynamics of arterial cells during atherogenesis. VII. Comparison of loss of endothelial cells over abdominal aortic intimal cellular masses (ICM) with that over non-ICM areas in swine fed a hyperlipidemic diet for 60 days

Abstract

One of the earliest events in the development of an intimal atherosclerotic lesion may be an alteration in the endothelial cell barrier, allowing the entry of injurious or mitogenic substances, resulting in proliferation of arterial smooth muscle cells. Intimal smooth muscle cell proliferation in atherosclerosis appears to begin in intimal cellular masses (ICM), which are normally occurring intimal structures made up of 2 to 10 layers of smooth muscle cells found in predominantly in relation to branching points of arteries. If endothelial cell loss is a precursor of overt atherosclerotic lesions, the loss should be greater over ICM than over areas showing no ICM. This study presents evidence suggesting that in swine the endothelial cell barrier is compromised over intimal cell masses more than in non-ICM regions prior to development of overt atherosclerotic lesions. The method used was to label swine aortic endothelium and ICM using tritiated thymidine. All swine were then placed on a low-fat, low-cholesterol diet for 15 days, at which time five baseline animals were sacrificed; the remaining five were placed on a hyperlipidemic (HL) diet and sacrificed 60 days later. The rate of endothelial cell growth, division patterns, and loss of labeled endothelial cells in the HL swine was determined by comparing the findings in this group with the baseline group. Smooth muscle cells of ICM and non-ICM medial regions were similar in regard to labeling indices in the baseline swine and in regard to division patterns in 60 day HL diet swine, except one which was an active lesion. None of the remaining ICM were active lesions and thus were suitable for study of the covering endothelial cells in what can be assumed to be the prodromal stage of lesion development. The approximate area of the abdominal aorta occupied by intimal cell mass was considerably more than we had anticipated, averaging more than 20%. The labeling indices and endothelial cell division patterns over ICM and non-ICM in the baseline swine were similar. In contrast, the labeling index was lower, and grain count changes indicated more endothelial cell divisions over ICM areas than over non ICM areas in the hypercholesterolemic swine. Calculation showed that the percentage of endothelial cell loss over ICM in the HL swine was more than twice as great as endothelial cell loss over non-ICM areas. The results of this study suggest the possibility that in the assumed prodromal phase of experimental aortic atherosclerosis in young swine, the endothelial cell barrier may be compromised selectively over areas of predilection for the future development of atherosclerotic lesions. This occurs before there is any evidence of focal increase in intimal smooth muscle cell proliferation, which we regard as the earliest detectable stage of lesion development.