Aortic intimal response to endothelial removal in cebus and squirrel monkeys

Whereas squirrel monkeys have an inherent susceptibility to atherosclerosis, cebus monkeys are relatively resistant. To assess whether this difference might lie in their response to endothelial injury, the acute morphologic changes in the aortic intima after endothelial removal were examined in the two species. The endothelium of the lower thoracic aorta was removed with an embolectomy catheter, and the intimal response was compared with the uninjured upper thoracic aorta in each monkey. By 21 days after aortic denudation, regrowth of the endothelium (assessed by in vivo Evans' blue dye staining) was significantly greater in squirrel monkeys (90% +/- 5% of aortic surface) than in cebus (56% +/- 11%). Squirrel monkeys had comparable sudanophilic surface in the nonballooned, control aorta (25% +/- 7%) and the ballooned, lower thoracic segment (17% +/- 6%). Cebus monkey aortas had no sudanophilia in either segment. The intima/media ratios (IMR) in all regions of the aorta were significantly greater in squirrel monkeys than in cebus, but in both species the IMR of the ventral ballooned segment was two to three times the IMR of the nonballooned control segment. In the dorsal aorta, where endothelial regrowth was more rapid, the IMR was similar to the control aortic segment. By electron microscopy the thickened aortic intima in both species contained a marked increase in modified smooth muscle cells, but lipid accumulation did not result from endothelial removal or regrowth in either species. Thus, although the squirrel monkey aorta had atherosclerotic lesions before endothelial removal, the acute intimal response to endothelial injury was similar in degree and kind in both cebus and squirrel monkeys. This suggests that factors other than those controlling the initial intimal thickening following endothelial injury are responsible for the observed difference in arterial lipid accumulation between cebus and squirrel monkeys.