Invasion of atheromatous plaques into tunica media causes coronary outward remodeling in WHHLMI rabbits

To clarify the mechanism of coronary outward remodeling, we examined atherosclerotic coronary arteries morphologically using WHHLMI rabbits that develop coronary atherosclerosis spontaneously. Perfusion-fixed coronary segments of WHHLMI rabbits were prepared at 500 μm intervals. After immunohistochemical staining and histopathological staining, the areas and lengths of the arterial wall and the lesions were measured. Obvious outward remodeling was observed in coronary sections with greater than 40% cross-sectional narrowing. In coronary sections with greater than 40% cross-sectional narrowing, the tunica media was thick at the shoulder of atheromatous plaque and was thin beneath the atheromatous plaques. Macrophages infiltrated those attenuated tunica media expressed matrix metalloproteinases and oxidized LDL was accumulated in those areas. In those areas, collagen fibers and the internal elastic lamina had disappeared partly and apoptotic smooth muscle cells were observed. Proliferation of smooth muscle cells was observed at the attenuated tunica media and adjacent adventitia. The present results suggest that invasion of atheromatous plaques into the tunica media causes coronary outward remodeling.