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Figure 1 The transition from the normal artery wall to the nascent atherosclerotic lesion. The normal muscular artery has a trilaminar structure. A monolayer of endothelial cells overlies the intimal layer and abuts a basement membrane. In human arteries, the intima normally contains a few resident smooth muscle cells and a layer of extracellular matrix. The internal elastic lamina constitutes the boundary between the intimal layer and the tunica media, normally filled with quiescent smooth muscle cells embedded in an elastin-rich extracellular matrix. When molecules associated with risk factors stimulate oxidative or inflammatory stress, they induce the expression of adhesion molecules for leukocytes and chemoattractants that draw the bound leukocytes into the intimal layer. This diagram does not depict the adventitia, the outermost layer of the blood vessel.