To better understand the pathological processes that occur with atherosclerosis, an understanding of the structure of arteries is required. Human arteries are composed of three layers, the intima, the media and the adventitia. The intima is the innermost layer of the artery, composed of EC’s, SMC’s, macrophages and extracellular matrix (ECM) components. An internal elastic lamina separates the intima from the media, which is made up mainly of SMC. The adventitia is separated from the media by external elastic lamina and is mainly composed of fibroblasts and connective tissue.
The initiation and progression of atherosclerotic plaques generally takes place over many years during which the affected individual remains symptom free. Therefore, when a patient becomes symptomatic, the disease is already well established. These plaques occur at specific sites within arteries and these sites are dictated by fluid shear stress, the frictional force generated by blood flow over the vascular endothelium (Hwang et al, 2003). Regions of branched and curved arteries experience the greatest disturbed blood flow and it is at these sites that high incidences of plaque formation is found (VanderLaan et al, 2004). Relatively straight arteries however, experience the least shear stress and are usually protected from plaque development. Explanations for why high fluid stress sites are more “lesion-prone” is currently speculative.
The initial factors which result in the initiation of plaque formation are currently unknown. A common hypothesis is that plaque formation occurs as a result of EC damage leading to cellular dysfunction (Shimokawa, 1999, Davignon and Ganz, 2004). The source of the initial damage to EC’s is also currently unknown, but hypertension, viruses, toxins, smoking have all been suggested. Cellular dysfunction results in subsequent recruitment and accumulation of leukocytes and monocytes which would otherwise have resisted any adhesive interactions (Bobryshev et al, 2005). These adhered monocytes then differentiate into macrophages, engulf lipids, become foam cells and form fatty streaks. As the progression of the plaque continues, SMC’s migrate from the intima and synthesis extracellular matrix proteins in the intima (Boyle et al, 1997). Progressive macrophage accumulation, SMC migration and proliferation and extracellular matrix protein synthesis result in the formation of an advanced lesion.
A schematic representation of the structure of an artery, showing the intima, media and adventitia (commons.wikimedia.org)