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The endothelium is a single, continuous layer of cells that lines the inside of arteries and provides the principal barrier against the entry of cholesterol and inflammatory cells into the arterial wall. The maintenance of this layer of cells is vital to the prevention of atherosclerotic lesion formation in arteries. Because of their location, endothelial cells experience shearing forces imposed by the flowing blood. Researchers have demonstrated that endothelial cells exposed to continuous flow in vitro elongate and align in the direction of flow; this response is thought to have a streamlining effect and to help cells to anchor themselves to their substrate and to neighboring cells.
Since atherosclerotic lesions in coronary arteries are associated with heart attacks and often form near branches and bends, where the flow field is complex, it is important to know whether the endothelial cells of coronary arteries fail to align in such areas. This knowledge could provide insight into the role of flow patterns in lesion development, as well as the cellular mechanisms linking the two.
Post mortem analysis of pig coronary arteries is performed in this study. While still attached to the heart, the coronary arteries are formalin fixed, and silicone casts of the arteries are made by injecting casting material into them. The arteries are opened up and stained using an antibody for the adhesion molecule PECAM-1, which reveals the borders of endothelial cells. Photomicrographs are taken under fluorescence optics, and image processing is used to automatically identify, and characterize the orientation and shape of, the cells in the images. The left panel of the following image shows one field of such cells stained using this immunofluorescence technique; the cells detected in the immunofluorescence image are shown in the right panel.
The organization of the cells in a field is determined by making various morphometical measurements such as cell eccentricity (defined as the ratio of the lengths of the major and minor axes of a cell) and orientation. Initial studies have revealed that, progressing distally from the inlet of the right coronary artery, as flow patterns likely become more regular, cell eccentricity increases and cells align more homogeneously in the direction of the vessel axis. In more recent studies, endothelial morphometry has been related to local arterial permeability, as measured by Evans blue dye uptake, and to local fluid shear stress obtained from computational fluid dynamics (CFD) calculations based on the vessel geometry as defined by the silicone cast. This analysis showed correlations between endothelial cell area and time-average shear stress, and possibly a relation between permeability and shear stress that varies along the length of the vessel. No relation between permeability and cell morphometry was seen in these initial studies. The following image demonstrates the experimental approach by presenting a microscopic field of cells and its corresponding location on a color coded map of the shear stress field in the same coronary artery.
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