Smooth Muscle and Endothelial CellsSmooth muscle cells are found in variable amounts in fatty streaks, but are the main cell type in fibrous plaques. They contain receptors for PDGF and LDL. The phenotype of these intimal smooth muscle cells is altered from a contractile to a secretory type. Production of an extracellular matrix and further accumulation of lipid and foam cells enlarge these lesions until the core becomes ischemic and eventually necrotic. Following arterial injury, smooth muscle cell proliferation begins in the media. Cells then migrate into the intima where replication continues. The observation that smooth muscle cells may elaborate both PDGF and PDGF receptors heralds a new understanding in the complex, local hormonal environment in atherosclerotic plaque. 7–11 The fibrous plaque consists of multiple layers of smooth muscle cells, each of which lies in a panelled arrangement comprising alternating layers of basement membrane and proteoglycan. Deep to this cellular area is a region of cholesterol crystals, necrotic debris, and calcification. There is increasing interest in circulating stem cells as possible precursors of the transformed smooth muscle cells at sites of arterial injury.
Endothelial cells regulate which plasma constituents reach the subendothelial space. They bind LDLs through a specific receptor, transcytose, and modify it so it is recognized and ingested by cells such as macrophages. They further produce and release growth factors and inhibitors, which may have profound effects on smooth muscle cells and macrophages.
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