Myofibroblasts appear in the light microscope as spindle-shaped cells with acidophilic cytoplasm . In ultrastructural microscopy these cells show features of both fibroblasts and smooth muscle cells. Because the differences between smooth muscle cells and myofibroblasts are subtle, a definition of myofibroblasts is needed to identify some features by ultrastructural methods, such as peripheral bundles of microfilaments with dense bodies, a well-developed rough endoplasmic reticulum, a notched nucleus, fibronectin fibrils on the surface, and fibronexus junctions connected with the surrounding extracellular matrix [25-28]. Because these cells are immunohistochemically positive for a-smooth muscle actin (a-SMA), a-SMA is a useful marker of myofibroblasts [2, 29]. Desmin is not a useful marker of myofibroblasts because these cells are negative for desmin in the human pancreas, although positive in rats . We take account of these staining patterns in diagnosis of the myofibroblast in human pancreas specimens .
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