Myosin is a cytoplasmic actin based motor protein, about 480 kDa, particularly associated with striated muscle cells, smooth muscle cells, and myoepithelial cells. It consist of 2 heavy chains (HC; 223 kDa) and four light chains (LC). Different isotypes of HC are seen in different cell types. In striated muscle cells, HC can be type I = slow myosin, or type II = fast myosin. In smooth muscle cells and myoepithelial cells, there are also two isotypes. Smooth muscle heavy chain myosin (SMH; 200-209 kDa) is a relatively specific marker for terminal smooth muscle cell differentiation, even though myofibroblasts and other cells show limited expression. Thus, SMH has been demonstrated in endothelial cells of lymph node postcapillary venules, splenic sinus lining cells, and dendritic follicular cells in lymph node germinal centres and splenic B-cell areas .
SMH can be demonstrated in the majority of smooth muscle cell tumours and myoepithelial tumours.
For the identification of smooth muscle cell tumours and myoepithelial tumours, SMH should be used in a panel of muscle cell markers. In neoplastic breast lesions, the myoepithelial cells expressing SMH are usually preserved in papillomas, benign sclerosing changes and in situ ductal carcinoma, while not expressed in invasive ductal carcinoma. Myoepithelial markers therefore are important tools in the differential diagnosis. Apart from SMH, alpha-smooth muscle actin (ASMA), S-100 protein, calponin, p63, CD10 and High molecular weight cytokeratins are or have been used as myoepithelial cell markers. SMH in combination with p63 have shown the highest combination of sensitivity and specificity.
Tonsil was found to be the preferred positive and negative tissue control for SMH. Virtually all smooth muscle cells in vessels must show a moderate to strong cytoplasmic staining reaction, while the follicular dendritic cells in the germinal centres must show an at least weak to moderate but distinct staining reaction.
No staining should be seen in lymphocytes and epithelial cells.
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