The expression patterns of a variety of cytoskeletal antigens were studied in normal human tissues (placenta, umbilical cord, myometrium, colon, mammary gland, testis, skeletal muscle, myocardium) as well as in abnormal human tissues (palmar fibromatosis, fibrocystic disease of the mammary gland, mammary carcinoma). The immunohistochemical binding patterns of the monoclonal antibody GB 42 were compared to those of commerical antibodies directed against vimentin, desmin, smooth muscle myosin, pan actin, α-smooth muscle actin and γ-smooth muscle actin. Methods applied comprised immunohistochemistry on cryostat sections and paraffin sections. Immunogold immunocytochemistry was performed on Lowicryl sections. The patterns of GB 42-binding were confirmed biochemically by SDS-PAGE and Western-blotting, and quantitative amino acid analysis. Our data suggest that the monoclonal antibody GB 42 recognizes an actin isoform which is identical to, or closely related to, γ-smooth muscle actin. Unlike the commercially available antibody against γ-smooth muscle actin, GB 42 does not cross-react with α-skeletal or α-cardiac actins. The GB 42-antigen is expressed in smooth muscle cells, myoepithelial cells and in later stages of differentiation of myofibroblasts, in all the tissues investigated. Throughout the development of smooth muscle cells and myofibroblasts, the appearance of the GB 42-antigen occurs after the expression of vimentin, desmin and α-smooth muscle actin, but prior to the expression of smooth muscle myosin. GB 42 is a reliable marker for higher stages of differentiation of smooth muscle cells and myofibro
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