Desmin is an intermediate filament protein (53 kDa) expressed in all striated muscle cells and most smooth muscle cells. In the developing striated muscle cells it replaces vimentin, linked to the Z-disc. In the smooth muscle cells desmin is associated with cytoplasmic dense bodies and subplasmalemmal dense plaques. In myofibroblasts and vascular smooth muscle cells desmin is co-expressed with vimentin. However, in the arterial smooth muscle cells, desmin may be scarce or absent, while vimentin expression is retained. Mesothelial cells may express desmin, particularly in effusions and cultures. Desmin play no role in contractility but serves to maintain the orientation of actin and myosin filaments and may also play a role in nuclear transcription.
Desmin is detected in most tumours of myogenic origin, e.g., leiomyosarcoma and rhabdomyosarcoma. Low differentiated myogenic tumours may lack desmin. Triton tumour, glomus tumour, alveolar soft part sarcoma, and PEComa frequently express desmin. Malignant fibrous histiocytoma, fibrosarcoma, liposarcoma and carcinosarcoma often show focal desmin positivity (as a result of a partial myogenic differentiation). Malignant mesothelioma is positive in about 10% of the cases. A few non-myogenic tumours such as lung adenocarcinoma, malignant melanoma and gliomas are occasionally reported desmin positive. However, in some cases, staining may be due to cross reaction with another intermediate filaments. Gastrointestinal stromal tumour (GIST) is virtually always desmin negative (positivity reported in 5%, possibly in some cases due to staining of entrapped smooth muscle cells).
Desmin is used in a panel for identification of leiomyosarcoma, rhabdomyosarcoma and other tumours with myoid differentiation, and for classification of mesenchymal, pleomorphic, spindle cell and round cell neoplasms. Desmin may also be used in the differential diagnosis of reactive mesothelium (mostly desmin positive) and epithelioid malignant mesothelioma (mostly desmin negative).
Appendix is recommendable as positive and negative tissue control. Virtually all the smooth muscle cells in the lamina muscularis mucosae and muscularis propria must show a moderate to strong cytoplasmic staining reaction, while an at least weak to moderate staining reaction must be seen in most smooth muscle cells in the vessels and in dispersed myofibroblasts lining the epithelial cells.
No staining reaction must be seen in the appendiceal epithelial cells.
Tonsil can be used primarily as supplementary negative tissue control. No staining reaction must be seen in lymphocytes and epithelial cells.
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