On the basis of their chemical compositionthey can be separated into five major groups: vimentin, keratins, desmin, glial fibrillary acidic protein, and neurofilaments.
Vimentin is a filamentous protein universally expressed in mesenchymal cells and in some epithelial cells and their neoplasms. In epithelial cells, it is typically coexpressed with other epithelial markers, such as keratins. For these two reasons, the specific diagnostic applicability of vimentin in the differential diagnosis of tumors is minimal.
It is most often used to verify the antigenicity of cells in question when other markers are negative.
Cytokeratin is a collective term for a group of 19 polypeptides. Keratins can be separated into two major categories, A and B (acidic and basic), on the basis of their electrophoretic behavior. Cytoplasmic keratin molecules consist of pairs of acidic and basic components.
The basic component is typically the larger molecule of the pair. Different epithelia have different keratin profiles. Keratins are markers of epithelial differentiation.
Tumors such as chordoma and, to some extent, chondrosarcoma express keratins. RNA that code for keratins are expressed in a wide range of mesenchymal cells and tumors of mesenchymal origin.
In skeletal pathology, keratins are used in the diagnosis of adamantinoma, chordoma, chondrosarcoma, epithelioid hemangioendothelioma, and sarcomatoid carcinoma and in the differential diagnosis of metastatic neoplasms.
Desmin is a filamentous molecule expressed in smooth, skeletal, and cardiac muscle fibers. Desmin is a marker of muscle differentiation, but it is also present in other cells that have contractile properties such as myofibroblasts.
Desmin is also expressed in some fetal cells such as embryonal mesothelium, stromal cells of fetal kidney, and chorionic villi. In general pathology, desmin is used as a marker for the diagnosis of tumors that exhibit muscle-predominately skeletal differentiation.
It is typically used in the differential diagnosis of primary and metastatic spindle-cell neoplasms and in the documentation of rhabdomyoblastic differentiation in dedifferentiated chondrosarcoma. It is also frequently used in the differential diagnosis of small-cell tumors of bones.
Neurofilaments protein is expressed in most neuronal cells. It is subdivided into three distinct polypeptides that differ in their molecular weight. Typically, all three polypeptides are expressed, but some neuronal cells may lack all or some of the neurofilament proteins. In general pathology, neurofilaments are used as markers of neural differentiation.
With reference to bone tumors, neurofilament proteins are used in the differential diagnosis of primary and metastatic small-cell tumors (e.g., neuroblastoma). Poorly differentiated tumor cells may express undetectable levels of neurofilament protein. Moreover, fixation and paraffin embedding significantly reduce the stainability of cells for this marker.