Ultrastructural and immunocytochemical investigations reveal that a third major filamentous structure is present in eukaryotic cells. In addition to the thin (actin) and thick (myosin) filaments, cells contain a class of intermediate-sized filaments with an average diameter of 10–12 nm (Figure 2–37 and Table 2–4). Several proteins that form intermediate filaments have been isolated and localized by immunocytochemical means.
Electron micrograph of a skin epithelial cell showing intermediate filaments of keratin associated with desmosomes.
Keratins (Gr. keras, horn) are a family of approximately 20 proteins found in epithelia. They are encoded by a family of genes and have different chemical and immunological properties. This diversity of keratin is related to the various roles these proteins play in the epidermis, nails, hooves, horns, feathers, scales, and the like that provide animals with defense against abrasion and loss of water and heat.
Vimentin filaments are characteristic of cells of mesenchymal origin. (Mesenchyme is an embryonic tissue.) Vimentin is a single protein (56–58 kDa) and may copolymerize with desmin or glial fibrillary acidic protein.
Desmin (skeletin) is found in smooth muscle and in the Z disks of skeletal and cardiac muscle (53–55 kDa).
Glial filaments (glial fibrillary acidic protein) are characteristic of astrocytes but are not found in neurons, muscle, mesenchymal cells, or epithelia (51 kDa).
Neurofilaments consist of at least three high-molecular-weight polypeptides (68, 140, and 210 kDa). Intermediate filament proteins have different chemical structures and different roles in cellular function.
The presence of a specific type of intermediate filament in tumors can reveal which cell originated the tumor, information important for diagnosis and treatment (see Table 1–1). Identification of intermediate filament proteins by means of immunocytochemical methods is a routine procedure.
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