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Intermediate Filaments

Intermediate filaments (IFs) [1] are the third type of fibrous cytoskeletal components. Their diameter is about 10 nm so they are typically intermediate in size between microfilaments and microtubules. Unlike the microfilaments and microtubules, the intermediate filaments are made of several different proteins. Therefore, the intermediate filaments can be divided into five major types. Type I and II are composed of acidic and basic keratin, respectively. They are produced by different types of epithelial

cells (i.e., bladder, skin). Type III encompasses intermediate filaments distributed in a number of cell types, including vimentin in fibroblasts, endothelial cells and leukocytes; desmin in muscle; glial fibrillary acidic factor in astrocytes and other types of glia; and peripherin in peripheral nerve fibers. Type IV are neurofilaments and type V are made of laminin (Fig. 2.18).

Distribution of intermediate filaments (composed of vimentin) in primary astrocytes. Scale bar

Figure 2.18 Distribution of intermediate filaments (composed of vimentin) in primary astrocytes. Scale bar: 10 pm. Unpublished data courtesy of Katarzyna Pogoda, IFJ PAN.

Each microtubule is typically composed of 13 protofilaments arranged around the circumference with diameter of about 24 nm and length in the range of 200 nm to 25 pm. A microtubule extends by the addition of tubulin proteins to one of its ends. Microtubules are found in the cytoplasm of all eukaryotic cells where they are often observed to spread out radially from a centrosome located near the nucleus. From this center, they provide a strong frame that supports the cell and determines its shape. They also serve as tracks along which cellular organelles can migrate. Microtubules can also form specialized structures such as centrioles, cilia, and flagella. Both cilia and flagella are cellular appendages consisting of a core of microtubules enclosed in an extension of the plasma membrane, playing an important role in cellular locomotion.

 
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