Intermediate Filaments

In the remainder of this chapter, we consider the properties of intermediate filaments (IFs) and the cytoskeletal structures that they form in cells. Intermediate filaments are found in nearly all animals but not in in plants and fungi. The association of intermediate filaments with the nuclear and plasma membranes suggests that their principal function is structural (Figure 19-31). In epithelium, for instance, intermediate filaments provide mechanical support for the plasma membrane where it comes into contact with other cells or with the extracellular matrix. In epidermal cells (outer layer of skin) and the axons of neurons (Figure 19-32), intermediate filaments are at least 10 times as abundant as microfilaments or microtubules, the other components of the cytoskeleton.

Much of the following discussion about intermediate filaments will seem familiar because their cellular organization is similar to that of the actin microfilaments discussed in preceding sections. These two types of cytoskeletal fibers are

▲ EXPERIMENTAL FIGURE 19-31 Staining with fluorochrome-tagged antibodies reveals cellular distribution of keratin and lamin intermediate filaments. In this fluorescent micrograph of a PtK2 cell doubly stained with anti-keratin and anti-lamin antibodies, a meshwork of lamin intermediate filaments (blue) can be seen underlying the nuclear membrane. The cytoplasmic keratin cytoskeleton (red) extends from the nuclear membrane to the plasma membrane. [Courtesy of R. D. Goldman.]

also similar in that they are usually associated with cell membranes. Unlike microfilaments and microtubules, however, intermediate filaments do not contribute to cell motility. There are no known examples of IF-dependent cell movements or of motor proteins that move along intermediate filaments.

► EXPERIMENTAL FIGURE 19-32 Deep-etching reveals microtubules and intermediate filaments in a neuronal axon. Neurofilaments and microtubules in a quick-frozen frog axon are visualized by the deep-etching technique. Several 24-nm-diameter microtubules run longitudinally; thinner, 10-nm-diameter intermediate filaments also run longitudinally. Occasional connections link the two types of cytoskeletal fibers. [From N. Hirokawa, 1982, J. Cell Biol. 94:129; courtesy of N. Hirokawa.]

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