Figure 245

Electron micrograph of the apical part of an epithelial cell. This electron micrograph, obtained using the quick-freeze deep-etch technique, shows the terminal web (TW) of an epithelial cell and underlying intermediate filaments (IF). The long straight actin filament cores or rootlets (R) extending from the microvilli are cross-linked by a dense network of actin filaments containing numerous actin-binding proteins. The network of intermediate filaments can be seen beneath the terminal web anchoring the actin filaments of the microvilli, x 47,000. (From Hirokawa N, et al. J Cell Biol 1983;96:1325.)

Intermediate filaments are a heterogeneous group of cytoskeletal elements found in various cell types

Intermediate filaments are organized into four major classes on the basis of protein composition and cellular distribution (Table 2.1):

• Keratins (cytokeratins), the most diverse group of intermediate filaments, contain more than 50 different iso-forms. Keratin filaments are formed from a variety of different keratin subunits and are found in different cells of epithelial origin. Specialized keratins called hard keratins are found in skin appendages such as hair and nails. Keratin filaments span the cytoplasm of epithelial cells and, via desmosomes, connect with keratin filaments in neighboring cells. Keratin subunits do not coassemble with

staggered tetramer
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