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Epithelial cells are subject to significant mechanical stress and therefore require a structure to protect them from permanent damage. This structure is the cytoskeleton and it is formed by three classes of proteins: microfilaments (principally actin), microtubules (principally tubulin), and intermediate filaments (IFs) (Steinert and Roop, 1988). The IF proteins differ from the actin and tubulin proteins in several important ways. The IFs are a highly diverse family of proteins. In contrast, there are only approximately 6 forms of actin and six forms each of the 2 types of tubulin (α and,β). IF proteins are non-polar, elongated molecules. In contrast, the actin and tubulin proteins are polar and globular molecules. The sizes of the cytoskeletal proteins range from 6 nm (actin) and 10 nm (IFs) to 23 nm (tubulin).