Cytokeratins of the Liver and Intestine Epithelial Cells During Development and Disease

A large part of the cytoplasm of the cells consists of components forming cytoskeleton. The constituents of the cytoskeleton in epithelial cells are actin-containing microfilaments, tubulin-containing microtubules and intermediate size filaments. The intermediate filaments are called as cytokeratins (CK). Thus, cytokeratins are a family of many different filamentforming proteins (polypeptides) with specific physicochemical properties and are normal components of epithelial cell cytoskeleton. CK are expressed in various types of epithelia in different combinations. Cytokeratins account for about 80% of the total protein content in differentiated cells of stratified epithelia (Pekny and Lane 2007). In both human and murine stratified epidermis, CK account for 25–35% of the extracted proteins(Bowden, Quinlan et al. 1984). The expression of proteins forming intermediate filaments can change when epithelial cells develop into mesenchymal cells and vice versa(Moll, Moll et al. 1984). For example, during neural tube formation, CK-producing ectodermal cells change into vimentinproducing mesenchymal cells, whereas during the formation of renal tubules vimentinproducing mesenchymal cells change into CK-producing epithelial cells(Moll, et al. 1984). Different types of cytokeratins are distinguished according to various characteristics, such as physicochemical properties, or according to the cells and tissues that produce certain CK. In simple, non-stratified epithelia these proteins are different than those in stratified epithelia. Epithelial cells in simple as well as in stratified epithelia always synthesize particular CK on a regular basis. These cytokeratins are referred to as the primary keratins of epithelial cells, such as CK8/CK18 in simple epithelia (Pekny and Lane 2007) or CK5/CK14 in stratified(Moll, Franke et al. 1982). In addition or instead, these epithelial cells can also produce secondary CK, such as CK7/CK19 in simple epithelia or CK15 and CK6/CK16 in stratified epithelia.