<P> Stratified epithelium differs from simple epithelium in that it is multilayered . It is therefore found where body linings have to withstand mechanical or chemical insult such that layers can be abraded and lost without exposing subepithelial layers . Cells flatten as the layers become more apical, though in their most basal layers the cells can be squamous, cuboidal or columnar . </P> <P> Stratified epithelia (of columnar, cuboidal or squamous type) can have the following specializations: </P> <Table> <Tr> <Th> Specialization </Th> <Th> Description </Th> </Tr> <Tr> <Td> Keratinized </Td> <Td> In this particular case, the most apical layers (exterior) of cells are dead and lose their nucleus and cytoplasm, instead contain a tough, resistant protein called keratin . This specialization makes the epithelium waterproof, so is found in the mammalian skin . The lining of the esophagus is an example of a non-keratinized or "moist" stratified epithelium . </Td> </Tr> <Tr> <Td> Parakeratinized </Td> <Td> In this case, the most apical layers of cells are filled with keratin, but they still retain their nuclei . These nuclei are pyknotic, meaning that they are highly condensed . Parakeratinized epithelium is sometimes found in the oral mucosa and in the upper regions of the esophagus . </Td> </Tr> <Tr> <Td> Transitional </Td> <Td> Transitional epithelia are found in tissues that stretch and it can appear to be stratified cuboidal when the tissue is relaxed, or stratified squamous when the organ is distended and the tissue stretches . It is sometimes called urothelium since it is almost exclusively found in the bladder, ureters and urethra . </Td> </Tr> </Table> <Tr> <Th> Specialization </Th> <Th> Description </Th> </Tr>

Other examples of epithelial tissues that are secretory in function