The skin is made up of a number of layers of dead cells. This layer is called stratified squamous keratinized epithelium. The main advantage of keratinization in the skin is to prevent water from entering ie, it provides a waterproof barrier. This type of keratinized epithelium is found only in the skin. In certain areas like the oesophagus, trachea and small intestine the layer of cells is squamous epithelium but they do not consist of a keratinized coating.

One of the most important requirement of such internal connecting tubes is the increased lubrication. Hence mucous should be produced and secreted by the walls of the oesophagus, intestines, tracheal walls anus and vagina. Keratin is a thick structural protein which prevents the tissues from being stressed. This is highly essential in superficial organs such as skin.