Epithelial cells form the cover of all internal and exterior body surfaces. They are positioned in close density and are characterised by plenty of cellular contacts. They do not contain blood vessels. Their polarity is another property which all of them have in common. They have an outer apical surface which is oriented to the exterior (e.g. of skin) or to the lumen (e.g. of intestine or glands) and a basal surface connected through basal lamina to the tissue layers beneath. The polarity of epithelial cells is characterised by structural and functional differences between their apical and basal membranes.