Epithelium is made up of the ensemble of epithelial cells (which are regular square shaped cells fitting closely together and having little intercellural space). It does not contain blood vessels, cells are closely attached to each other. The function of epithelium is to dissociate organs and to cover their surface continuously. It covers the inside and outside surface of our body (the surface layer of skin, the digestive organ (mucous membrane), the respiratory and secretory system), there is a connective tissue substance below. It contains nerve endings.
Classification of epitheliums according to their function:
- covering or protective epithelium (e.g. the mucous membrane of skin): having dissociative and protective role,
- pigment epithelium (e.g. eye, skin): taking part in the protection against ultraviolet rays,
- glandular epithelium (e.g. saliva, bile, perspiration): is made up of special epithelial cells which, apart from secretion, produces excretion, too. (Glands that secrete their products onto the body surface (e.g.perspiration) or into some cavity ( e.g. gastric juices) are called exocrine glands and those which secrete their products directly into the bloodstream (e.g. hormones) are called endocrine glands).
- sensory epithelium (e.g. in the olfactory region of nasal cavity, at the taste buds of tongue) transports stimulus towards the nervous system, serves as a transition to the nervous tissue, it is continued in nerve fibers.
- absorbing epithelium (e.g. within the walls of stomach and bowels) make different substances get into the body.
Classification of epitheliums according to the shape of cells:
- squamous,
- cuboidal,
- columnar.
Classification of epitheliums according to the number of layers:
- single layer: squamous-, cuboidal, columnar epithelium;
- stratified squamous keratinized epithelium (e.g. the skin)
- stratified squamous non – keratinized epithelium (e.g. mucous membrane, nail bed).
