Epithelium

In biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue. It lies on top of connective tissue, and the two layers are separated by a basement membrane.

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue.

Epithelium is often defined by the expression of the adhesion molecule e-cadherin (as opposed to n-cadherin, which is used by cells of the connective tissue).

Functions of epithelial cells include secretion, selective absorption, protection, transcellular transport and detection of sensation and they commonly as a result present extensive apical-basolateral polarity (e.g. different membrane proteins expressed) and specialisation.

Classification (structural)

Epithelial tissue can be structurally divided into two groups depending on the number of layers of which it is composed. Epithelial tissue which is only one cell thick is known as simple epithelium. If it is two or more cells thick, it is known as stratified epithelium. However, when taller simple epithelial cells (see columnar, below) the variably placed nuclei of are viewed in cross section, they can be confused with stratified epithelia. This kind of epithelium is therefore described as "pseudostratified" epithelium.

Regardless of the type, any epithelium is separated from the underlying tissue by a thin sheet of connective tissue; basement membrane. The basement membrane provides structural support for the epithelium and also binds it to neighbouring structures.

Simple epithelium

Simple epithelium is one cell thick; that is, every cell is in contact with the underlying basal lamina. Simple epithelium can be subdivided further according to the shape and function of its cells.

Stratified Epithelium

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.

In addition though, stratified epithelia (of columnar, cuboidal or squamous type) can have the following specialisations:

Functions

• Protection: Epithelial cells protect underlying tissue from mechanical injury, harmful chemicals and pathogens and excessive water loss.

• Sensation: Sensory stimuli are detected by specialised epithelial cells. Specialised epithelial tissue containing sensory nerve endings is found in the skin, eyes, ears and nose and on the tongue.

• Secretion: In glands, epithelial tissue is specialised to secrete specific chemical substances such as enzymes, hormones and lubricating fluids.

• Absorption: Certain epithelial cells lining the small intestine absorb nutrients from the digestion of food.

• Excretion: Epithelial tissues in the kidney excrete waste products from the body and reabsorb needed materials from the urine. Sweat is also excreted from the body by epithelial cells in the sweat glands.

• Diffusion: Simple epithelium promotes the diffusion of gases, liquids and nutrients. Because they form such a thin lining, they are ideal for the diffusion of gases (e.g. walls of capillaries and lungs).

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