Epithelium

Epithelium is one of the four basic types of animal tissue, along with connective tissue, muscle tissue, and nervous tissue. Epithelial tissues line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs.

Structure

Epithelial tissue is composed of cells laid together in sheets with the cells tightly connected to one another. Epithelial layers are avascular, but innervated. Epithelial cells have two surfaces that differ in both structure and function. One surface is not in contact with other cells and is exposed to either the external environment or the lumen of an internal organ. It is called the apical surface. The other surface is attached to a basement membrane, which is, in turn, attached to an underlying connective tissue.

Functions

Epithelial tissues have several functions, which include protection against abrasion, radiation damage, chemical stress and invasion by pathogens. A single organ can have different types of epithelial tissue based on the substances to which different surfaces are exposed. Shapes of epithelial cells include squamous (flattened and thin), cuboidal (cube-shaped), and columnar (taller than wide).

Types

Epithelial tissues can be classified into two groups: simple epithelium and stratified epithelium. A simple epithelium consists of a single layer of cells with each cell extending from the basement membrane to the free surface. A stratified epithelium consists of several stacked layers of cells. Only one layer is in contact with the basement membrane, while the other layers adhere to one another to maintain structural integrity.

Clinical significance

Epithelial tissue has a number of functions, which include secretion, selective absorption, protection, transcellular transport, and sensing. Epithelial tissues contain no blood vessels, so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.

See also

• Cell junction
• Endothelium
• Mesothelium
• Glandular epithelium
• Pseudostratified columnar epithelium

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