Epithelium

Epithelium
Epithelium
	Types of epithelium
Identifiers
MeSH	D004848
TH	H2.00.02.0.00002
FMA	9639
	Anatomical terms of microanatomy

Epithelium or epithelial tissue is a thin, continuous, protective layer of compactly packed cells with little extracellular matrix. 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. An example is the epidermis, the outermost layer of the skin. Epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. These tissues also lack blood or lymph supply. The tissue is supplied by nerves.

There are three principal shapes of epithelial cell: squamous (scaly), columnar, and cuboidal. These can be arranged in a singular layer of cells as simple epithelium, either simple squamous, simple columnar, or simple cuboidal, or in layers of two or more cells deep as stratified (layered), or compound, either squamous, columnar or cuboidal. In some tissues, a layer of columnar cells may appear to be stratified due to the placement of the nuclei. This sort of tissue is called pseudostratified. All glands are made up of epithelial cells. Functions of epithelial cells include diffusion, filtration, secretion, selective absorption, germination, and transcellular transport. Compound epithelium has protective functions.

Epithelial layers contain no blood vessels (avascular), so they must receive nourishment via diffusion of substances from the underlying connective tissue, through the basement membrane.^[1]^[2]^: 3 Cell junctions are especially abundant in epithelial tissues.

^ Eurell JA, Frappier BL, eds. (2006). Dellmann's Textbook of Veterinary Histology. Wiley-Blackwell. p. 18. ISBN 978-0-7817-4148-4.
^ Freshney RI (2002). "Introduction". In Freshney RI, Freshney M (eds.). Culture of epithelial cells. John Wiley & Sons. ISBN 978-0-471-40121-6.

[Eurell_2006-1] Eurell JA, Frappier BL, eds. (2006). Dellmann's Textbook of Veterinary Histology. Wiley-Blackwell. p. 18. ISBN 978-0-7817-4148-4.

[Freshney_2002-2] Freshney RI (2002). "Introduction". In Freshney RI, Freshney M (eds.). Culture of epithelial cells. John Wiley & Sons. ISBN 978-0-471-40121-6.

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