AskDefine | Define epithelial

Dictionary Definition

epithelial adj : of or belonging to the epithelium; "epithelial layer"

User Contributed Dictionary

English

Adjective

  1. of or pertaining to the epithelium

Extensive Definition

In biology and medicine, epithelium is a tissue composed of layers of cells that line the cavities and surfaces of structures throughout the body. It is also the type of tissue of which many glands are formed. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.
Mucous membranes lining the inside of the mouth, the oesophagus, and part of the rectum are lined by nonkeratinized stratified squamous epithelium. Other, open to outside body cavities are lined by simple squamous or columnar epithelial cells.
Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily-regenerated epithelial cells.
Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.
Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.
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.

Classification

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells)

  • Squamous: All Squamous cells are flat cells with an irregular flattened shape. A one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane, and the endothelium of capillaries, and is a minimal barrier to diffusion. Squamous cells can be found included in the filtration tubules of the kidneys, and the major cavities of the body. These cells are relatively inactive metabolically, and are associated with the diffusion of water, electrolytes, and other substances.
  • Cuboidal: As the name suggests, these cells have a shape similar to a cube, meaning its width is the same size as its height. The nuclei of these cells are usually located in the center. The cuboidal epithelium forms the smallest duct glands and many kidney tubules.
  • Columnar: These cells are taller than they are wide. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The small intestine is a tubular organ lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cell has tiny hairlike projections called microvilli. They increase the surface area for absorption.
  • Transitional: This is a specialized type of epithelium found lining organs that can stretch, such as the urothelium that lines the bladder and ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.

Stratification

  • Simple: There is a single layer of cells.
  • Stratified: More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.
  • Pseudostratified with cilia: This is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, but the position of the nuclei gives the impression that it is stratified. If a specimen looks stratified, but you can identify cilia, the specimen is pseudostratified ciliated epithelium. Stratified epithelium cannot have cilia but may be very rarely found in the fetal esophagus. A cell that contains hairs will be around ten times stronger than a regular cell.

Specializations

  • Keratinized cells contain keratin (a cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier.

Examples

Cell junctions

A cell junction is a structure within a tissue of a multicellular organism. Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they built up the paracellular barrier of epithelia and control the paracellular transport.

Secretory epithelia

As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands: endocrine glands and exocrine glands. Endocrine glands are glands that secrete their product directly onto a surface rather than through a duct. This group contains the glands of the Endocrine system

Embryology

In general, there are epithelial tissues deriving from all of the embryological germ layers:
However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas. Also, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers.

Additional images

Image:Tkanka nablonkowa.png

References

  • Molecular Biology of the Cell, 4th edition, Alberts et al., 2002
epithelial in Afrikaans: Epiteelweefsel
epithelial in Bulgarian: Епителна тъкан
epithelial in Catalan: Teixit epitelial
epithelial in Czech: Epitelová tkáň
epithelial in Danish: Epitel
epithelial in German: Epithel
epithelial in Urdu: ظہارہ
epithelial in Spanish: Epitelio
epithelial in Esperanto: Epitelio
epithelial in Basque: Epitelio ehun
epithelial in French: Épithélium
epithelial in Korean: 상피세포
epithelial in Croatian: Epitelno tkivo
epithelial in Indonesian: Jaringan epitel
epithelial in Italian: Tessuto epiteliale
epithelial in Hebrew: אפיתל
epithelial in Latvian: Epitēlijaudi
epithelial in Lithuanian: Epitelinis audinys
epithelial in Malay (macrolanguage): Epitelium
epithelial in Dutch: Epitheel
epithelial in Japanese: 上皮細胞
epithelial in Norwegian: Epitel
epithelial in Norwegian Nynorsk: Epitel
epithelial in Polish: Tkanka nabłonkowa
epithelial in Portuguese: Epitélio
epithelial in Russian: Эпителий
epithelial in Simple English: Epithelium
epithelial in Slovak: Epitel
epithelial in Slovenian: Krovno tkivo
epithelial in Serbian: Епителско ткиво
epithelial in Serbo-Croatian: Epitelsko tkivo
epithelial in Finnish: Epiteelikudos
epithelial in Swedish: Epitel
epithelial in Thai: เนื้อเยื่อบุผิว
epithelial in Vietnamese: Biểu mô
epithelial in Tajik: Бофтаи эпителӣ
epithelial in Turkish: Epitel
epithelial in Ukrainian: Епітелій
epithelial in Chinese: 上皮組織
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