Epithelium
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{{otheruses4|the epithelium as it relates to animal anatomy|the fungal structure of the same name|Pileipellis}}
[[Image:Illu epithelium.jpg|thumb|350px|Types of epithelium]]
In biology and medicine, '''epithelium''' is a [[Biological tissue|tissue]] composed of layers of [[cell (biology)|cell]]s that line the cavities and surfaces of structures throughout the body. It is also the type of tissue of which many [[gland]]s are formed. Epithelium lines both the outside ([[skin]]) and the inside cavities and [[lumen (anatomy)|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 [[keratinized|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 [[lung]]s, 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 vessel]]s, 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 [[Cell nucleus|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 (anatomy)|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===
* ''[[Keratin]]ized'' cells contain [[keratin]] (a [[cytoskeleton|cytoskeletal]] protein). While keratinized epithelium occurs mainly in the skin, it is also found in the [[mouth]] and [[nose]], providing a tough, impermeable barrier.
* ''[[Cilia]]ted'' cells have [[Apical membrane|apical]] [[plasma membrane]] extensions composed of [[microtubule]]s capable of beating rhythmically to move [[mucus]] or other substances through a duct. Cilia are common in the [[respiratory system]] and the lining of the [[oviduct]].
==Examples==
{| class="wikitable" class="sortable wikitable"
| '''System''' || '''Tissue''' || '''Epithelium''' || '''Subtype'''
|-
| circulatory || [[blood vessels]] || Simple squamouis || [[endothelium]]
|-
| digestive || [[Duct (anatomy)|ducts]] of [[submandibular gland]]s || Stratified columnar || -
|-
| digestive || attached [[gingiva]] || Stratified squamous, keratinized || -
|-
| digestive || dorsum of [[tongue]]|| Stratified squamous, keratinized || -
|-
| digestive || [[hard palate]] || Stratified squamous, keratinized || -
|- o
| digestive || [[oesophagus]] || Stratified squamous, non-keratinised || -
|-
| digestive || [[stomach]] || Simple columnar, non-ciliated || -
|-
| digestive || [[small intestine]] || Simple columnar, non-ciliated || [[intestinal epithelium]]
|-
| digestive || [[large intestine]] || Simple columnar, non-ciliated || [[intestinal epithelium]]
|-
| digestive || [[rectum]] || Stratified squamous, non-keratinised || -
|-
| digestive || [[anus]] || Stratified squamous, keratinised || -
|-
| digestive || [[gallbladder]] || Simple columnar, non-ciliated || -
|-
| endocrine || [[thyroid]] follicles || Simple cuboidal || -
|-
| nervous || [[ependyma]] || Simple cuboidal || -
|-
| lymphatic || [[lymph vessel]] || Simple squamous || [[endothelium]]
|-
| integumentary || [[skin]] - dead superficial layer || Stratified squamous, keratinized || -
|-
| integumentary || [[sweat gland]] ducts || Stratified cuboidal || -
|-
| integumentary || [[mesothelium]] of [[body cavities]] || Simple squamous || [[mesothelium]]
|-
| reproductive - female || [[ovaries]] || Simple cuboidal || [[germinal epithelium (female)]]
|-
| reproductive - female || [[Fallopian tubes]] || Simple columnar, ciliated || -
|-
| reproductive - female || [[uterus]] || Simple columnar, ciliated || -
|-
| reproductive - female || [[endometrium]] || Simple columnar || -
|-
| reproductive - female || [[cervix]] ([[endocervix]]) || Simple columnar || -
|-
| reproductive - female || [[cervix]] ([[ectocervix]]) || Stratified squamous, non-keratinised || -
|-
| reproductive - female || [[vagina]] || Stratified squamous, non-keratinised || -
|-
| reproductive - female || [[labia majora]] || Stratified squamous, keratinised || -
|-
| reproductive - male || [[tubuli recti]] || Simple cuboidal || [[germinal epithelium (male)]]
|-
| reproductive - male || [[rete testis]] || Simple cuboidal || -
|-
| reproductive - male || [[ductuli efferentes]] || Pseudostratified columnar || -
|-
| reproductive - male || [[epididymis]] || Pseudostratified columnar, with [[stereocilia]] || -
|-
| reproductive - male || [[vas deferens]] || Pseudostratified columnar || -
|-
| reproductive - male || [[ejaculatory duct]] || Simple columnar || -
|-
| reproductive - male (gland) || [[bulbourethral glands]] || Simple columnar || -
|-
| reproductive - male (gland) || [[seminal vesicle]] || Pseudostratified columnar || -
|-
| respiratory || [[oropharynx]] || Stratified squamous, non-keratinised || -
|-
| respiratory || [[larynx]] || Pseudostratified columnar, ciliated || [[respiratory epithelium]]
|-
| respiratory || [[larynx - True Vocal Cords]] || Stratified squamous, non-keratinised || -
|-
| respiratory || [[Vertebrate trachea|trachea]] || Pseudostratified columnar, ciliated || [[respiratory epithelium]]
|-
| respiratory || [[respiratory bronchioles]] || Simple cuboidal || -
|-
| sensory || [[cornea]] || Stratified squamous, non-keratinised || [[corneal epithelium]]
|-
| sensory || [[nose]] || Pseudostratified columnar || [[olfactory epithelium]]
|-
| urinary || [[kidney]] - [[proximal convoluted tubule]] || Simple columnar, ciliated || -
|-
| urinary || [[kidney]] - [[ascending thin limb]] || Simple squamous || -
|-
| urinary || [[kidney]] - [[distal convoluted tubule]] || Simple columnar, non-ciliated || -
|-
| urinary || [[kidney]] - [[collecting duct]] || Simple cuboidal || -
|-
| urinary || [[renal pelvis]] || Transitional || [[urothelium]]
|-
| urinary || [[ureter]] || Transitional || [[urothelium]]
|-
| urinary || [[urinary bladder]] || Transitional || [[urothelium]]
|-
| urinary || [[prostatic urethra]] || Transitional || [[urothelium]]
|-
| urinary || [[membranous urethra]] || Pseudostratified columnar, non-ciliated || -
|-
| urinary || [[penile urethra]] || Pseudostratified columnar, non-ciliated || -
|-
| urinary || [[external urethral orifice]] || Stratified squamous || -
|}
==Cell junctions==
{{Main|Cell junction}}
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]]:
* from [[ectoderm]] (e.g., the [[Epidermis (skin)|epidermis]]);
* from [[endoderm]] (e.g., the lining of the [[gastrointestinal tract]]);
* from [[mesoderm]] (e.g., the inner linings of [[body cavities]]).
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 [[sarcoma]]s, whereas true epithelial cancers are called [[carcinoma]]s. 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==
<gallery>
Image:Tkanka nablonkowa.png
</gallery>
==Links==
*[http://www.antibodypatterns.com/gpc.php Parietal cell antibody]
*[http://www.ii.bham.ac.uk/clinicalimmunology/CISimagelibrary/GPC.htm Antibody to GPC]
*[http://www.cholesteatoma.net An Example (cholesteatoma)]
==References==
*''Molecular Biology of the Cell'', 4th edition, Alberts ''et al.'', 2002
{{Biological tissue}}
{{Epithelial tissue}}
{{Tissue layers}}
[[Category:Tissues]]
[[af:Epiteelweefsel]]
[[bg:Епителна тъкан]]
[[ca:Teixit epitelial]]
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[[it:Tessuto epiteliale]]
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