Difference between revisions of "Epithelium" - New World Encyclopedia

From New World Encyclopedia
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[[Image:Illu epithelium.jpg|thumb|400px|Types of epithelium]]
 
[[Image:Illu epithelium.jpg|thumb|400px|Types of epithelium]]
  
−
'''Epithelium''' is a [[biological tissue|tissue]] (collection of interconnected [[cell (biology)|cells]] that perform a similar function within an organism) that covers organ systems of the bodies of [[animal]]s, including both outide surfaces (the [[skin]]) and inside cavities and [[lumen (anatomy)|lumen]] (interior of a vessel within the body, such as the small central space in an artery or vein through which blood flows). Epithelial cells are close together, with very little intercellular material. The epithelium is generally free of blood vessels, with very few exceptions. Functions of epithelial cells include protection, secretion, absorption, transcellular transport, [[sense|sensation]] detection, and selective permeability.  
+
'''Epithelium''' is a [[biological tissue|tissue]] (collection of interconnected [[cell (biology)|cells]] that perform a similar function within an organism) that covers organs and surfaces of the bodies of [[animal]]s, including both outide surfaces (the [[skin]]) and inside cavities and [[lumen (anatomy)|lumen]] (interior of a vessel, such as the small central space in an artery or vein through which blood flows). Epithelial cells are close together, with very little intercellular material. The epithelium is generally free of blood vessels, with very few exceptions. Functions of epithelial cells include protection, secretion, absorption, transcellular transport, [[sense|sensation]] detection, and selective permeability.  
  
−
Epithelium is one of four primary body tissues of animals, including the [[human body]] and lower multicellular organisms, such as [[insect]]s. The other three basic tissues are ''connective tissue'' (holds everything together), ''[[muscle]] tissue'' (contain contractile filaments that move past each other and change the size of the cell), and ''nervous tissue'' (forming the [[brain]], spinal cord, and peripheral nervous system). (Plants also have tissues, broadly categorized into epidermis, vascular tissue, and ground tissue.)
+
Epithelium is one of four primary body tissues of animals, including the [[human body]] and lower multicellular organisms, such as [[insect]]s. The other three basic tissues are ''[[muscle]] tissue'' (contain contractile filaments that move past each other and change the size of the cell), ''nervous tissue'' (forming the [[brain]], spinal cord, and peripheral nervous system), and ''connective tissue'' (holds everything together).
  
−
The outermost layer of our [[skin]], the epidermis, is composed of stratified [[squamous]] epithelial cells (with an underlying basal lamina), as are the [[mucous membranes]] lining the inside of mouths and body cavities. 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 outermost layer of our [[skin]], the epidermis, is composed of stratified [[squamous]] epithelial cells (with an underlying basal lamina), as are the [[mucous membrane]]s lining the inside of mouths and body cavities. 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.  
  
 
[[Endothelium]] (the inner lining of [[blood vessel]]s) is a specialized form of epithelium.
 
[[Endothelium]] (the inner lining of [[blood vessel]]s) is a specialized form of epithelium.
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==Classification==
 
==Classification==
 
Epithelial cells are classified by the following three factors:
 
Epithelial cells are classified by the following three factors:
−
* '''shape'''
+
* ''Shape''
−
* '''Stratification'''
+
* ''Stratification''
 
* ''Specializations''
 
* ''Specializations''
  
 
===Shape===
 
===Shape===
−
* '''Squamous''': Squamous cells are flat cells with an irregular flattened shape
+
* '''Squamous''': Squamous cells are flat cells with an irregular flattened shape. The one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane in the [[lung]]s, and the [[endothelium]] of capillaries, and is a minimal barrier to [[diffusion]]. Places where squamous cells can be found also include the filtration tubules of the [[kidneys]] and the major cavities of the body. These cells are relatively inactive [[metabolism|metabolically]] and are associated with the diffusion of water, [[electrolytes]], and other substances.   
−
. The one-cell layer of simple squamous epithelium that forms the alveoli of the respiratory membrane, and the [[endothelium]] of capillaries, and is a minimal barrier to [[diffusion]]. Places where squamous cells can be found include the [[alveoli]] of the [[lungs]], 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 about the same size as its height. The [[Cell nucleus|nuclei]] of these cells are usually located in the center.
−
* '''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.
+
* '''Columnar''': These cells are taller than they are wide. The nucleus is also closer to the base of the cell. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. 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, which increase the surface area for absorption.
−
* '''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 [[mammal]]s. 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.
−
* '''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===
 
===Stratification===
−
* '''Simple''': There is a single layer of cells.
+
* '''Simple''': There is a single layer of cells. As noted above, simple squamous epithelium forms the alveoli of the respiratory membrane, as they offer minimal barrier to diffusion.
−
* '''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.
+
* '''Stratified''': There is more than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina (layer on which epithelium sits and which is secreted by the epithelial cells). Stratified cells can usually withstand large amounts of stress. For example, stratified squamous epithelial cells make up the outer layer of [[skin]].
 
* '''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 since stratified epithelium cannot have cilia.
 
* '''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 since stratified epithelium cannot have cilia.
  
 
===Specializations===
 
===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.
+
* '''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.
  
−
* ''[[Cilia]]ted'' cells have 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]].
+
* '''Ciliated''' cells have apical [[plasma membrane]] extensions composed of microtubules 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.
 
*
 
*
  
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| '''System''' || '''Tissue''' || '''Epithelium''' || '''Subtype'''
 
| '''System''' || '''Tissue''' || '''Epithelium''' || '''Subtype'''
 
  |-  
 
  |-  
−
  | circulatory ||  [[blood vessels]] || Simple squamous  || [[endothelium]]
+
  | circulatory ||  [[blood]] vessels || Simple squamous  || [[endothelium]]
 
  |-  
 
  |-  
−
  | digestive || [[Duct (anatomy)|ducts]] of [[submandibular gland]]s || Stratified columnar || -
+
  | digestive || [[Duct (anatomy)|ducts]]* of submandibular glands || Stratified columnar || -
 
  |-  
 
  |-  
−
  | digestive || attached [[gingiva]] || Stratified squamous, keratinized || -
+
  | digestive || attached [[gingiva]]* || Stratified squamous, keratinized || -
 
  |-  
 
  |-  
 
  | digestive || dorsum of [[tongue]]|| Stratified squamous, keratinized || -  
 
  | digestive || dorsum of [[tongue]]|| Stratified squamous, keratinized || -  
 
  |-  
 
  |-  
−
  | digestive || [[hard palate]] || Stratified squamous, keratinized || -  
+
  | digestive || [[hard palate]]* || Stratified squamous, keratinized || -  
 
  |-  
 
  |-  
 
  | digestive || [[esophagus]] || Stratified squamous, non-keratinised || -  
 
  | digestive || [[esophagus]] || Stratified squamous, non-keratinised || -  
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  | endocrine || [[thyroid]] follicles || Simple cuboidal || -
 
  | endocrine || [[thyroid]] follicles || Simple cuboidal || -
 
  |-  
 
  |-  
−
  | nervous || [[ependyma]] || Simple cuboidal || -
+
  | nervous || [[ependyma]]* || Simple cuboidal || -
 
  |-  
 
  |-  
−
  | lymphatic || [[lymph vessel]] || Simple squamous  || [[endothelium]]
+
  | lymphatic || [[lymph]] vessel || Simple squamous  || [[endothelium]]*
 
  |-  
 
  |-  
 
  | integumentary || [[skin]] - dead superficial layer || Stratified squamous, keratinized || -  
 
  | integumentary || [[skin]] - dead superficial layer || Stratified squamous, keratinized || -  
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  | integumentary || [[mesothelium]] of [[body cavities]] || Simple squamous || -
 
  | integumentary || [[mesothelium]] of [[body cavities]] || Simple squamous || -
 
  |-  
 
  |-  
−
  | reproductive - female  || [[ovaries]] || Simple cuboidal  || [[germinal epithelium (female)]]
+
  | reproductive - female  || [[ovary|ovaries]] || Simple cuboidal  || [[germinal epithelium (female)]]*
 
  |-  
 
  |-  
−
  | reproductive - female  || [[Fallopian tubes]] || Simple columnar, ciliated || -
+
  | reproductive - female  || [[Fallopian tube]]s || Simple columnar, ciliated || -
 
  |-  
 
  |-  
 
  | reproductive - female  || [[uterus]] || Simple columnar, ciliated || -
 
  | reproductive - female  || [[uterus]] || Simple columnar, ciliated || -
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  | reproductive - female  || [[endometrium]] || Simple columnar || -
 
  | reproductive - female  || [[endometrium]] || Simple columnar || -
 
  |-  
 
  |-  
−
  | reproductive - female  || [[cervix]] ([[endocervix]]) || Simple columnar || -  
+
  | reproductive - female  || [[cervix]] ([[endocervix]]*) || Simple columnar || -  
 
  |-  
 
  |-  
−
  | reproductive - female  || [[cervix]] ([[ectocervix]]) || Stratified squamous, non-keratinised || -  
+
  | reproductive - female  || [[cervix]]* ([[ectocervix]]*) || Stratified squamous, non-keratinised || -  
 
  |-  
 
  |-  
 
  | reproductive - female  || [[vagina]] || Stratified squamous, non-keratinised || -  
 
  | reproductive - female  || [[vagina]] || Stratified squamous, non-keratinised || -  
 
  |-  
 
  |-  
−
  | reproductive - female  || [[labia majora]] || Stratified squamous, keratinised || -  
+
  | reproductive - female  || [[labia majora]]* || Stratified squamous, keratinised || -  
 
  |-  
 
  |-  
−
  | reproductive - male || [[tubuli recti]] || Simple cuboidal  || [[germinal epithelium (male)]]
+
  | reproductive - male || [[tubuli recti]]* || Simple cuboidal  || [[germinal epithelium (male)]]*
 
  |-  
 
  |-  
−
  | reproductive - male || [[rete testis]] || Simple cuboidal || -
+
  | reproductive - male || [[rete testis]]* || Simple cuboidal || -
 
  |-  
 
  |-  
−
  | reproductive - male || [[ductuli efferentes]] || Pseudostratified columnar || -
+
  | reproductive - male || [[ductuli efferentes]]* || Pseudostratified columnar || -
 
  |-  
 
  |-  
−
  | reproductive - male || [[epididymis]] || Pseudostratified columnar, with [[stereocilia]] || -
+
  | reproductive - male || [[epididymis]]* || Pseudostratified columnar, with [[stereocilia]]* || -
 
  |-  
 
  |-  
−
  | reproductive - male || [[vas deferens]] || Pseudostratified columnar || -
+
  | reproductive - male || [[vas deferens]]* || Pseudostratified columnar || -
 
  |-  
 
  |-  
−
  | reproductive - male || [[ejaculatory duct]] || Simple columnar || -
+
  | reproductive - male || [[ejaculatory duct]]* || Simple columnar || -
 
  |-  
 
  |-  
−
  | reproductive - male (gland) || [[bulbourethral glands]] || Simple columnar || -
+
  | reproductive - male (gland) || [[bulbourethral glands]]* || Simple columnar || -
 
  |-  
 
  |-  
−
  | reproductive - male (gland) || [[seminal vesicle]] || Pseudostratified columnar || -
+
  | reproductive - male (gland) || [[seminal vesicle]]* || Pseudostratified columnar || -
 
  |-  
 
  |-  
−
  | respiratory || [[oropharynx]] || Stratified squamous, non-keratinised  || -
+
  | respiratory || [[oropharynx]]* || Stratified squamous, non-keratinised  || -
 
  |-  
 
  |-  
−
  | respiratory || [[larynx]] || Pseudostratified columnar, ciliated  || [[respiratory epithelium]]
+
  | respiratory || [[larynx]] || Pseudostratified columnar, ciliated  || [[respiratory epithelium]]*
 
  |-  
 
  |-  
−
  | respiratory || [[trachea]] || Pseudostratified columnar, ciliated  || [[respiratory epithelium]]
+
  | respiratory || [[trachea]] || Pseudostratified columnar, ciliated  || [[respiratory epithelium]]*
 
  |-  
 
  |-  
−
  | respiratory || [[respiratory bronchioles]] || Simple cuboidal  || -
+
  | respiratory || [[respiratory bronchioles]]* || Simple cuboidal  || -
 
  |-  
 
  |-  
−
  | sensory || [[cornea]] || Stratified squamous, non-keratinised || [[corneal epithelium]]
+
  | sensory || [[cornea]] || Stratified squamous, non-keratinised || [[corneal epithelium]]*
 
  |-  
 
  |-  
−
  | sensory || [[nose]] || Pseudostratified columnar || [[olfactory epithelium]]
+
  | sensory || [[nose]] || Pseudostratified columnar || [[olfactory epithelium]]*
 
  |-  
 
  |-  
−
  | urinary || [[kidney]] - [[proximal convoluted tubule]] || Simple columnar, ciliated  || -
+
  | urinary || [[kidney]] - [[proximal convoluted tubule]]* || Simple columnar, ciliated  || -
 
  |-  
 
  |-  
−
  | urinary || [[kidney]] - [[ascending thin limb]] || Simple squamous || -
+
  | urinary || [[kidney]]* - [[ascending thin limb]]* || Simple squamous || -
 
  |-  
 
  |-  
−
  | urinary || [[kidney]] - [[distal convoluted tubule]] || Simple columnar, non-ciliated  || -
+
  | urinary || [[kidney]] - [[distal convoluted tubule]]* || Simple columnar, non-ciliated  || -
 
  |-  
 
  |-  
−
  | urinary || [[kidney]] - [[collecting duct]] || Simple cuboidal || -
+
  | urinary || [[kidney]] - [[collecting duct]]* || Simple cuboidal || -
 
  |-  
 
  |-  
−
  | urinary || [[renal pelvis]] || Transitional || [[urothelium]]
+
  | urinary || [[renal pelvis]]* || Transitional || [[urothelium]]*
 
  |-  
 
  |-  
−
  | urinary || [[ureter]] || Transitional || [[urothelium]]
+
  | urinary || [[ureter]] || Transitional || [[urothelium]]*
 
  |-  
 
  |-  
−
  | urinary || [[urinary bladder]] || Transitional  || [[urothelium]]
+
  | urinary || [[urinary bladder]] || Transitional  || [[urothelium]]*
 
  |-  
 
  |-  
−
  | urinary || [[prostatic urethra]] || Transitional  || [[urothelium]]
+
  | urinary || [[prostatic urethra]]* || Transitional  || [[urothelium]]*
 
  |-  
 
  |-  
−
  | urinary || [[membranous urethra]] || Pseudostratified columnar, non-ciliated || -  
+
  | urinary || [[membranous urethra]]* || Pseudostratified columnar, non-ciliated || -  
 
  |-  
 
  |-  
−
  | urinary || [[penile urethra]] || Pseudostratified columnar, non-ciliated || -  
+
  | urinary || [[penile urethra]]* || Pseudostratified columnar, non-ciliated || -  
 
  |-  
 
  |-  
−
  | urinary || [[external urethral orifice]] || Stratified squamous || -
+
  | urinary || [[external urethral orifice]]* || Stratified squamous || -
 
|}
 
|}
  
−
==Cell junctions==
+
==Embryology, secretory epithelia, and cell junctions==
−
{{Main|Cell junction}}
+
There are epithelial tissues deriving from all three of the embryological germ layers:
−
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]].
 
−
 
 
−
==Embryology==
 
−
There are epithelial tissues deriving from all of the embryological [[germ layers]]:
 
 
* from [[ectoderm]] (e.g., the [[epidermis]]);
 
* from [[ectoderm]] (e.g., the [[epidermis]]);
  
Line 157: Line 149:
 
* from [[mesoderm]] (e.g., the inner linings of [[body cavities]]).
 
* from [[mesoderm]] (e.g., the inner linings of [[body cavities]]).
  
−
==Additional images==
+
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]]. Exocrine glands are glands that secrete their products into ducts (duct glands). Typical exocrine glands include sweat glands, salivary glands, [[mammary gland]]s, and many glands of the [[digestive system]].
−
<gallery>
+
 
−
Image:Tkanka nablonkowa.png
+
Cell junctions are especially abundant in epithelial tissues. A cell junction is a structure within a tissue of a multicellular organism that consists of protein complexes and that provides contact between neighboring cells or between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the [[paracellular transport]]*.
−
</gallery>
 
  
 
==References==
 
==References==
−
*''Molecular Biology of the Cell'', 4th edition, Alberts ''et al.'', 2002
+
* Alberts, B. 2002. ''Molecular Biology of the Cell'', fourth edition. Garland Science. ISBN 0815340729
 +
* Lodish, H., A. Beck, L. S. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell. 2000. ''Molecular Cell Biology'', 4th edition. New York: W. H. Freeman. ISBN 0716731363.
 +
* Towle, A. 1989. ''Modern Biology''. Austin, TX: Holt, Rinehart and Winston.  ISBN 0030139198
 +
 
 +
 
  
 
[[Category:Life sciences]]
 
[[Category:Life sciences]]
−
 
  
 
{{credit2|Epithelium|105629392|Tissue_(biology)|111883294}}
 
{{credit2|Epithelium|105629392|Tissue_(biology)|111883294}}

Revision as of 21:59, 1 March 2007

Types of epithelium

Epithelium is a tissue (collection of interconnected cells that perform a similar function within an organism) that covers organs and surfaces of the bodies of animals, including both outide surfaces (the skin) and inside cavities and lumen (interior of a vessel, such as the small central space in an artery or vein through which blood flows). Epithelial cells are close together, with very little intercellular material. The epithelium is generally free of blood vessels, with very few exceptions. Functions of epithelial cells include protection, secretion, absorption, transcellular transport, sensation detection, and selective permeability.

Epithelium is one of four primary body tissues of animals, including the human body and lower multicellular organisms, such as insects. The other three basic tissues are muscle tissue (contain contractile filaments that move past each other and change the size of the cell), nervous tissue (forming the brain, spinal cord, and peripheral nervous system), and connective tissue (holds everything together).

The outermost layer of our skin, the epidermis, is composed of stratified squamous epithelial cells (with an underlying basal lamina), as are the mucous membranes lining the inside of mouths and body cavities. 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.

Endothelium (the inner lining of blood vessels) is a specialized form of epithelium.

Classification

Epithelial cells are classified by the following three factors:

  • Shape
  • Stratification
  • Specializations

Shape

  • Squamous: Squamous cells are flat cells with an irregular flattened shape. The one-cell layer of simple squamous epithelium forms the alveoli of the respiratory membrane in the lungs, and the endothelium of capillaries, and is a minimal barrier to diffusion. Places where squamous cells can be found also include 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 about the same size as its height. The nuclei of these cells are usually located in the center.
  • Columnar: These cells are taller than they are wide. The nucleus is also closer to the base of the cell. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. 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, which 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. As noted above, simple squamous epithelium forms the alveoli of the respiratory membrane, as they offer minimal barrier to diffusion.
  • Stratified: There is more than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina (layer on which epithelium sits and which is secreted by the epithelial cells). Stratified cells can usually withstand large amounts of stress. For example, stratified squamous epithelial cells make up the outer layer of skin.
  • 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 since stratified epithelium cannot have cilia.

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.
  • Ciliated cells have apical plasma membrane extensions composed of microtubules 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

System Tissue Epithelium Subtype
circulatory blood vessels Simple squamous endothelium
digestive ducts of submandibular glands Stratified columnar -
digestive attached gingiva Stratified squamous, keratinized -
digestive dorsum of tongue Stratified squamous, keratinized -
digestive hard palate Stratified squamous, keratinized -
digestive esophagus Stratified squamous, non-keratinised -
digestive stomach Simple columnar, non-ciliated -
digestive small intestine Simple columnar, non-ciliated -
digestive large intestine Simple columnar, non-ciliated -
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 -
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 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 -

Embryology, secretory epithelia, and cell junctions

There are epithelial tissues deriving from all three of the embryological germ layers:

  • from mesoderm (e.g., the inner linings of body cavities).

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. Exocrine glands are glands that secrete their products into ducts (duct glands). Typical exocrine glands include sweat glands, salivary glands, mammary glands, and many glands of the digestive system.

Cell junctions are especially abundant in epithelial tissues. A cell junction is a structure within a tissue of a multicellular organism that consists of protein complexes and that provides contact between neighboring cells or between a cell and the extracellular matrix, or they build up the paracellular barrier of epithelia and control the paracellular transport.

References
ISBN links support NWE through referral fees

  • Alberts, B. 2002. Molecular Biology of the Cell, fourth edition. Garland Science. ISBN 0815340729
  • Lodish, H., A. Beck, L. S. Zipursky, P. Matsudaira, D. Baltimore, and J. Darnell. 2000. Molecular Cell Biology, 4th edition. New York: W. H. Freeman. ISBN 0716731363.
  • Towle, A. 1989. Modern Biology. Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198

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