Difference between revisions of "Epithelium"

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'''Definition'''
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An '''epithelium''' is a type of animal [[tissue]] made up of densely packed cells (called [[epithelial cell]]s) that rest on a [[basement membrane]]. Its function is to act as a covering or lining of various bodily surfaces and cavities. The epithelium is one of the five main types of animal tissues; the other four are [[nerve tissue]], [[connective tissue]],  [[muscle tissue]], and [[vascular tissue]]. The epithelium, in turn, is classified based on the number of layers that make up the tissue: ''simple'', ''stratified'', and ''pseudostratified''. It may also be classified histologically, according to the cell shape: ''squamous'', ''columnar'', and ''cuboidal''. The epithelium is found lining the body cavities and vessels, e.g. digestive tract and reproductive tract. It is primarily involved in providing protection of the underlying structures, secretory functions, transcellular transport, and selective absorption.
  
''noun, plural: epithelia''
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== Epithelium definition ==
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[[File: Epithelium_-types.jpg|thumb|right|alt=Epithelium types illustration|The different types of epithelium]]
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Epithelium (plural: epithelia) is a membranous animal tissue consisting of [[epithelial cell]]s closely packed together and joined by cell junctions. Underneath the epithelial cell layer(s) is a basement membrane.
  
A type of [[animal]] [[tissue]] made up of densely packed cells that rest on a [[basement membrane]] to act as a covering or lining of various bodily surfaces and cavities
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== Etymology ==
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The term ''epithelium'' came from New Latin ''epithēlium'', which in turn came from the Ancient Greek ''ἐπί'' (epí), meaning “on" or "atop" and ''θηλή'' (thēlḗ), meaning “nipple”. The term ''epithelial'' is a descriptive word that means ''of, pertaining to, relating to, or characterized by an epithelium'', as in ''[[epithelial cell]]'', ''[[epithelial body]]''. Synonym: epithelial tissue.
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== Types of epithelial tissues ==
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The epithelium may be classified according to the number of layers that make it up: [[simple epithelium]], [[stratified epithelium]], and [[pseudostratified epithelium]].
  
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A ''simple epithelial tissue'' (also called ''simple epithelium'') is an epithelial tissue made up of a single layer of epithelial cells. The cells lie in direct contact with the basement membrane.
  
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A stratified epithelial tissue (also called ''stratified epithelium'') is an epithelial tissue comprised of more than one layer of epithelial cells, with only the basal layer being in contact with the basement membrane, and often confers protective or mechanical function. A special type of stratified epithelium is the ''transitional epithelium''. It consists of several layers of epithelial cells that can transition from squamous to cuboidal. The cells may contract or expand depending on the tension on the epithelium.<sup>(2)</sup>
  
'''Supplement'''
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A ''pseudostratified epithelial tissue'' (also called ''pseudostratified epithelium'') is an epithelial tissue comprised of a single layer of epithelial cells that are in direct contact with the basement membrane. However, it appears stratified due to the positioning of the cells.
  
Tissues of animals may be classified into the following fundamental types: epithelial tissue or [[epithelium]] , [[nerve tissue]],  [[connective tissue]],  [[muscle tissue]], and [[vascular tissue]]. The epithelial tissue (also called epithelium) is a membranous animal tissue made up of one or more layers of cells closely packed together and serves as a covering of internal and external surfaces of the body, including bodily cavities and vessels. It is primarily involved in protecting the underlying structures, secretion, [[regulation]], and [[absorption]]. This tissue may be classified histologically according to the cell shape:
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'''Comparison of the three major types of epithelium'''
* [[squamous epithelium]]
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{| class="wikitable" style="background: #fff;"
* [[columnar epithelium ]]
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! Simple epithelium
* [[cuboidal epithelium]]
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! Stratified epithelium
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! Pseudostratified epithelium
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|-
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|style="vertical-align:top;padding:15px;"| Single layer of epithelial cells
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|style="vertical-align:top; padding:15px;"| Several layers of epithelial cells
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|style="vertical-align:top; padding:15px;"| Single layer of epithelial cells of varying cell sizes, resulting in looking like a stratified.
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|-
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|style="vertical-align:top; padding:15px;"| Epithelial cell shapes:
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* Squamous
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* Cuboidal
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* Columnar
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|style="vertical-align:top; padding:15px;"| Epithelial cell shapes:
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* Squamous
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* Cuboidal
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* Columnar
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|style="vertical-align:top; padding:15px;"| Epithelial cell shape:
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* Columnar
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|-
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|style="vertical-align:top; padding:15px;"| Functions:
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* Secretions, especially mucus and enzymes
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* Transport of materials via diffusion
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* Absorption
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|style="vertical-align:top; padding:15px;"| Functions:
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* Protection, e.g. from abrasion
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* Secretion
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|style="vertical-align:top; padding:15px;"| Functions:
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* Secretion of mucus
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* In the presence of cilia, moves mucus
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|-
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|style="vertical-align:top; padding:15px;"| Locations:
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*Linings of heart, blood vessels, and lymphatic vessels
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* Air sacs and bronchi
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* Secretory parts and ducts of glands
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* Kidney tubules
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* Linings of bladder and uterus
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* Linings of  the digestive tract
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|style="vertical-align:top; padding:15px;"| Locations:
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* Linings of esophagus and mouth
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* In the vagina
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* Sweat glands, salivary glands, mammary glands
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* Male urethra
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* Transitional epithelium in the urinary bladder
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|style="vertical-align:top; padding:15px;"| Locations:
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* Linings of the trachea
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* Portions of the upper respiratory tract
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|}
  
The epithelium may also be grouped according to the number of layers that make it up : [[simple epithelium]], [[stratified epithelium]], and [[pseudostratified epithelium]].
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== Special types ==
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Special types of epithelium are the mesothelium and the endothelium. The ''[[mesothelium]]'' is the epithelial tissue in serous membranes lining the ventral body cavity and covering its organs, such as the linings in the pleural cavity, peritoneal cavity, and pericardial cavity. The ''[[endothelium]]'' is the type of epithelium that lines the blood vessels and the lymphatic vessels. Nevertheless, in pathology, these two are not considered as true epithelium since they have pathological features that are markedly different from those of other epithelial tissues. For instance, the cancer of the endothelium or of the mesothelium is referred to as sarcomas and the cancer involving the rest of the epithelial tissues as carcinomas.
  
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== Structure ==
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The epithelium consists mainly of epithelial cells, which could be in a single layer (as in simple epithelium and pseudostratified) or in multiple layers (as in stratified epithelium). Underneath is a basement membrane that separates the epithelium from the connective tissue.
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=== Epithelial cells ===
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This tissue may be classified histologically according to the cell shape: (1) [[squamous epithelium]], (2) [[columnar epithelium]], and (3) [[cuboidal epithelium]]. Squamous epithelial cells are thin, flat cells looking like scales. They appear polygonal in shape when viewed from above. Columnar epithelial cells are rectangular, i.e. the cells appear taller than wide. Cuboidal epithelial cells are cube-shaped, i.e. the height and width of the cells are approximately the same.
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=== Basement membrane ===
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In the epithelium, the [[basement membrane]] is a thin, fibrous extracellular matrix located at the base, particularly under the epithelial cells. In [[metazoan]]s, the basement membrane is the layer that separates the epithelial tissue from the connective tissue.<sup>(1)</sup> The basement membrane is comprised of two layers: ''[[basal lamina]]'' and ''[[reticular lamina]]''. The basal lamina is produced by the secretions of the epithelial cells whereas the reticular lamina is produced by the secretions of the [[fibroblast]]s of the [[connective tissue]].
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=== Cell junctions ===
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''[[Cell junction]]s'', such as ''tight junctions'', ''adherens junctions'', ''[[desmosome]]s'', ''hemidesmosomes'', and ''gap junctions'', connect adjacent cells together. They are protein complexes that provide a ''bridge'' between contiguous cells or between a cell and the [[extracellular matrix]].
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== Functions ==
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The epithelium is primarily involved in providing protection to the underlying structures against abrasion, radiation, desiccation, and other physical trauma. Also, it prevents pathogens and toxins from easily entering the body. The skin, in particular, when disrupted, e.g. by a cut, provides a point of entry for pathogens and parasites. Thus, when the epithelial tissue is damaged, the body repairs it immediately through a cascade reaction involving several clotting factors and fibrin to seal it. Another crucial function of the skin is to provide sensation. The epithelial cells, in turn, are involved in various biological activities that enable secretion, regulation, and [[absorption]]. Glandular tissues are made up of epithelium. The two major types of glands are ''endocrine glands'' and ''exocrine glands''.
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== Locations ==
 
Epithelial tissues may be found lining animal organs such as those of the [[reproductive tract]] and the [[digestive tract]].  
 
Epithelial tissues may be found lining animal organs such as those of the [[reproductive tract]] and the [[digestive tract]].  
  
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== Development ==
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During embryonic development, the epithelial tissues may come from any of the three germ layers. Certain cells from the ectoderm, for instance, may give rise to epidermis whereas certain cells from the endoderm may become the lining of the digestive tract. Certain cells from the mesoderm are fated to become the inner lining of the body cavities.
  
''Synonym(s):''
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== Related terms ==
* [[epithelial tissue]]
 
''See also:''
 
* [[tissue]]
 
* [[endothelium]]
 
''Related term(s):''
 
 
* [[Barretts epithelium]]
 
* [[Barretts epithelium]]
 
* [[Crevicular epithelium]]
 
* [[Crevicular epithelium]]
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* [[Enamel epithelium]]
 
* [[Enamel epithelium]]
 
* [[Inner dental epithelium]]
 
* [[Inner dental epithelium]]
* [[Lining epithelium]]
 
 
* [[Muscle epithelium]]
 
* [[Muscle epithelium]]
 
* [[Olfactory epithelium]]
 
* [[Olfactory epithelium]]
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* [[Stratified squamous epithelium]]
 
* [[Stratified squamous epithelium]]
 
* [[Surface epithelium]]
 
* [[Surface epithelium]]
''Related form(s):'' 
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* epithelial (''adjective'', of, pertaining to, relating to, or characterized by, epithelium, as in ''[[epithelial cell]]'', ''[[epithelial body]]'')
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== See also ==
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* [[Tissue]]
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* [[Endothelium]]
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* [[Mesothelium]]
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== References ==
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# Kierszenbaum, Abraham; Tres, Laura (2012). Histology and Cell Biology, An Introduction to Pathology (3rd ed.).
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# Pratt, Rebecca. "Epithelial Cells". AnatomyOne. Amirsys, Inc. Archived from the original on 2012-12-19. Retrieved 2012-09-28.
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== Further readings ==
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* Klatt, E. C. (2019). Epithelium. Retrieved from Utah.edu website: https://webpath.med.utah.edu/HISTHTML/NORMAL/NORMAL05.html
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* Brelje, C. (2019). Epithelium. Retrieved from Histologyguide.com website: http://www.histologyguide.com/slidebox/02-epithelium.html
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* SIU SOM Histology INTRO. (2010). Retrieved from Siumed.edu website: http://www.siumed.edu/~dking2/intro/epith.htm
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* 4.2 Epithelial Tissue – Anatomy and Physiology. (2013, March 6). Retrieved from Opentextbc.ca website: https://opentextbc.ca/anatomyandphysiology/chapter/4-2-epithelial-tissue/
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© Biology Online. Content provided and moderated by '''[https://www.biology-online.org/about/ Biology Online Editors]'''
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Latest revision as of 08:26, 30 November 2019

An epithelium is a type of animal tissue made up of densely packed cells (called epithelial cells) that rest on a basement membrane. Its function is to act as a covering or lining of various bodily surfaces and cavities. The epithelium is one of the five main types of animal tissues; the other four are nerve tissue, connective tissue, muscle tissue, and vascular tissue. The epithelium, in turn, is classified based on the number of layers that make up the tissue: simple, stratified, and pseudostratified. It may also be classified histologically, according to the cell shape: squamous, columnar, and cuboidal. The epithelium is found lining the body cavities and vessels, e.g. digestive tract and reproductive tract. It is primarily involved in providing protection of the underlying structures, secretory functions, transcellular transport, and selective absorption.

Epithelium definition

Epithelium types illustration
The different types of epithelium

Epithelium (plural: epithelia) is a membranous animal tissue consisting of epithelial cells closely packed together and joined by cell junctions. Underneath the epithelial cell layer(s) is a basement membrane.

Etymology

The term epithelium came from New Latin epithēlium, which in turn came from the Ancient Greek ἐπί (epí), meaning “on" or "atop" and θηλή (thēlḗ), meaning “nipple”. The term epithelial is a descriptive word that means of, pertaining to, relating to, or characterized by an epithelium, as in epithelial cell, epithelial body. Synonym: epithelial tissue.

Types of epithelial tissues

The epithelium may be classified according to the number of layers that make it up: simple epithelium, stratified epithelium, and pseudostratified epithelium.

A simple epithelial tissue (also called simple epithelium) is an epithelial tissue made up of a single layer of epithelial cells. The cells lie in direct contact with the basement membrane.

A stratified epithelial tissue (also called stratified epithelium) is an epithelial tissue comprised of more than one layer of epithelial cells, with only the basal layer being in contact with the basement membrane, and often confers protective or mechanical function. A special type of stratified epithelium is the transitional epithelium. It consists of several layers of epithelial cells that can transition from squamous to cuboidal. The cells may contract or expand depending on the tension on the epithelium.(2)

A pseudostratified epithelial tissue (also called pseudostratified epithelium) is an epithelial tissue comprised of a single layer of epithelial cells that are in direct contact with the basement membrane. However, it appears stratified due to the positioning of the cells.

Comparison of the three major types of epithelium

Simple epithelium Stratified epithelium Pseudostratified epithelium
Single layer of epithelial cells Several layers of epithelial cells Single layer of epithelial cells of varying cell sizes, resulting in looking like a stratified.
Epithelial cell shapes:
  • Squamous
  • Cuboidal
  • Columnar
Epithelial cell shapes:
  • Squamous
  • Cuboidal
  • Columnar
Epithelial cell shape:
  • Columnar
Functions:
  • Secretions, especially mucus and enzymes
  • Transport of materials via diffusion
  • Absorption
Functions:
  • Protection, e.g. from abrasion
  • Secretion
Functions:
  • Secretion of mucus
  • In the presence of cilia, moves mucus
Locations:
  • Linings of heart, blood vessels, and lymphatic vessels
  • Air sacs and bronchi
  • Secretory parts and ducts of glands
  • Kidney tubules
  • Linings of bladder and uterus
  • Linings of the digestive tract
Locations:
  • Linings of esophagus and mouth
  • In the vagina
  • Sweat glands, salivary glands, mammary glands
  • Male urethra
  • Transitional epithelium in the urinary bladder
Locations:
  • Linings of the trachea
  • Portions of the upper respiratory tract

Special types

Special types of epithelium are the mesothelium and the endothelium. The mesothelium is the epithelial tissue in serous membranes lining the ventral body cavity and covering its organs, such as the linings in the pleural cavity, peritoneal cavity, and pericardial cavity. The endothelium is the type of epithelium that lines the blood vessels and the lymphatic vessels. Nevertheless, in pathology, these two are not considered as true epithelium since they have pathological features that are markedly different from those of other epithelial tissues. For instance, the cancer of the endothelium or of the mesothelium is referred to as sarcomas and the cancer involving the rest of the epithelial tissues as carcinomas.

Structure

The epithelium consists mainly of epithelial cells, which could be in a single layer (as in simple epithelium and pseudostratified) or in multiple layers (as in stratified epithelium). Underneath is a basement membrane that separates the epithelium from the connective tissue.

Epithelial cells

This tissue may be classified histologically according to the cell shape: (1) squamous epithelium, (2) columnar epithelium, and (3) cuboidal epithelium. Squamous epithelial cells are thin, flat cells looking like scales. They appear polygonal in shape when viewed from above. Columnar epithelial cells are rectangular, i.e. the cells appear taller than wide. Cuboidal epithelial cells are cube-shaped, i.e. the height and width of the cells are approximately the same.

Basement membrane

In the epithelium, the basement membrane is a thin, fibrous extracellular matrix located at the base, particularly under the epithelial cells. In metazoans, the basement membrane is the layer that separates the epithelial tissue from the connective tissue.(1) The basement membrane is comprised of two layers: basal lamina and reticular lamina. The basal lamina is produced by the secretions of the epithelial cells whereas the reticular lamina is produced by the secretions of the fibroblasts of the connective tissue.

Cell junctions

Cell junctions, such as tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions, connect adjacent cells together. They are protein complexes that provide a bridge between contiguous cells or between a cell and the extracellular matrix.

Functions

The epithelium is primarily involved in providing protection to the underlying structures against abrasion, radiation, desiccation, and other physical trauma. Also, it prevents pathogens and toxins from easily entering the body. The skin, in particular, when disrupted, e.g. by a cut, provides a point of entry for pathogens and parasites. Thus, when the epithelial tissue is damaged, the body repairs it immediately through a cascade reaction involving several clotting factors and fibrin to seal it. Another crucial function of the skin is to provide sensation. The epithelial cells, in turn, are involved in various biological activities that enable secretion, regulation, and absorption. Glandular tissues are made up of epithelium. The two major types of glands are endocrine glands and exocrine glands.

Locations

Epithelial tissues may be found lining animal organs such as those of the reproductive tract and the digestive tract.

Development

During embryonic development, the epithelial tissues may come from any of the three germ layers. Certain cells from the ectoderm, for instance, may give rise to epidermis whereas certain cells from the endoderm may become the lining of the digestive tract. Certain cells from the mesoderm are fated to become the inner lining of the body cavities.

Related terms

See also

References

  1. Kierszenbaum, Abraham; Tres, Laura (2012). Histology and Cell Biology, An Introduction to Pathology (3rd ed.).
  2. Pratt, Rebecca. "Epithelial Cells". AnatomyOne. Amirsys, Inc. Archived from the original on 2012-12-19. Retrieved 2012-09-28.

Further readings



© Biology Online. Content provided and moderated by Biology Online Editors