View source for Medicine:Metaplasia

{{Infobox medical condition (new) 
| name            = Metaplasia 
| image           = Pancreatic acinar metaplasia - high mag.jpg 
| caption         = [[Micrograph]] of a [[Biology:Stomach|gastro]]-[[Biology:Esophagus|esophageal]] junction with pancreatic acinar metaplasia.  The esophageal mucosa (stratified squamous epithelium) is seen on the right.  The gastric mucosa (simple columnar epithelium) is seen on the left. The metaplastic epithelium is at the junction (center of image) and has an intensely eosinophilic (bright pink) cytoplasm. H&E stain. 
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'''Metaplasia''' ({{lang-gr|"change in form"}}) is the transformation of a [[Biology:Cell type|cell type]] to another cell type.<ref>{{Cite journal |last=Slack |first=Jonathan M. W. |date=2007-03-21 |title=Metaplasia and transdifferentiation: from pure biology to the clinic |url=https://www.nature.com/articles/nrm2146 |journal=Nature Reviews Molecular Cell Biology |language=en |volume=8 |issue=5 |pages=369–378 |doi=10.1038/nrm2146 |issn=1471-0072}}</ref>  The change from one type of cell to another may be part of a normal maturation process, or caused by some sort of abnormal stimulus.  In simplistic terms, it is as if the original cells are not robust enough to withstand their environment, so they transform into another cell type better suited to their environment. If the stimulus causing metaplasia is removed or ceases, tissues return to their normal pattern of differentiation.  Metaplasia is not synonymous with [[Biology:Dysplasia|dysplasia]], and is not considered to be an actual cancer.<ref>{{cite web|last=Abrams|first=Gerald|title=Neoplasia I|url=http://open.umich.edu/education/med/m1/patientspop-genetics/fall2008/materials|access-date=23 January 2012}}</ref>  It is also contrasted with heteroplasia, which is the spontaneous abnormal growth of [[Biology:Cell biology|cytologic]] and histologic elements. Today, metaplastic changes are usually considered to be an early phase of [[Biology:Carcinogenesis|carcinogenesis]], specifically for those with a history of cancers or who are known to be susceptible to carcinogenic changes. Metaplastic change is thus often viewed as a [[Medicine:Precancerous condition|premalignant condition]] that requires immediate intervention, either surgical or medical, lest it lead to cancer via [[Biology:Malignant transformation|malignant transformation]].

== Causes ==
{{-plasia}}
When cells are faced with physiological or pathological stresses, they respond by adapting in any of several ways, one of which is metaplasia. It is a benign (i.e. non-cancerous) change that occurs as a response to change of milieu (physiological metaplasia) or chronic physical or chemical irritation. One example of pathological irritation is cigarette smoke, which causes the mucus-secreting ciliated pseudostratified columnar respiratory epithelial cells that line the airways to be replaced by stratified squamous epithelium, or a stone in the bile duct that causes the replacement of the secretory columnar epithelium with stratified squamous epithelium ([[Medicine:Squamous metaplasia|squamous metaplasia]]). Metaplasia is an adaptation that replaces one type of epithelium with another that is more likely to be able to withstand the stresses it is faced with. It is also accompanied by a loss of endothelial function, and in some instances considered undesirable; this undesirability is underscored by the propensity for metaplastic regions to eventually turn cancerous if the irritant is not eliminated.

The cell of origin for many types of metaplasias are controversial or unknown.  For example, there is evidence supporting several different hypotheses of origin in Barrett's esophagus.  They include direct [[Biology:Transdifferentiation|transdifferentiation]] of squamous cells to columnar cells, the stem cell changing from esophageal type to intestinal type, migration of gastric [[Biology:Stomach#Sections|cardiac]] cells, and a population of resident embryonic cells present through adulthood.

== Significance in disease ==

Normal physiological metaplasia, such as that of the endocervix, is highly desirable.

The medical significance of metaplasia is that in some sites where pathological irritation is present, cells may progress from metaplasia, to develop dysplasia, and then malignant neoplasia (cancer). Thus, at sites where abnormal metaplasia is detected, efforts are made to remove the causative irritant, thereby decreasing the risk of progression to malignancy. The metaplastic area must be carefully monitored to ensure that dysplastic change does not begin to occur. A progression to significant dysplasia indicates that the area could need removal to prevent the development of cancer.

== Examples ==
[[File:Barretts esophagus alcian blue high mag.jpg|right|thumb|A micrograph of Barrett's esophagus.]]
Barrett's esophagus is an abnormal change in the cells of the lower esophagus, thought to be caused by damage from chronic stomach acid exposure.

The following table lists some common tissues susceptible to metaplasia, and the stimuli that can cause the change:

{| class="wikitable"
|-
! Tissue
! Normal
! Metaplasia
! Stimulus
|-
| Airways
| Pseudostratified columnar epithelium
| Squamous epithelium
| Cigarette smoke
|-
| [[Biology:Urinary bladder|Urinary bladder]]
| Transitional epithelium
| Squamous epithelium
| Bladder stone
|-
| [[Biology:Esophagus|Esophagus]]
| Squamous epithelium
| Columnar epithelium (Barrett's Esophagus)
| Gastro-esophageal reflux
|-
| [[Medicine:Cervix|Cervix]]
| Glandular epithelium
| Squamous epithelium
| Low pH of [[Biology:Vagina|vagina]]
|}

===Intestinal metaplasia===

Intestinal metaplasia is a [[Medicine:Precancerous condition|premalignant condition]] that increases the risk for subsequent [[Medicine:Stomach cancer|gastric cancer]].<ref name = Krishnan2020>Krishnan V, Lim DXE, Hoang PM, Srivastava S, Matsuo J, Huang KK, Zhu F, Ho KY, So JBY, Khor C, Tsao S, Teh M, Fock KM, Ang TL, Jeyasekharan AD, Tan P, Yeoh KG, Ito Y. DNA damage signalling as an anti-cancer barrier in gastric intestinal metaplasia. Gut. 2020 Oct;69(10):1738-1749. doi: 10.1136/gutjnl-2019-319002. Epub 2020 Jan 14. PMID 31937549; PMCID: PMC7497583</ref>  Intestinal metaplasia lesions with an active [[Biology:DNA damage (naturally occurring)|DNA damage]] response will likely undergo extended latency in the premalignant state until further damaging hits override the DNA damage response leading to clonal expansion and progression.<ref name = Krishnan2020/>  The DNA damage response includes expression of proteins that detect DNA damages and activate downstream responses like [[Biology:DNA repair|DNA repair]], [[Biology:Cell cycle checkpoint|cell cycle checkpoint]]s or [[Biology:Apoptosis|apoptosis]].<ref name = Krishnan2020/>

==See also==
* [[Biology:Epigenetics|Epigenetics]]
* [[Biology:Induced stem cells|Induced stem cells]]
* [[Biology:List of biological development disorders|List of biological development disorders]]
*[[Biology:Pleomorphism (cytology)|Pleomorphism]]
* [[Biology:Reprogramming|Reprogramming]]
* [[Biology:Transdifferentiation|Transdifferentiation]]

== Notes ==
*''The AMA Home Medical Encyclopedia'', Random House, p.&nbsp;683
*''Robbins and Cotran - Pathologic Basis of Disease'', 7th Edition, Saunders, p.&nbsp;10
* Prof. Dr. Clark S., Australian Cancer institute, premalignant conditions. 1st edition pages(321-376). Reviewed.

== References ==
{{Reflist}}

== External links ==
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{{Pathology}}

[[Category:Histopathology]]
[[Category:Oncology]]
[[Category:Induced stem cells]]

{{Sourceattribution|Metaplasia}}