Editing Apoptosis (section)

{{Short description|Form of programmed cell death}}
{{Infobox anatomy
| Name        = Apoptosis
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| Image       = Apoptosis DU145 cells mosaic.jpg
| Caption     = An [[etoposide]]-treated [[DU145|DU145 prostate cancer cell]] exploding into a cascade of apoptotic bodies. The sub images were extracted from a 61-hour [[time-lapse microscopy]] video, created using [[quantitative phase-contrast microscopy]]. The optical thickness is color-coded. With increasing thickness, color changes from gray to yellow, red, purple and finally black. [http://www.cellimagelibrary.org/images/43705<br />See the video at The Cell: An Image Library]
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'''Apoptosis''' (from {{langx|grc|[[wikt:ἀπόπτωσις|ἀπόπτωσις]]|apóptōsis|falling off}}) is a form of [[programmed cell death]] that occurs in [[multicellular organism]]s and in some eukaryotic, single-celled microorganisms such as [[yeast]].<ref>{{cite book|vauthors=Green D|title=Means to an End: Apoptosis and other Cell Death Mechanisms|year=2011|publisher=Cold Spring Harbor Laboratory Press|location=Cold Spring Harbor, NY|isbn=978-0-87969-888-1|url=https://books.google.com/books?id=s8jBcQAACAAJ|access-date=2020-05-25|archive-date=2020-07-26|archive-url=https://web.archive.org/web/20200726071718/https://books.google.com/books?id=s8jBcQAACAAJ|url-status=live}}</ref> [[Biochemistry|Biochemical]] events lead to characteristic cell changes ([[Morphology (biology)|morphology]]) and death.<ref name="pmid14499155">{{cite journal |vauthors=Böhm I, Schild H |title=Apoptosis: the complex scenario for a silent cell death |journal=Mol Imaging Biol |volume=5 |issue=1 |pages=2–14 |year=2003 |pmid=14499155 |doi=10.1016/S1536-1632(03)00024-6}}</ref> These changes include [[Bleb (cell biology)|blebbing]], [[Plasmolysis|cell shrinkage]], [[Karyorrhexis|nuclear fragmentation]], [[Pyknosis|chromatin condensation]], [[Apoptotic DNA fragmentation|DNA fragmentation]], and [[mRNA]] decay. The average adult human loses 50 to 70 [[1,000,000,000|billion]] cells each day due to apoptosis.{{efn|Note that the average human adult has more than 13 trillion cells ({{val|1.3|e=13}}),{{sfn|Alberts|p=2}} of which at most only 70 billion ({{val|7.0|e=10}}) die per day. That is, about 5 out of every 1,000 cells (0.5%) die each day due to apoptosis.}} For the average human child between 8 and 14 years old, each day the approximate loss is 20 to 30 billion cells.<ref>{{cite book | vauthors = Karam JA |title=Apoptosis in Carcinogenesis and Chemotherapy|year=2009|publisher=Springer|location=Netherlands|isbn=978-1-4020-9597-9}}</ref>

In contrast to [[necrosis]], which is a form of traumatic cell death that results from acute cellular injury, apoptosis is a highly regulated and controlled process that confers advantages during an organism's life cycle. For example, the separation of fingers and toes in a developing human [[embryo]] occurs because cells between the digits undergo a form of apoptosis that is genetically determined.<ref name="Elmore">{{cite journal |vauthors=Elmore S |title=Apoptosis: a review of programmed cell death |journal=Toxicol Pathol |volume=35 |issue=4 |pages=495–516 |date=June 2007 |pmid=17562483 |pmc=2117903 |doi=10.1080/01926230701320337 |url=}}</ref> Unlike necrosis, apoptosis produces cell fragments called [[Extracellular vesicle#Apoptotic bodies|apoptotic bodies]] that [[phagocyte]]s are able to engulf and remove before the contents of the cell can spill out onto surrounding cells and cause damage to them.<ref>{{cite book | vauthors = Alberts B, Johnson A, Lewis J, Raff M, Roberts K, Walter P |title=Molecular Biology of the Cell (textbook) |edition=5th |publisher=[[Garland Science]] |page=1115 |chapter=Chapter 18 Apoptosis: Programmed Cell Death Eliminates Unwanted Cells |year=2008 |isbn=978-0-8153-4105-5|title-link=Molecular Biology of the Cell (textbook) }}</ref>

Because apoptosis cannot stop once it has begun, it is a highly regulated process. Apoptosis can be initiated through one of two pathways. In the ''intrinsic pathway'' the cell kills itself because it senses [[Cellular stress response|cell stress]], while in the ''extrinsic pathway'' the cell kills itself because of signals from other cells. Weak external signals may also activate the intrinsic pathway of apoptosis.<ref>{{cite journal | vauthors = Raychaudhuri S | title = A minimal model of signaling network elucidates cell-to-cell stochastic variability in apoptosis | journal = PLOS ONE | volume = 5 | issue = 8 | pages = e11930 | date = August 2010 | pmid = 20711445 | pmc = 2920308 | doi = 10.1371/journal.pone.0011930 | arxiv = 1009.2294 | doi-access = free | bibcode = 2010PLoSO...511930R }}</ref> Both pathways induce cell death by activating [[caspase]]s, which are [[protease]]s, or enzymes that degrade proteins. The two pathways both activate initiator caspases, which then activate executioner caspases, which then kill the cell by degrading proteins indiscriminately.

In addition to its importance as a biological phenomenon, defective apoptotic processes have been implicated in a wide variety of diseases. Excessive apoptosis causes [[atrophy]], whereas an insufficient amount results in uncontrolled cell proliferation, such as [[cancer]]. Some factors like [[Fas receptor]]s and caspases promote apoptosis, while some members of the [[Bcl-2 family]] of proteins inhibit apoptosis.<ref>{{cite journal | vauthors = Elmore S | title = Apoptosis: A Review of Programmed Cell Death | journal = Toxicologic Pathology | volume = 35 | issue = 4 | pages = 495–516 | date = June 2007 | pmid = 17562483 | pmc = 2117903 | doi = 10.1080/01926230701320337 }}</ref>