Editing Platelet (section)

{{short description|Component of blood aiding in coagulation}}
{{Other uses}}
{{Infobox cell
| Name        = Platelets
| Latin       = thrombocytus
| Image       = Platelets2.JPG
| Caption     = Image from a [[light microscope]] (500 ×) from a [[Giemsa stain|Giemsa-stained]] peripheral [[blood film|blood smear]] showing platelets (small purple dots) surrounded by [[red blood cell]]s (large gray circular structures)
| Width       =
| Image2      =
| Caption2    =
| Precursor   = [[Megakaryocyte]]s
| System      =
| Function = Formation of blood clots; prevention of bleeding
}}

'''Platelets''' or '''thrombocytes''' ({{etymology|grc|''{{Wikt-lang|grc|θρόμβος}}'' ({{grc-transl|θρόμβος}})|clot||''{{Wikt-lang|grc|κύτος}}'' ({{grc-transl|κύτος}})|cell}}) are a part of [[blood]] whose function (along with the [[coagulation#Coagulation factors|coagulation factors]]) is to react to [[bleeding]] from [[blood vessel]] injury by clumping to form a [[thrombus|blood clot]].<ref>{{cite journal |vauthors=Laki K |title=Our ancient heritage in blood clotting and some of its consequences |journal=Annals of the New York Academy of Sciences |volume=202 |issue=1 |pages=297–307 |date=December 1972 |pmid=4508929 |doi=10.1111/j.1749-6632.1972.tb16342.x |bibcode=1972NYASA.202..297L |s2cid=45051688}}</ref> Platelets have no [[cell nucleus]]; they are fragments of [[cytoplasm]] from [[megakaryocyte]]s which reside in [[bone marrow]] or [[Lung|lung tissue]],<ref>{{cite journal |last1=Lefrançais |first1=Emma |last2=Ortiz-Muñoz |first2=Guadalupe |last3=Caudrillier |first3=Axelle |last4=Mallavia |first4=Beñat |last5=Liu |first5=Fengchun |last6=Sayah |first6=David M. |last7=Thornton |first7=Emily E. |last8=Headley |first8=Mark B. |last9=David |first9=Tovo |last10=Coughlin |first10=Shaun R. |last11=Krummel |first11=Matthew F. |date=April 2017 |title=The lung is a site of platelet biogenesis and a reservoir for haematopoietic progenitors |journal=Nature |language=en |volume=544 |issue=7648 |pages=105–9 |doi=10.1038/nature21706 |pmid=28329764 |pmc=5663284 |bibcode=2017Natur.544..105L |issn=1476-4687}}</ref> and then enter the circulation. Platelets are found only in mammals, whereas in other [[vertebrate]]s (e.g. [[bird]]s, [[amphibian]]s), thrombocytes circulate as intact [[agranulocyte|mononuclear cell]]s.<ref name=Michelson/>{{rp|3}}

[[File:Platelet Response Animation.gif|thumb|The [[ligand]]s, denoted by letter L, signal for platelets (P) to migrate towards the wound (Site A). As more platelets gather around the opening, they produce more ligands to amplify the response. The platelets congregate around the wound in order to create a cap to stop blood flow out of the tissue.]]

One major function of platelets is to contribute to [[hemostasis]]: the process of stopping bleeding at the site where the lining of vessels ([[endothelium]]) has been interrupted. Platelets gather at the site and, unless the interruption is physically too large, they plug the hole. First, platelets attach to substances outside the interrupted endothelium: ''[[adhesion (medicine)|adhesion]]''. Second, they change shape, turn on receptors and secrete [[ligand (biochemistry)|chemical messengers]]: ''activation''. Third, they connect to each other through receptor bridges: ''aggregation''.<ref name="pmid16036569">{{cite journal |vauthors=Yip J, Shen Y, Berndt MC, Andrews RK |title=Primary platelet adhesion receptors |journal=IUBMB Life |volume=57 |issue=2 |pages=103–8 |date=February 2005 |pmid=16036569 |doi=10.1080/15216540500078962 |s2cid=12054259|doi-access=free }}</ref> Formation of this [[platelet plug]] (primary hemostasis) is associated with activation of the [[coagulation cascade]], with resultant [[fibrin]] deposition and linking (secondary hemostasis). These processes may overlap: the spectrum is from a predominantly platelet plug, or "white clot" to a predominantly fibrin, or "red clot" or the more typical mixture. Berridge adds ''retraction'' and ''[[platelet inhibition]]'' as fourth and fifth steps,<ref>{{cite book |last1=Berridge |first1=Michael J. |chapter=Module 11: Cell Stress, Inflammatory Responses and Cell Death |chapter-url=https://portlandpress.com/DocumentLibrary/Umbrella/Cell%20Signaling/csb0001011.full.pdf |title=Cell Signalling Biology |date=1 October 2014 |volume=6 |pages=11-1–11-30 |doi=10.1042/csb0001011 |publisher=Portland Press|doi-broken-date=1 November 2024 }} {{open access}}</ref> while others would add a sixth step, ''wound repair''.{{Citation needed|date=June 2024}} Platelets participate in both innate<ref name=":0">{{cite journal |vauthors=Gaertner F, Massberg S |title=Blood coagulation in immunothrombosis-At the frontline of intravascular immunity |journal=Seminars in Immunology |volume=28 |issue=6 |pages=561–9 |date=December 2016 |pmid=27866916 |doi=10.1016/j.smim.2016.10.010}}</ref> and adaptive<ref>{{cite journal |vauthors=Hampton T |title=Platelets' Role in Adaptive Immunity May Contribute to Sepsis and Shock |journal=JAMA |volume=319 |issue=13 |pages=1311–2 |date=April 2018 |pmid=29614158 |doi=10.1001/jama.2017.12859}}</ref> intravascular immune responses.

In addition to facilitating the clotting process, platelets contain [[cytokine]]s and [[growth factor]]s which can promote wound healing and regeneration of damaged tissues.<ref name="pmid34924312">{{cite journal | vauthors = Cecerska-Heryć E, Goszka M, Dołęgowska B | title=Applications of the regenerative capacity of platelets in modern medicine | journal=Cytokine & Growth Factor Reviews | volume=64 | pages=84–94 | year=2022 | doi= 10.1016/j.cytogfr.2021.11.003 | pmid=34924312}}</ref><ref name="pmid32145082">{{cite journal | vauthors = Xu J, Gou L, Qiu S | title=Platelet-rich plasma and regenerative dentistry | journal=Australian Dental Journal | volume=65 | issue=2 | pages=131–142 | year=2020 | doi= 10.1111/adj.12754 | pmc=7384010 | pmid=32145082}}</ref>