View source for Platelet

{{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 it. 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=12 July 2025 }} {{open access}}</ref> while others would add a sixth step, ''wound repair''.<ref>{{Cite journal |last=Etulain |first=Julia |date=2018-08-18 |title=Platelets in wound healing and regenerative medicine |journal=Platelets |volume=29 |issue=6 |pages=556–568 |doi=10.1080/09537104.2018.1430357 |issn=0953-7104 |pmid=29442539 |hdl=11336/93928 |hdl-access=free }}</ref> 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>

== Term ==
The term ''thrombocyte'' (clot cell) came into use in the early 1900s and is sometimes used as a synonym for platelet; but not generally in the scientific literature, except as a root word for other terms related to platelets (e.g. ''thrombocytopenia'' meaning low platelets).<ref name="Michelson" />{{rp|v3}} The term thrombocytes are proper for mononuclear cells found in the blood of non-mammalian vertebrates: they are the functional equivalent of platelets, but circulate as intact cells rather than cytoplasmic fragments of bone marrow megakaryocytes.<ref name="Michelson" />{{rp|3}}

In some contexts, the word ''thrombus'' is used interchangeably with the word ''clot'', regardless of its composition (white, red, or mixed). In other contexts it is used to contrast a normal from an abnormal clot: ''thrombus'' arises from physiologic hemostasis, ''thrombosis'' arises from a pathologic and excessive quantity of clot.<ref>{{cite journal |vauthors=Furie B, Furie BC |date=August 2008 |title=Mechanisms of thrombus formation |journal=The New England Journal of Medicine |volume=359 |issue=9 |pages=938–949 |doi=10.1056/NEJMra0801082 |pmid=18753650}}</ref> In a third context it is used to contrast the result from the process: ''thrombus'' is the result, ''thrombosis'' is the process.

==Structure==
Structurally the platelet can be divided into four zones, from peripheral to innermost:<ref>{{Cite journal |last1=Geraldo |first1=Reinaldo Barros |last2=Sathler |first2=Plínio Cunha |last3=Lourenço |first3=André Luiz |last4=Saito |first4=Max Seidy |last5=Cabral |first5=Lucio M. |last6=Rampelotto |first6=Pabulo Henrique |last7=Castro |first7=Helena Carla |date=2014-10-07 |title=Platelets: still a therapeutical target for haemostatic disorders |journal=International Journal of Molecular Sciences |volume=15 |issue=10 |pages=17901–17919 |doi=10.3390/ijms151017901 |doi-access=free |issn=1422-0067 |pmc=4227196 |pmid=25295482}}</ref>
* Peripheral zone — rich in [[glycoprotein]]s required for platelet adhesion, activation and aggregation. For example, [[GPIb|GPIb/IX/V]]; [[GPVI]]; [[GpIIb/IIIa|GPIIb/IIIa]]
* Sol-gel zone — rich in [[microtubule]]s and [[microfilament]]s, allowing platelets to maintain a discoid shape
* Organelle zone — rich in platelet granules. [[Alpha granule]]s contain clotting mediators such as [[factor V]], [[factor VIII]], [[fibrinogen]], [[fibronectin]], platelet-derived growth factor, and [[chemotactic agent]]s. Delta granules, or [[dense granule|dense bodies]], contain [[ADP-ribosylation|ADP]], [[calcium]], and [[serotonin]], which are platelet-activating mediators.
* Membranous zone — membranes derived from [[megakaryocyte]] smooth [[endoplasmic reticulum]] organized into a dense tubular system that is responsible for [[thromboxane A2]] synthesis. This dense tubular system is connected to the surface platelet membrane to aid thromboxane A2 release.

===Shape===

Circulating inactivated platelets are biconvex discoid (lens-shaped) structures,<ref>{{cite journal |vauthors=Jain NC |title=A scanning electron microscopic study of platelets of certain animal species |journal=Thrombosis et Diathesis Haemorrhagica |volume=33 |issue=3 |pages=501–7 |date=June 1975 |pmid=1154309}}</ref><ref name=Michelson>{{cite book |last1=Michelson |first1=Alan D. |title=Platelets |year=2013 |publisher=Academic |isbn=978-0-12-387837-3 |oclc=820818942  |edition=3rd}}</ref>{{rp|117–118}} 2–3&nbsp;μm in greatest diameter.<ref>{{cite journal |vauthors=Paulus JM |title=Platelet size in man |journal=Blood |volume=46 |issue=3 |pages=321–336 |date=September 1975 |pmid=1097000 |doi=10.1182/blood.V46.3.321.321 |doi-access=free}}</ref> Activated platelets have cell membrane projections covering their surface.

In a first approximation, the shape can be considered similar to [[spheroid#Oblate spheroids|oblate spheroids]], with a semiaxis ratio of 2 to 8.<ref>{{cite journal |vauthors=Frojmovic MM |title=Geometry of normal mammalian platelets by quantitative microscopic studies |journal=Biophysical Journal |volume=16 |issue=9 |pages=1071–89 |date=1976 |doi=10.1016/s0006-3495(76)85756-6 |pmid=786400 |pmc=1334946 |bibcode=1976BpJ....16.1071F}}</ref> This approximation can be used to model the hydrodynamic and optical properties of a population, as well as to restore the geometric parameters of individual measured platelets by [[flow cytometry]].<ref>{{cite journal |vauthors=Moskalensky AE, Yurkin MA, Konokhova AI, Strokotov DI, Nekrasov VM, Chernyshev AV, Tsvetovskaya GA, Chikova ED, Maltsev VP |title=Accurate measurement of volume and shape of resting and activated blood platelets from light scattering |journal=Journal of Biomedical Optics |date=2013 |volume=18 |issue=1 |article-number=017001 |doi=10.1117/1.JBO.18.1.017001 |pmid=23288415 |bibcode=2013JBO....18a7001M |s2cid=44626047|doi-access=free }}</ref> More accurate biophysical models of platelet surface morphology that model its shape from first principles, make it possible to obtain a more realistic platelet geometry in a calm and activated state.<ref>{{cite journal |vauthors=Moskalensky AE, Yurkin MA, Muliukov AR, Litvinenko AL, Nekrasov VM, Chernyshev AV, Maltsev VP |title=Method for the simulation of blood platelet shape and its evolution during activation |journal=PLOS Computational Biology |volume=14 |issue=3 |article-number=e1005899 |date=2018 |doi=10.1371/journal.pcbi.1005899 |pmid=29518073 |pmc=5860797 |bibcode=2018PLSCB..14E5899M |doi-access=free }}</ref>

===Development===
[[File:Blood cells differentiation chart.jpg|thumb|Platelets derive from multipotent marrow stem cells.]]
* Megakaryocyte and platelet production is regulated by [[thrombopoietin]], a hormone produced in the kidneys and liver.
* Each megakaryocyte produces between 1,000 and 3,000 platelets during its lifetime.
* An average of 10<sup>11</sup> platelets are produced daily in a healthy adult.
* Reserve platelets are stored in the spleen and are released when needed by splenic contraction induced by the sympathetic nervous system.
[[File:1908 Platelet Development.jpg|thumb|Platelets extruded from megakaryocytes]]
* The average life span of circulating platelets is 8 to 9 days.<ref>{{cite journal |vauthors=Harker LA, Roskos LK, Marzec UM, Carter RA, Cherry JK, Sundell B, Cheung EN, Terry D, Sheridan W |title=Effects of megakaryocyte growth and development factor on platelet production, platelet life span, and platelet function in healthy human volunteers |journal=Blood |volume=95 |issue=8 |pages=2514–22 |date=April 2000 |pmid=10753829 |doi=10.1182/blood.V95.8.2514}}</ref> Life span of individual platelets is controlled by the internal apoptotic regulating pathway, which has a Bcl-x<sub>L</sub> timer.<ref>{{cite journal |vauthors=Mason KD, Carpinelli MR, Fletcher JI, Collinge JE, Hilton AA, Ellis S, Kelly PN, Ekert PG, Metcalf D, Roberts AW, Huang DC, Kile BT |title=Programmed anuclear cell death delimits platelet life span |journal=Cell |volume=128 |issue=6 |pages=1173–86 |date=March 2007 |pmid=17382885 |doi=10.1016/j.cell.2007.01.037 |s2cid=7492885 |doi-access=free}}</ref>
* Old platelets are destroyed by [[phagocytosis]] in the spleen and liver.

==Hemostasis==
{{Main|Hemostasis}}
[[File:Blausen 0740 Platelets.png|thumb|3D rendering of four inactivated and three activated platelets]]
The fundamental function of platelets is to clump together to stop acute bleeding. This process is complex, as more than 193 proteins and 301 interactions are involved in platelet dynamics.<ref name="pmid16036569"/>  Despite much overlap, platelet function can be modeled in three steps:

===Adhesion===
[[Thrombus]] formation on an intact [[endothelium]] is prevented by [[nitric oxide]],<ref>{{cite journal |vauthors=Palmer RM, Ferrige AG, Moncada S |title=Nitric oxide release accounts for the biological activity of endothelium-derived relaxing factor |journal=Nature |volume=327 |issue=6122 |pages=524–6 |date=1987 |pmid=3495737 |doi=10.1038/327524a0 |bibcode=1987Natur.327..524P |s2cid=4305207}}</ref> [[prostacyclin]],<ref name="Jones-2012">{{cite book <!-- Citation bot bypass-->|vauthors=Jones CI, Barrett NE, Moraes LA, Gibbins JM, Jackson DE |chapter=Endogenous inhibitory mechanisms and the regulation of platelet function |volume=788 |pages=341–366 |date=2012 |pmid=22130718 |doi=10.1007/978-1-61779-307-3_23 |isbn=978-1-61779-306-6 |series=Methods in Molecular Biology |title=Platelets and Megakaryocytes }}</ref> and [[CD39]].<ref>{{cite journal |vauthors=Marcus AJ, Broekman MJ, Drosopoulos JH, Olson KE, Islam N, Pinsky DJ, Levi R |title=Role of CD39 (NTPDase-1) in thromboregulation, cerebroprotection, and cardioprotection |journal=Seminars in Thrombosis and Hemostasis |volume=31 |issue=2 |pages=234–246 |date=April 2005 |pmid=15852226 |doi=10.1055/s-2005-869528|s2cid=41764516 }}</ref>

[[Endothelial cells]] attach to the subendothelial [[collagen]] by [[von Willebrand factor]] (VWF), which these cells produce. VWF is also stored in the [[Weibel-Palade bodies]] of the endothelial cells and secreted constitutively into the blood. Platelets store vWF in their alpha granules.

When the endothelial layer is disrupted, collagen and VWF anchor platelets to the subendothelium. Platelet [[glycoprotein Ib-IX-V Receptor Complex|GP1b-IX-V]] receptor binds with VWF; and GPVI receptor and integrin α2β1 bind with collagen.<ref name=Dubois06>{{cite journal |vauthors=Dubois C, Panicot-Dubois L, Merrill-Skoloff G, Furie B, Furie BC |title=Glycoprotein VI-dependent and -independent pathways of thrombus formation in vivo |journal=Blood |volume=107 |issue=10 |pages=3902–6 |date=May 2006 |pmid=16455953 |pmc=1895285 |doi=10.1182/blood-2005-09-3687}}</ref>

===Activation===
[[File:Red White Blood cells.jpg|thumb|Scanning electron micrograph of blood cells. From left to right: human [[red blood cell|erythrocyte]], activated platelet, [[leukocyte]].]]

====Inhibition====
Factors from the lining of vessels stop platelets from activating. An intact endothelial lining ''inhibits'' platelet activation by producing [[nitric oxide]], endothelial-[[apyrase|ADPase]], and [[prostacyclin|PGI<sub>2</sub>]] (prostacyclin). Endothelial-ADPase degrades the platelet activator [[adenosine diphosphate|ADP]].<ref>{{Cite journal |last=van Hinsbergh |first=Victor W. M. |date=2010 |title=Endothelium--role in regulation of coagulation and inflammation |journal=Seminars in Immunopathology |volume=34 |issue=1 |pages=93–106 |doi=10.1007/s00281-011-0285-5 |issn=1863-2300 |pmc=3233666 |pmid=21845431}}</ref>

Resting platelets maintain active calcium [[efflux (microbiology)|efflux]] via a [[cyclic AMP]]-activated calcium pump. Intracellular calcium concentration determines platelet activation status, as it is the [[second messenger]] that drives platelet conformational change and degranulation. Endothelial [[prostacyclin]] binds to [[prostacyclin receptor|prostanoid]] receptors on the surface of resting platelets. This event stimulates the coupled [[Gs alpha subunit|Gs]] protein to increase [[adenylate cyclase]] activity and increases the production of cAMP, further promoting the efflux of calcium and reducing intracellular calcium availability for platelet activation.<ref>{{Cite journal |last1=Nagy |first1=Zoltan |last2=Smolenski |first2=Albert |date=July 2018 |title=Cyclic nucleotide-dependent inhibitory signaling interweaves with activating pathways to determine platelet responses |journal=Research and Practice in Thrombosis and Haemostasis |volume=2 |issue=3 |pages=558–571 |doi=10.1002/rth2.12122 |pmid=30046761 |pmc=6046581 |issn=2475-0379}}</ref>

ADP binds to [[purinergic receptor]]s on the platelet surface. Since the thrombocytic purinergic receptor [[P2Y12]] is coupled to [[Gi alpha subunit|Gi]] proteins, ADP reduces platelet adenylate cyclase activity and cAMP production, leading to accumulation of calcium inside the platelet by inactivating the cAMP calcium efflux pump. The other ADP-receptor [[P2Y1]] couples to Gq that activates phospholipase C-beta 2 ([[PLCB2]]), resulting in [[inositol 1,4,5-trisphosphate]] (IP3) generation and intracellular release of more calcium. This together induces platelet activation. Endothelial ADPase degrades ADP and prevents this from happening. [[Clopidogrel]] and related antiplatelet medications also work as purinergic receptor [[P2Y12]] [[receptor antagonist|antagonists]].<ref>{{Cite journal |last1=Dorsam |first1=Robert T. |last2=Kunapuli |first2=Satya P. |date=February 2004 |title=Central role of the P2Y12 receptor in platelet activation |journal=The Journal of Clinical Investigation |volume=113 |issue=3 |pages=340–345 |doi=10.1172/JCI20986 |issn=0021-9738 |pmc=324551 |pmid=14755328}}</ref> Data suggest that ADP activates the [[PI3K/Akt]] pathway during a first wave of aggregation, leading to thrombin generation and [[protease-activated receptor 1|PAR‐1]] activation, which evokes a second wave of aggregation.<ref name="JiangXu2013">{{cite journal |last1=Jiang |first1=L. |last2=Xu |first2=C. |last3=Yu |first3=S. |last4=Liu |first4=P. |last5=Luo |first5=D. |last6=Zhou |first6=Q. |last7=Gao |first7=C. |last8=Hu |first8=H. |title=A critical role of thrombin/PAR-1 in ADP-induced platelet secretion and the second wave of aggregation |journal=Journal of Thrombosis and Haemostasis |volume=11 |issue=5 |year=2013 |pages=930–940 |issn=1538-7933 |doi=10.1111/jth.12168 |pmid=23406164 |doi-access=free}}</ref>

====Trigger (induction)====
Platelet activation begins seconds after adhesion occurs. It is triggered when ''collagen'' from the subendothelium binds with its receptors ([[GPVI]] receptor and integrin α2β1) on the platelet. GPVI is associated with the Fc receptor gamma chain and leads via the activation of a tyrosine kinase cascade finally to the activation of PLC-gamma2 ([[PLCG2]]) and more calcium release.<ref>{{Cite journal |last1=Gibbins |first1=J. M. |last2=Okuma |first2=M. |last3=Farndale |first3=R. |last4=Barnes |first4=M. |last5=Watson |first5=S. P. |date=1997-08-18 |title=Glycoprotein VI is the collagen receptor in platelets which underlies tyrosine phosphorylation of the Fc receptor gamma-chain |journal=FEBS Letters |volume=413 |issue=2 |pages=255–259 |doi=10.1016/s0014-5793(97)00926-5 |issn=0014-5793 |pmid=9280292 |bibcode=1997FEBSL.413..255G }}</ref>

[[Tissue factor]] also binds to [[factor VII]] in the blood, which initiates the extrinsic [[coagulation]] cascade to increase [[thrombin]] production. Thrombin is a potent platelet activator, acting through Gq and G12. These are [[G protein-coupled receptor]]s and they turn on calcium-mediated [[signaling pathways]] within the platelet, overcoming the baseline calcium efflux. Families of three G proteins (Gq, Gi, G12) operate together for full activation. Thrombin also promotes secondary fibrin-reinforcement of the [[platelet plug]]. Platelet activation in turn degranulates and releases [[factor V]] and [[fibrinogen]], potentiating the coagulation cascade. Platelet plugging and coagulation occur simultaneously, with each inducing the other to form the final fibrin-crosslinked thrombus.<ref>{{Cite journal |last=Offermanns |first=Stefan |date=2006-12-08 |title=Activation of Platelet Function Through G Protein–Coupled Receptors |url=https://www.ahajournals.org/doi/10.1161/01.RES.0000251742.71301.16 |journal=Circulation Research |volume=99 |issue=12 |pages=1293–1304 |doi=10.1161/01.RES.0000251742.71301.16|url-access=subscription }}</ref><ref>{{Cite journal |last=Monroe |first=Dougald M. |last2=Hoffman |first2=Maureane |last3=Roberts |first3=Harold R. |date=1 August 2002 |title=Platelets and Thrombin Generation |url=https://www.ahajournals.org/doi/10.1161/01.ATV.0000031340.68494.34 |journal=Arteriosclerosis, Thrombosis, and Vascular Biology |volume=22 |issue=9 |pages=1381–1389 |doi=10.1161/01.ATV.0000031340.68494.34|url-access=subscription }}</ref>

====Components (consequences)====

=====GPIIb/IIIa activation=====
Collagen-mediated GPVI signalling increases the platelet production of [[thromboxane A2]] (TXA2) and decreases the production of [[prostacyclin]]. This occurs by altering the metabolic flux of platelet's [[eicosanoid]] synthesis pathway, which involves enzymes [[phospholipase A2]], [[PTGS1|cyclo-oxygenase 1]], and [[thromboxane-A synthase]]. Platelets secrete thromboxane A2, which acts on the platelet's own [[thromboxane receptor]]s on the platelet surface (hence the so-called "out-in" mechanism), and those of other platelets. These receptors trigger intraplatelet signaling, which converts [[GPIIb/IIIa]] receptors to their active form to initiate ''aggregation''.<ref name="pmid16036569"/>

=====Granule secretion=====
[[File:Platelet structure.png|right|thumb|Diagram of the structure of a platelet showing the granules]]
Platelets contain [[dense granules]], lambda granules, and [[alpha granule]]s. Activated platelets secrete the contents of these granules through their canalicular systems to the exterior. Bound and activated platelets degranulate to release platelet [[chemotactic]] agents to attract more platelets to the site of endothelial injury. Granule characteristics:
* [[Platelet alpha-granule|α granules (alpha granules)]] — containing [[P-selectin]], [[platelet factor 4]], [[TGF beta 1|transforming growth factor-β1]], [[platelet-derived growth factor]], [[fibronectin]], [[B-thromboglobulin]], [[von Willebrand factor|vWF]], [[fibrinogen]], and [[coagulation factor]]s [[factor V|V]] and [[factor XIII|XIII]]
* [[dense granule|δ granules (delta or dense granules)]] — containing [[adenosine diphosphate|ADP]] or [[adenosine triphosphate|ATP]], [[calcium]], and [[serotonin]]
* γ granules (gamma granules) — similar to [[lysosome]]s and contain several hydrolytic enzymes
* λ granules (lambda granules) — contents involved in resorption during later stages of vessel repair

=====Morphology change=====
As shown by flow cytometry and [[electron microscopy]], the most sensitive sign of activation, when exposed to platelets using ADP, are morphological changes.<ref>{{cite journal |vauthors=Litvinov RI, Weisel JW, Andrianova IA, Peshkova AD, Minh GL |title=Differential Sensitivity of Various Markers of Platelet Activation with Adenosine Diphosphate |journal=BioNanoScience |volume=9 |issue=1 |pages=53–58 |date=2018 |doi=10.1007/s12668-018-0586-4 |pmid=31534882 |pmc=6750022}}</ref> Mitochondrial hyperpolarization is a key event in initiating morphology changes.<ref>{{cite journal |vauthors=Matarrese P, Straface E, Palumbo G, Anselmi M, Gambardella L, Ascione B, Del Principe D, Malorni W |title=Mitochondria regulate platelet metamorphosis induced by opsonized zymosan A — activation and long-term commitment to cell death |journal=The FEBS Journal |volume=276 |issue=3 |pages=845–856 |date=February 2009 |pmid=19143843 |doi=10.1111/j.1742-4658.2008.06829.x |doi-access=free}}</ref> Intraplatelet calcium concentration increases, stimulating the interplay between the microtubule/actin filament complex. The continuous changes in shape from the unactivated to the fully activated platelet are best seen via [[scanning electron microscopy]]. The three steps along this path are named ''early dendritic'', ''early spread,'' and ''spread''. The surface of the unactivated platelet looks similar to the surface of the brain–a wrinkled appearance from numerous shallow folds that increase the surface area; ''early dendritic'', an octopus with multiple arms and legs; ''early spread'', an uncooked frying egg in a pan, the "yolk" is the central body; and the ''spread'', a cooked fried egg with a denser central body.

These changes are all brought about by the interaction of the microtubule/actin complex with the platelet cell membrane and open canalicular system (OCS), which is an extension and invagination of that membrane. This complex runs just beneath these membranes and is the chemical motor that pulls the invaginated OCS out of the interior of the platelet, like turning pants pockets inside out, creating the dendrites. This process is similar to the mechanism of contraction in a [[muscle cell]].<ref>{{cite journal |vauthors=White JG |title=An overview of platelet structural physiology |journal=Scanning Microsc. |volume=1 |issue=4 |pages=1677–1700 |date=December 1987 |pmid=3324323}}</ref> The entire OCS thus becomes indistinguishable from the initial platelet membrane as it forms the "fried egg". This dramatic increase in surface area comes about with neither stretching nor adding phospholipids to the platelet membrane.<ref>{{cite journal |vauthors=Behnke O |title=The morphology of blood platelet membrane systems |journal=Series Haematologica |volume=3 |issue=4 |pages=3–16 |date=1970 |pmid=4107203}}</ref>

=====Platelet-coagulation factor interactions: coagulation facilitation=====
Platelet activation causes its membrane surface to become negatively charged. One of the signaling pathways turns on [[scramblase]], which moves negatively charged [[phospholipid]]s from the inner to the outer platelet membrane surface. These phospholipids then bind the [[tenase]] and [[prothrombinase]] complexes, two of the sites of interplay between platelets and the coagulation cascade. Calcium ions are essential for the binding of these coagulation factors.

In addition to interacting with vWF and fibrin, platelets interact with thrombin, Factors X, Va, VIIa, XI, IX, and prothrombin to complete formation via the coagulation cascade.<ref name=Bouchard10>{{cite journal |vauthors=Bouchard BA, Mann KG, Butenas S |title=No evidence for tissue factor on platelets |journal=Blood |volume=116 |issue=5 |pages=854–5 |date=August 2010 |pmid=20688968 |pmc=2918337 |doi=10.1182/blood-2010-05-285627}}</ref><ref>{{cite journal |vauthors=Ahmad SS, Rawala-Sheikh R, Walsh PN |title=Components and assembly of the factor X activating complex |journal=Seminars in Thrombosis and Hemostasis |volume=18 |issue=3 |pages=311–323 |date=1992 |pmid=1455249 |doi=10.1055/s-2007-1002570|s2cid=28765989 }}</ref> Human platelets do not express [[tissue factor]].<ref name=Bouchard10/> Rat platelets do express tissue factor protein and carry both tissue factor pre-mRNA and mature mRNA.<ref>{{cite journal |vauthors=Tyagi T, Ahmad S, Gupta N, Sahu A, Ahmad Y, Nair V, Chatterjee T, Bajaj N, Sengupta S, Ganju L, Singh SB, Ashraf MZ |title=Altered expression of platelet proteins and calpain activity mediate hypoxia-induced prothrombotic phenotype |journal=Blood |volume=123 |issue=8 |pages=1250–60 |date=February 2014 |pmid=24297866 |doi=10.1182/blood-2013-05-501924 |doi-access=free}}</ref>

===Aggregation===<!--Platelet aggregation redirects here-->
[[File:Platelets.jpg|thumb|Platelet clumps in a blood smear]]
Platelet aggregation begins minutes after activation, and occurs as a result of turning on the [[GPIIb/IIIa]] receptor, allowing these receptors to bind with [[von Willebrand factor|vWF]] or [[fibrinogen]].<ref name="pmid16036569"/> Each platelet has around 60,000 of these receptors.<ref>{{cite journal |vauthors=O'Halloran AM, Curtin R, O'Connor F, Dooley M, Fitzgerald A, O'Brien JK, Fitzgerald DJ, Shields DC |title=The impact of genetic variation in the region of the GPIIIa gene, on Pl expression bias and GPIIb/IIIa receptor density in platelets |journal=British Journal of Haematology |volume=132 |issue=4 |pages=494–502 |date=February 2006 |pmid=16412022 |doi=10.1111/j.1365-2141.2005.05897.x |s2cid=41983626}}</ref> When any one or more of at least nine different platelet surface receptors are turned on during activation, intraplatelet signaling pathways cause existing GpIIb/IIIa receptors to change shape — curled to straight — and thus become capable of binding.<ref name="pmid16036569"/>

Since fibrinogen is a rod-like protein with nodules on either end capable of binding GPIIb/IIIa, activated platelets with exposed GPIIb/IIIa can bind fibrinogen to aggregate. GPIIb/IIIa may also further anchor the platelets to subendothelial vWF for additional structural stabilisation.

Classically it was thought that this was the only mechanism involved in aggregation, but three other mechanisms have been identified which can initiate aggregation, depending on the velocity of blood flow (i.e. shear range).<ref>{{cite journal |vauthors=Coller BS, Cheresh DA, Asch E, Seligsohn U |title=Platelet vitronectin receptor expression differentiates Iraqi-Jewish from Arab patients with Glanzmann thrombasthenia in Israel |journal=Blood |volume=77 |issue=1 |pages=75–83 |date=January 1991 |pmid=1702031 |doi=10.1182/blood.V77.1.75.75 |doi-access=free}}</ref>

==Immune function==
Platelets have a central role in [[innate immunity]], initiating and participating in multiple inflammatory processes, directly binding and even destroying pathogens. Clinical data show that many patients with serious bacterial or viral infections have [[thrombocytopenia]], thus reducing their contribution to inflammation. Platelet-leukocyte aggregates (PLAs) found in circulation are typical in [[sepsis]] or [[inflammatory bowel disease]], showing the connection between thrombocytes and immune cells.<ref name=":1">{{cite journal |vauthors=Jenne CN, Urrutia R, Kubes P |title=Platelets: bridging hemostasis, inflammation, and immunity |journal=International Journal of Laboratory Hematology |volume=35 |issue=3 |pages=254–261 |date=June 2013 |pmid=23590652 |doi=10.1111/ijlh.12084 |doi-access=free}}</ref>

The platelet cell membrane has receptors for collagen. Following rupture of the blood vessel wall, platelets are exposed and adhere to the collagen in the surrounding tissue.

===Immunothrombosis===
As hemostasis is a basic function of thrombocytes in mammals, it also has its uses in possible infection confinement.<ref name=":0"/> In case of injury, platelets, together with the coagulation cascade, provide the first line of defense by forming a blood clot. Hemostasis and host defense were thus intertwined in evolution. For example, in the [[Atlantic horseshoe crab]] (estimated to be over 400&nbsp;million years old), the only blood cell type, the [[amebocyte]], facilitates both the hemostatic function and immune functions, including encapsulation, phagocytosis of [[pathogen]]s, and [[exocytosis]] of intracellular granules containing [[bactericide|bactericidal]] defense molecules. Blood clotting supports immune function by trapping the bacteria.<ref>{{citation |last=Levin |first=Jack |name-list-style=vanc |title=Platelets |chapter=The Evolution of Mammalian Platelets |date=2007 |pages=3–22 |publisher=Elsevier |isbn=978-0-12-369367-9 |doi=10.1016/B978-012369367-9/50763-1 }}</ref>

Thrombosis (blood coagulation in intact blood vessels) is usually viewed as a pathological immune response, leading to obturation of lumen of blood vessel and subsequent hypoxic tissue damage. In some cases, however, directed thrombosis (or ''immunothrombosis)'' can locally control the spread of an infection. The thrombosis is directed in concordance with platelets, [[neutrophil]]s and [[monocyte]]s. The process is initiated either by immune cells by activating their pattern recognition receptors (PRRs), or by platelet-bacterial binding. Platelets can bind to bacteria either directly through thrombocytic PRRs<ref name=":1"/> and bacterial surface proteins, or via plasma proteins that bind both to platelets and bacteria.<ref>{{cite journal |vauthors=Cox D, Kerrigan SW, Watson SP |title=Platelets and the innate immune system: mechanisms of bacterial-induced platelet activation |journal=Journal of Thrombosis and Haemostasis |volume=9 |issue=6 |pages=1097–1107 |date=June 2011 |pmid=21435167 |doi=10.1111/j.1538-7836.2011.04264.x |url=https://epubs.rcsi.ie/cgi/viewcontent.cgi?article=1041&context=mctart |doi-access=free}}</ref> Monocytes respond to bacterial [[pathogen-associated molecular pattern]]s (PAMPs), or [[damage-associated molecular pattern]]s (DAMPs) by activating the extrinsic pathway of coagulation. Neutrophils facilitate the blood coagulation by [[Neutrophil extracellular traps|NETosis]], while platelets facilitate neutrophils' NETosis. NETs bind tissue factor, binding the coagulation centers to the location of infection. They also activate the intrinsic coagulation pathway by providing a negatively charged surface for factor XII. Other neutrophil secretions, such as proteolytic enzymes which cleave coagulation inhibitors, also bolster the process.<ref name=":0"/>

In case of imbalance in the regulation of immunothrombosis, this process can become aberrant. Regulatory defects in immunothrombosis are suspected to be a major factor in pathological thrombosis in forms such as [[disseminated intravascular coagulation]] (DIC) or [[deep vein thrombosis]]. DIC in sepsis is a prime example of both the dysregulated coagulation process and an undue systemic inflammatory response. It results in a multitude of microthrombi. These are similar in composition to the thrombi produced in native immunothrombosis — they are made up of fibrin, platelets, neutrophils and NETs.<ref name=":0"/>

===Inflammation===
Platelets rapidly deploy to sites of injury or infection. There, they are thought to modulate inflammatory processes ''via'' interactions with [[leukocyte]]s and secretion of [[cytokine]]s, [[chemokine]]s, and other inflammatory mediators.<ref>{{cite journal |vauthors=Weyrich AS, Zimmerman GA |title=Platelets: signaling cells in the immune continuum |journal=Trends in Immunology |volume=25 |issue=9 |pages=489–495 |date=September 2004 |pmid=15324742 |doi=10.1016/j.it.2004.07.003}}</ref><ref name="pmid14500287">{{cite journal |vauthors=Wagner DD, Burger PC |title=Platelets in inflammation and thrombosis |journal=Arteriosclerosis, Thrombosis, and Vascular Biology |volume=23 |issue=12 |pages=2131–7 |date=December 2003 |pmid=14500287 |doi=10.1161/01.ATV.0000095974.95122.EC |doi-access=free}}</ref><ref name="pmid8662511">{{cite journal |vauthors=Diacovo TG, Puri KD, Warnock RA, Springer TA, von Andrian UH |title=Platelet-mediated lymphocyte delivery to high endothelial venules |journal=Science |volume=273 |issue=5272 |pages=252–5 |date=July 1996 |pmid=8662511 |doi=10.1126/science.273.5272.252 |bibcode=1996Sci...273..252D |s2cid=21334521}}</ref><ref name="pmid16258538">{{cite journal |vauthors=Iannacone M, Sitia G, Isogawa M, Marchese P, Castro MG, Lowenstein PR, Chisari FV, Ruggeri ZM, Guidotti LG |title=Platelets mediate cytotoxic T lymphocyte-induced liver damage |journal=Nature Medicine |volume=11 |issue=11 |pages=1167–9 |date=November 2005 |pmid=16258538 |pmc=2908083 |doi=10.1038/nm1317}}</ref><ref>{{cite journal |last1=Oehlers |first1=Stefan H. |last2=Tobin |first2=David M. |last3=Britton |first3=Warwick J. |last4=Shavit |first4=Jordan A. |last5=Nguyen |first5=Tuong |last6=Johansen |first6=Matt D. |last7=Johnson |first7=Khelsey E. |last8=Hortle |first8=Elinor |title=Thrombocyte inhibition restores protective immunity to mycobacterial infection in zebrafish |url= |journal=The Journal of Infectious Diseases |volume=220 |issue=3 |pages=524–534 |language=en |doi=10.1093/infdis/jiz110 |pmid=30877311 |pmc=6603966 |year=2019}}</ref> Platelets also secrete [[platelet-derived growth factor]] (PDGF).

Platelets modulate neutrophils by forming platelet-leukocyte aggregates (PLAs). These formations induce upregulated production of the [[complement receptor]] αmβ2 ([[integrin alpha M|Mac-1]]) integrin in neutrophils. Interaction with PLAs also induces degranulation and increased phagocytosis in neutrophils.

Platelets are the largest source of soluble [[CD40L|CD40L (CD154)]] which induces production of [[reactive oxygen species]] (ROS) and upregulates expression of adhesion molecules (such as [[E-selectin]], [[ICAM-1]], and [[VCAM-1]]) in neutrophils. CD40L also activates macrophages and activates cytotoxic response in [[T lymphocyte|T]] and [[B lymphocytes]].<ref name=":1" />

Mammalian platelets [[Enucleation (microbiology)|lacking nucleus]] are able to conduct autonomous locomotion.<ref>{{cite journal |vauthors=Gaertner F, Ahmad Z, Rosenberger G, Fan S, Nicolai L, Busch B, Yavuz G, Luckner M, Ishikawa-Ankerhold H, Hennel R, Benechet A, Lorenz M, Chandraratne S, Schubert I, Helmer S, Striednig B, Stark K, Janko M, Böttcher RT, Verschoor A, Leon C, Gachet C, Gudermann T, Mederos Y, Schnitzler M, Pincus Z, Iannacone M, Haas R, Wanner G, Lauber K, Sixt M, Massberg S |title=Migrating Platelets Are Mechano-scavengers that Collect and Bundle Bacteria |journal=Cell |volume=171 |issue=6 |pages=1368–82 |date=November 2017 |pmid=29195076 |doi=10.1016/j.cell.2017.11.001 |doi-access=free}}</ref> Platelets are active scavengers, scaling walls of blood vessels and reorganising the thrombus. They are able to recognize and adhere to many surfaces, including bacteria, and can envelop them in their open canalicular system (OCP), leading to a proposal to name the process as ''covercytosis'' (OCS) rather than phagocytosis, as OCS is merely an invagination of outer plasma membrane. These platelet-bacteria bundles provide an interaction platform for neutrophils that destroy bacteria using [[Neutrophil extracellular traps|NETs]] and phagocytosis.

Platelets also participate in chronic inflammatory disease, such as [[synovitis]] or [[rheumatoid arthritis]].<ref>{{cite journal |vauthors=Boilard E, Nigrovic PA, Larabee K, Watts GF, Coblyn JS, Weinblatt ME, Massarotti EM, Remold-O'Donnell E, Farndale RW, Ware J, Lee DM |title=Platelets amplify inflammation in arthritis via collagen-dependent microparticle production |journal=Science |volume=327 |issue=5965 |pages=580–3 |date=January 2010 |pmid=20110505 |pmc=2927861 |doi=10.1126/science.1181928 |bibcode=2010Sci...327..580B}}</ref> Platelets are activated by collagen receptor [[GPVI|glycoprotein IV]] (GPVI). Proinflammatory platelet microvesicles trigger constant cytokine secretion from neighboring [[fibroblast-like synoviocyte]]s, most prominently [[interleukin 6|Il-6]] and [[interleukin 8|Il-8]]. Inflammatory damage to the surrounding extracellular matrix continuously reveals more collagen, binding receptors on platelets and maintaining microvesicle production.

===Adaptive immunity===
Activated platelets are able to participate in [[adaptive immunity]], interacting with [[antibody|antibodies]]. They are able to specifically bind [[immunoglobulin G|IgG]] through [[FcγRIIA]], a receptor for IgG's constant fragment (Fc). When activated and bound to IgG-[[opsonin|opsonised]] bacteria, platelets release reactive oxygen species (ROS), antimicrobial peptides, [[defensins]], kinocidins and [[proteases]], killing the bacteria directly.<ref name=":2">{{cite journal |vauthors=Palankar R, Kohler TP, Krauel K, Wesche J, Hammerschmidt S, Greinacher A |title=Platelets kill bacteria by bridging innate and adaptive immunity via platelet factor 4 and FcγRIIA |journal=Journal of Thrombosis and Haemostasis |volume=16 |issue=6 |pages=1187–97 |date=June 2018 |pmid=29350833 |doi=10.1111/jth.13955 |doi-access=free}}</ref> Platelets also secrete proinflammatory and procoagulant mediators such as inorganic [[polyphosphates]] or [[platelet factor 4]] (PF4), connecting innate and adaptive immune responses.<ref name=":2"/><ref>{{cite journal |vauthors=McMorran BJ, Wieczorski L, Drysdale KE, Chan JA, Huang HM, Smith C, Mitiku C, Beeson JG, Burgio G, Foote SJ |title=Platelet factor 4 and Duffy antigen required for platelet killing of Plasmodium falciparum |journal=Science |volume=338 |issue=6112 |pages=1348–51 |date=December 2012 |pmid=23224555 |doi=10.1126/science.1228892 |bibcode=2012Sci...338.1348M |s2cid=206544569 |url=https://zenodo.org/record/261563}}</ref>

==Measurement and testing==

===Measurement===
Platelet concentration in the blood (i.e. platelet count), can be measured manually using a [[hemocytometer]], or by placing blood in an automated platelet analyzer using particle counting, such as a [[Coulter counter]] or optical methods.<ref name="Stiff">{{cite book |last=Stiff |first=Patrick J. |url=https://www.ncbi.nlm.nih.gov/books/NBK262/ |title=Clinical Methods: The History, Physical, and Laboratory Examinations |date=1990 |publisher=Butterworths |isbn=978-0-409-90077-4 |editor-last=Walker |editor-first=H. Kenneth |edition=3rd |location=Boston |pmid=21250105 |editor-last2=Hall |editor-first2=W. Dallas |editor-last3=Hurst |editor-first3=J. Willis}}</ref> Most common [[complete blood count|blood testing methods]] include platelet count in their measurements, usually reported as [[complete blood count#Reference ranges|PLT]].<ref name="NHLBI">{{cite web |publisher=National Heart, Lung, and Blood Institute (NHLBI) |title=Platelet Disorders: Thrombocytopenia |date=24 March 2022 |url=https://www.nhlbi.nih.gov/health/thrombocytopenia |access-date=2022-11-18}}</ref>

Platelet concentrations vary between individuals and over time, with the population average between 250,000 and 260,000 cells per mm<sup>3</sup> (equivalent to per microliter), but the typical laboratory accepted normal range is between 150,000 and 400,000 cells per mm<sup>3</sup> or 150–400 billion per liter.<ref name="NHLBI"/><ref name="Stiff"/>

[[File:First and second wave of platelet aggregation.png|thumb|On for example optical densitometry, a first and second wave of platelet aggregation is seen, in this case for an [[adenosine diphosphate|ADP]]-initiated aggregation. ]]

On a stained [[blood smear]], platelets appear as dark purple spots, about 20% of the diameter of red blood cells. The smear reveals size, shape, qualitative number, and [[clumping (biology)|clumping]]. A healthy adult typically has 10 to 20 times more red blood cells than platelets.

===Bleeding time===
[[Bleeding time]] was developed as a test of platelet function by Duke in 1910.<ref name="Lind-485">{{cite book |last1=Lind |first1=Stuart E. |last2=Kurkjian |first2=Carla D. |editor1-last=Michelson |editor1-first=Alan D. |title=Platelets |date=2011 |publisher=Elsevier |isbn=978-0-08-046586-9 |oclc=162572838 |page=485 |edition=2nd |chapter-url=https://books.google.com/books?id=GnIQGmiSylkC&pg=PA485 |chapter=The bleeding time}}</ref> Duke's test measured the time taken for bleeding to stop from a standardized wound in the ear lobe that was blotted every 30 seconds, considering less than 3 minutes as normal.<ref>{{cite journal |vauthors=Duke WW |title=The relation of blood platelets to hemorrhagic disease |journal=JAMA |date=1910 |volume=55 |issue=14 |pages=1185–92 |doi=10.1001/jama.1910.04330140029009 |url=https://zenodo.org/record/1447283}}</ref> Bleeding time has low sensitivity and specificity for mild to moderate platelet disorders and is no longer recommended for screening.<ref>Mehic D, Assinger A, Gebhart J. Utility of Global Hemostatic Assays in Patients with Bleeding Disorders of Unknown Cause. Hamostaseologie. 2024 Jul 1. doi: 10.1055/a-2330-9112. Epub ahead of print. PMID 38950624.</ref>

===Multiple electrode aggregometry===
{{Main|Multiple electrode aggregometry}}
In [[multiple electrode aggregometry]], anticoagulated whole blood is mixed with saline and a platelet agonist in a single-use cuvette with two pairs of electrodes. The increase in impedance between the electrodes as platelets aggregate onto them, is measured and visualized as a curve.<ref>{{cite book |first1=Marco |last1=Ranucci |first2=Paolo |last2=Simioni |title=Point-of-Care Tests for Severe Hemorrhage: A Manual for Diagnosis and Treatment |url=https://books.google.com/books?id=7klECwAAQBAJ&pg=PA40 |year=2016 |publisher=Springer |isbn=978-3-319-24795-3 |pages=40–42}}</ref><ref>{{cite book |first1=Carlo |last1=Marcucci |first2=Patrick |last2=Schoettker |title=Perioperative Hemostasis: Coagulation for Anesthesiologists |url=https://books.google.com/books?id=e7WNBAAAQBAJ&pg=PA54 |year= 2014 |publisher=Springer |isbn=978-3-642-55004-1 |pages=54–56}}</ref>

{{Anchor|LTA}}

===Light transmission aggregometry===
In light transmission aggregometry (LTA), [[platelet-rich plasma]] is placed between a light source and a [[photocell]]. Unaggregated plasma allows relatively little light to pass through. After adding an agonist, the platelets aggregate, increasing light transmission, which is detected by a photocell.<ref name="Cuker2014">{{cite journal |last1=Cuker |first1=Adam |title=Light Transmission Aggregometry |journal=The Hematologist |volume=11 |issue=2 |year=2014 |issn=1551-8779 |doi=10.1182/hem.V11.2.2555}}</ref>

===Whole blood impedance aggregometry===
Whole blood impedance aggregometry (WBA) measures the change in electrical impedance between two electrodes when platelet aggregation is induced by an agonist. Whole blood lumiaggregometry may increase the test sensitivity to impairment of platelet granule secretion.<ref>{{cite journal |vauthors=McGlasson DL, Fritsma GA |title=Whole blood platelet aggregometry and platelet function testing |journal=Semin Thromb Hemost |volume=35 |issue=2 |pages=168–180 |date=March 2009 |pmid=19408190 |doi=10.1055/s-0029-1220325 }}</ref>

===PFA-100===
The [[PFA-100]] (Platelet Function Assay — 100) is a system for analysing platelet function in which citrated whole blood is aspirated through a disposable cartridge containing an aperture within a membrane coated with either collagen and epinephrine or collagen and ADP. These agonists induce platelet adhesion, activation and aggregation, leading to rapid occlusion of the aperture and cessation of blood flow termed the closure time (CT). An elevated CT with EPI and collagen can indicate intrinsic defects such as [[von Willebrand disease]], [[uremia]], or circulating platelet inhibitors. A follow-up test involving collagen and ADP is used to indicate if the abnormal CT with collagen and EPI was caused by the effects of acetyl sulfosalicylic acid (aspirin) or medications containing inhibitors.<ref>{{cite web |url=http://www.pathology.vcu.edu/media/pathology/clinical/coag/PFA-100FAQ.pdf |title=Platelet Function Assay FAQ |website=Department of Pathology |publisher=Virginia Commonwealth University |access-date=2017-03-27}}</ref> The PFA-100 is highly sensitive to von Willebrand disease, but is only moderately sensitive to defects in platelet function.<ref>{{cite journal |vauthors=Favaloro EJ, Pasalic L, Lippi G |title=Towards 50 years of platelet function analyser (PFA) testing |journal=Clin Chem Lab Med |volume=61 |issue=5 |pages=851–860 |date=April 2023 |pmid=35859143 |doi=10.1515/cclm-2022-0666 }}</ref>

==Clinical significance==
Spontaneous and excessive bleeding can occur because of platelet disorders. This bleeding can be caused by deficient numbers of platelets, dysfunctional platelets, or platelet densities over 1&nbsp;million/microliter. (The excessive numbers create a relative von Willebrand factor deficiency due to sequestration.)<ref>{{cite journal |vauthors=Murakawa M, Okamura T, Tsutsumi K, Tanoguchi S, Kamura T, Shibuya T, Harada M, Niho Y |date=1992 |title=Acquired von Willebrand's disease in association with essential thrombocythemia: regression following treatment |journal=Acta Haematologica |volume=87 |issue=1–2 |pages=83–87 |doi=10.1159/000204725 |pmid=1585777}}</ref><ref>{{cite journal |vauthors=van Genderen PJ, Leenknegt H, Michiels JJ, Budde U |date=September 1996 |title=Acquired von Willebrand disease in myeloproliferative disorders |journal=Leukemia & Lymphoma |volume=22 |issue=Suppl 1 |pages=79–82 |doi=10.3109/10428199609074364 |pmid=8951776}}</ref>

Bleeding due to a platelet disorder or a coagulation factor disorder can be distinguished by the characteristics and location of the bleeding.<ref name="Michelson" />{{rp|815, Table 39-4}} Platelet bleeding involves bleeding from a cut that is prompt and excessive, but can be controlled by pressure; spontaneous bleeding into the skin which causes a purplish stain named by its size: [[petechiae]], [[purpura]], [[ecchymoses]]; bleeding into mucous membranes causing bleeding gums, nose bleed, and gastrointestinal bleeding; menorrhagia; and intraretinal and intracranial bleeding.

Excessive numbers of platelets, and/or normal platelets responding to abnormal vessel walls, can result in [[venous thrombosis]] and [[arterial thrombosis]]. The symptoms depend on the thrombosis site.

==Disorders==
Platelet disorders can occur because there are not enough platelets, too many platelets, or the platelets do not function properly.<ref name="Michelson" />{{rp|vii}}

Low platelet concentration is called [[thrombocytopenia]], and is due to either decreased production, increased destruction of platelets, or platelets being sequestered in another part of the body. Elevated platelet concentration is called [[thrombocytosis]], and is either [[congenital]], reactive (to [[cytokine]]s), or due to unregulated production: one of the [[myeloproliferative neoplasm]]s or certain other myeloid [[neoplasm]]s.

Normal platelets can respond to an abnormality on the vessel wall rather than to hemorrhage, resulting in inappropriate platelet adhesion/activation and [[thrombosis]]: the formation of a clot within an intact vessel. This type of thrombosis arises by mechanisms different from those of a normal clot: extending the fibrin of [[venous thrombosis]]; extending an unstable or ruptured arterial plaque, causing [[arterial thrombosis]]; and microcirculatory thrombosis. An arterial [[thrombus]] may partially obstruct blood flow, causing downstream [[ischemia]], or may completely obstruct it, causing downstream [[infarction|tissue death]].:<ref name="Michelson" />{{rp|vii}}

{{Comprehensive table of platelet aggregation disorders and agonists}}
===Thrombocytopenia===
* [[Immune thrombocytopenia]] (ITP) — formerly known as immune thrombocytopenic purpura and idiopathic thrombocytopenic purpura
* [[Splenomegaly]]
** [[Gaucher's disease]]
* Familial thrombocytopenia<ref>{{cite journal |last1=Warren |first1=JT |last2=Di Paola |first2=J |title=Genetics of inherited thrombocytopenias. |journal=Blood |date=2 June 2022 |volume=139 |issue=22 |pages=3264–77 |doi=10.1182/blood.2020009300 |pmid=35167650 |pmc=9164741}}</ref><ref>{{cite journal |last1=Pecci |first1=A |last2=Balduini |first2=CL |title=Inherited thrombocytopenias: an updated guide for clinicians. |journal=Blood Reviews |date=July 2021 |volume=48 |article-number=100784 |doi=10.1016/j.blre.2020.100784 |pmid=33317862 |s2cid=229178137}}</ref>
* [[Chemotherapy]]
* [[Babesiosis]]
* [[Dengue fever]]
* [[Onyalai]]
* [[Thrombotic thrombocytopenic purpura]]
* [[HELLP syndrome]]
* [[Hemolytic–uremic syndrome]]
* [[Drug-induced thrombocytopenic purpura]] (five known drugs — most problematic is [[heparin-induced thrombocytopenia]] (HIT)
* Pregnancy-associated
* Neonatal alloimmune associated
* [[Aplastic anemia]]
* Transfusion-associated
* [[Pseudothrombocytopenia]]
* [[Vaccine-induced immune thrombotic thrombocytopenia]] (VITT)

===Altered platelet function===
* Congenital
** Disorders of adhesion
*** [[Bernard–Soulier syndrome]]
** Disorders of activation
*** Disorders of granule amount or release
*** [[Hermansky–Pudlak syndrome]]
*** [[Gray platelet syndrome]]
*** ADP receptor defect
*** Decreased cyclooxygenase activity
*** [[Platelet storage pool deficiency]]
** Disorders of aggregation
*** [[Glanzmann's thrombasthenia]]
*** [[Wiskott–Aldrich syndrome]]
** Disorders of coagulant activity
*** [[COAT platelet defect]]
*** [[Scott syndrome]]
* Acquired
** Disorders of adhesion
*** [[Paroxysmal nocturnal hemoglobinuria]]
*** [[Asthma]]<ref name="pmid17654302">{{cite journal |vauthors=Kornerup KN, Page CP |title=The role of platelets in the pathophysiology of asthma |journal=Platelets |volume=18 |issue=5 |pages=319–328 |date=August 2007 |pmid=17654302 |doi=10.1080/09537100701230436 |s2cid=7923694}}</ref>
*** [[Aspirin-exacerbated respiratory disease]] (AERD/Samter's triad)<ref name="pmid22262771">{{cite journal |vauthors=Laidlaw TM, Kidder MS, Bhattacharyya N, Xing W, Shen S, Milne GL, Castells MC, Chhay H, Boyce JA |title=Cysteinyl leukotriene overproduction in aspirin-exacerbated respiratory disease is driven by platelet-adherent leukocytes |journal=Blood |volume=119 |issue=16 |pages=3790–8 |date=April 2012 |pmid=22262771 |pmc=3335383 |doi=10.1182/blood-2011-10-384826}}</ref>
*** [[Cancer]]<ref>{{cite journal |vauthors=Erpenbeck L, Schön MP |title=Deadly allies: the fatal interplay between platelets and metastasizing cancer cells |journal=Blood |volume=115 |issue=17 |pages=3427–36 |date=April 2010 |pmid=20194899 |pmc=2867258 |doi=10.1182/blood-2009-10-247296}}</ref>
*** [[Malaria]]<ref>{{cite journal |vauthors=Pleass RJ |title=Platelet power: sticky problems for sticky parasites? |journal=Trends in Parasitology |volume=25 |issue=7 |pages=296–9 |date=July 2009 |pmid=19539528 |pmc=3116138 |doi=10.1016/j.pt.2009.04.002}}</ref>
*** Decreased cyclooxygenase activity

===Thrombocytosis and thrombocythemia===
* Reactive
** Chronic infection
** Chronic inflammation
** Malignancy
** Hyposplenism (post-splenectomy)
** Iron deficiency
** Acute blood loss
* [[Myeloproliferative neoplasm]]s — platelets are both elevated and activated
** [[Essential thrombocythemia]]
** [[Polycythemia vera]]
* Associated with other myeloid neoplasms
* Congenital

==Pharmacology==

===Anti-inflammatory drugs===
Some drugs used to treat inflammation have the unwanted side effect of suppressing normal platelet function. These are the [[non-steroidal anti-inflammatory drugs]] (NSAIDS). [[Aspirin]] irreversibly disrupts platelet function by inhibiting [[cyclooxygenase]]-1 (COX1), and hence normal hemostasis. The resulting platelets are unable to produce new cyclooxygenase because they have no DNA. Normal platelet function does not return until the use of aspirin has ceased and enough of the affected platelets have been replaced by new ones, which can take over a week. [[Ibuprofen]], another [[NSAID]], does not have such a long duration effect, with platelet function usually returning within 24 hours,<ref>{{cite journal |date=April 2005 |title=Summaries for patients. Platelet function after taking Ibuprofen for 1 week |journal=Annals of Internal Medicine |volume=142 |issue=7 |pages=I–54 |doi=10.7326/0003-4819-142-7-200504050-00004 |pmid=15809457 |doi-access=free}}<!-- |access-date=2008-08-26 --></ref> and taking ibuprofen before aspirin prevents the irreversible effects of aspirin.<ref name="pmid6411052">{{cite journal |vauthors=Rao GH, Johnson GG, Reddy KR, White JG |title=Ibuprofen protects platelet cyclooxygenase from irreversible inhibition by aspirin |journal=Arteriosclerosis |volume=3 |issue=4 |pages=383–8 |date=1983 |pmid=6411052 |doi=10.1161/01.ATV.3.4.383 |s2cid=3229482 |doi-access=free}}</ref>

===Drugs that suppress platelet function===
These drugs are used to prevent thrombus formation.

====Oral agents====
* [[Aspirin]]
* [[Cilostazol]]
* [[Clopidogrel]]
* [[Prasugrel]]
* [[Ticagrelor]]
* [[Ticlopidine]]
====Intravenous agents====
* [[Abciximab]]
* [[Eptifibatide]]
* [[Tirofiban]]
* [[Argatroban]]

===Drugs that stimulate platelet production===
* [[Desmopressin]]
* [[Factor VIIa]]
* [[Thrombopoietin mimetics]]
* [[Oprelvekin]]
* [[Romiplostim]]
* [[Eltrombopag]]

==Therapies==

===Transfusion===
{{Main|Platelet transfusion}}

====Indications====
[[Platelet transfusion]] is most frequently used to correct unusually low platelet counts, either to prevent spontaneous bleeding (typically at counts below 10 billion/L) or in anticipation of medical procedures that necessarily involve some bleeding. For example, in patients undergoing [[surgery]], a level below 50 billion/L is associated with abnormal surgical bleeding, and [[regional anaesthetic]] procedures such as [[epidural]]s are avoided for levels below 80 billion/L.<ref name="pmid19775301">{{cite journal |vauthors=van Veen JJ, Nokes TJ, Makris M |title=The risk of spinal haematoma following neuraxial anaesthesia or lumbar puncture in thrombocytopenic individuals |journal=British Journal of Haematology |volume=148 |issue=1 |pages=15–25 |date=January 2010 |pmid=19775301 |doi=10.1111/j.1365-2141.2009.07899.x |doi-access=free}}</ref> Platelets may also be transfused when the platelet count is normal but the platelets are dysfunctional, such as when an individual is taking aspirin or [[clopidogrel]].<ref>{{cite book |veditors=Roback J, Grossman B, Harris T, Hillyer C |title=Technical Manual |edition=17th |date=2011 |publisher=AABB |location=Bethesda MD |isbn=978-1-56395-315-6 |oclc=756764486 |author=American Association of Blood Banks |page=580}}</ref> Finally, platelets may be transfused as part of a [[massive transfusion protocol]], in which the three major blood components (red blood cells, plasma, and platelets) are transfused to address severe [[hemorrhage]]. Platelet transfusion is contraindicated in [[thrombotic thrombocytopenic purpura]] (TTP), as it fuels the [[coagulopathy]].  Platelet transfusion is generally ineffective, and thus contraindicated, for prophylaxis in [[immune thrombocytopenia]] (ITP), because the transfused platelets are immediately cleared; however, it is indicated to treat bleeding.<ref>{{cite journal |vauthors=Provan D, Arnold DM, Bussel JB, Chong BH, Cooper N, Gernsheimer T, Ghanima W, Godeau B, González-López TJ, Grainger J, Hou M, Kruse C, McDonald V, Michel M, Newland AC, Pavord S, Rodeghiero F, Scully M, Tomiyama Y, Wong RS, Zaja F, Kuter DJ |title=Updated international consensus report on the investigation and management of primary immune thrombocytopenia |journal=Blood Adv |volume=3 |issue=22 |pages=3780–3817 |date=November 2019 |pmid=31770441 |pmc=6880896 |doi=10.1182/bloodadvances.2019000812 }}</ref>

====Collection====
[[File:Platelet blood bag.jpg|thumb|Platelet concentrate]]
Platelets are either isolated from collected units of whole blood and pooled to make a therapeutic dose, or collected by [[Plateletpheresis|platelet apheresis]]: blood is taken from the donor, passed through a device which removes the platelets, and the remainder is returned to the donor in a closed loop. The industry standard is for platelets to be tested for bacteria before transfusion to avoid septic reactions, which can be fatal. Recently the AABB Industry Standards for [[blood bank|Blood Banks]] and Transfusion Services (5.1.5.1) has allowed use of pathogen reduction technology as an alternative to bacterial screenings in platelets.<ref>{{cite book |author=American Association of Blood Banks |author-link=AABB |chapter=5.1.5.1 |title=Standards for Blood Banks and Transfusion Services |publisher=AABB |location=Bethesda MD |date=2003 |edition=22nd |isbn=978-1-56395-173-2 |oclc=53010679 }}</ref>

Pooled whole-blood platelets, sometimes called "random" platelets, are separated by one of two methods.<ref name="pmid1731433">{{cite journal |vauthors=Högman CF |title=New trends in the preparation and storage of platelets |journal=Transfusion |volume=32 |issue=1 |pages=3–6 |date=January 1992 |pmid=1731433 |doi=10.1046/j.1537-2995.1992.32192116428.x |doi-access=free}}</ref> In the US, a unit of whole blood is placed into a large [[centrifuge]] in what is referred to as a "soft spin". At these settings, the platelets remain suspended in the plasma. The [[platelet-rich plasma]] (PRP) is removed from the red cells, then centrifuged at a faster setting to harvest the platelets from the plasma. In other regions of the world, the unit of whole blood is centrifuged using settings that cause the platelets to become suspended in the "[[buffy coat]]" layer, which includes the platelets and the white blood cells. The "buffy coat" is isolated in a sterile bag, suspended in a small amount of red blood cells and plasma, then centrifuged again to separate the platelets and plasma from the red and white blood cells. Regardless of the initial method of preparation, multiple donations may be combined into one container using a sterile connection device to manufacture a single product with the desired therapeutic dose.

Apheresis platelets are collected using a mechanical device that draws blood from the donor and centrifuges the collected blood to separate out the platelets and other components to be collected. The remaining blood is returned to the donor. The advantage to this method is that a single donation provides at least one therapeutic dose, as opposed to the multiple donations for whole-blood platelets. This means that a recipient is exposed to fewer donors and has less risk of transfusion-transmitted disease and other complications. Sometimes a person such as a [[cancer]] patient who requires routine transfusions of platelets receives repeated donations from a specific donor to minimize risk. Pathogen reduction of platelets using for example, [[pathogen reduction using riboflavin and UV light|riboflavin and UV light treatments]] can reduce the infectious load of pathogens contained in donated blood products.<ref name="pmid15157255">{{cite journal |vauthors=Ruane PH, Edrich R, Gampp D, Keil SD, Leonard RL, Goodrich RP |title=Photochemical inactivation of selected viruses and bacteria in platelet concentrates using riboflavin and light |journal=Transfusion |volume=44 |issue=6 |pages=877–885 |date=June 2004 |pmid=15157255 |doi=10.1111/j.1537-2995.2004.03355.x |s2cid=24109912}}</ref><ref name="pmid15934989">{{cite journal |vauthors=Perez-Pujol S, Tonda R, Lozano M, Fuste B, Lopez-Vilchez I, Galan AM, Li J, Goodrich R, Escolar G |title=Effects of a new pathogen-reduction technology (Mirasol PRT) on functional aspects of platelet concentrates |journal=Transfusion |volume=45 |issue=6 |pages=911–9 |date=June 2005 |pmid=15934989 |doi=10.1111/j.1537-2995.2005.04350.x |s2cid=23169569}}</ref> Another photochemical treatment process utilizing amotosalen and UVA light has been developed for the inactivation of viruses, bacteria, parasites, and leukocytes.<ref>{{cite journal |vauthors=Prowse CV |title=Component pathogen inactivation: a critical review |journal=Vox Sanguinis |volume=104 |issue=3 |pages=183–199 |date=April 2013 |pmid=23134556 |doi=10.1111/j.1423-0410.2012.01662.x |s2cid=38392712}}</ref> In addition, apheresis platelets tend to contain fewer contaminating red blood cells because the collection method is more efficient than "soft spin" centrifugation.

====Storage====
Platelets collected by either method have a typical shelf life of five days. This results in supply shortages, as testing donations often requires up to a full day. No effective preservative solutions have been devised for platelets.

Platelets are stored under constant agitation at {{convert|20|–|24|C|F}}. Units cannot be refrigerated as this causes platelets to change shape and lose function. Storage at room temperature provides an environment where any introduced bacteria may proliferate and subsequently cause [[bacteremia]]. The United States requires products to be tested for the presence of bacterial contamination before transfusion.<ref>{{cite book |author=AABB |title=Standards for Blood Banks and Transfusion Services |publisher=AABB |location=Bethesda MD |date=2009 |edition=26th |isbn=978-1-56395-289-0 |oclc=630715051}}</ref>
[[File:Platelets collected by using apheresis.jpg|thumb|right|Platelets collected by using [[apheresis]] at an [[American Red Cross]] donation center]]

====Delivery====
Platelets do not need to belong to the same A-B-O blood group as the recipient or be cross-matched to ensure immune compatibility between donor and recipient unless they contain a significant amount of red blood cells (RBCs). The presence of RBCs imparts a reddish-orange color to the product and is usually associated with whole-blood platelets. Some sites may type platelets, but this is not critical.

Prior to issuing platelets to the recipient, they may be irradiated to prevent [[transfusion-associated graft versus host disease]] or they may be washed to remove the plasma.

The change in the recipient's platelet count after transfusion is termed the "increment" and is calculated by subtracting the pre-transfusion platelet count from the post-transfusion count. Many factors affect the increment including body size, the number of platelets transfused, and clinical features that may cause premature destruction of the transfused platelets. When recipients fail to demonstrate an adequate post-transfusion increment, this is termed [[platelet transfusion refractoriness]].

Platelets, either apheresis-derived or random-donor, can be processed through a volume reduction process. In this process, the platelets are spun in a centrifuge and plasma is removed, leaving 10 to 100 mL of platelet concentrate. Such volume-reduced platelets are normally transfused only to neonatal and pediatric patients when a large volume of plasma could overload the child's small circulatory system. The lower volume of plasma also reduces the chances of an adverse transfusion reaction to plasma proteins.<ref name="pmid16537051">{{cite journal |vauthors=Schoenfeld H, Spies C, Jakob C |title=Volume-reduced platelet concentrates |journal=Current Hematology Reports |volume=5 |issue=1 |pages=82–88 |date=March 2006 |pmid=16537051}}</ref> Volume reduced platelets have a shelf life of four hours.<ref>[http://www.cbbsweb.org/enf/2001/pltwashvol.html CBBS: Washed and volume-reduced Plateletpheresis units] {{Webarchive|url=https://web.archive.org/web/20140414063023/http://www.cbbsweb.org/enf/2001/pltwashvol.html |date=2014-04-14}}. Cbbsweb.org (2001-10-25). Retrieved on 2011-11-14.</ref>

===Wound repair===

{{Main|Wound repair}}
The blood clot is only a temporary solution to stop bleeding; tissue repair is needed. Small interruptions in the endothelium are handled by physiological mechanisms; large interruptions by a trauma surgeon.<ref>{{cite book |last1=Nguyen |first1=D.T. |last2=Orgill |first2=D.P. |last3=Murphy |first3=G.F. |chapter=4. The Pathophysiologic Basis for Wound Healing and Cutaneous Regeneration |chapter-url=https://www.sciencedirect.com/science/article/pii/B9781845693633500043 |doi=10.1533/9781845695545.1.25 |oclc=844452405 |title=Biomaterials For Treating Skin Loss |publisher=CRC Press |date=2009 |isbn=978-1-4200-9989-8 |pages=25–57 |url=}}</ref> The fibrin is slowly dissolved by the fibrinolytic enzyme, [[plasmin]], and the platelets are cleared by [[phagocytosis]].<ref name="pmid4957257">{{cite journal |author4-link=James Fraser Mustard |vauthors=Movat HZ, Weiser WJ, Glynn MF, Mustard JF |title=Platelet phagocytosis and aggregation |journal=The Journal of Cell Biology |volume=27 |issue=3 |pages=531–543 |date=December 1965 |pmid=4957257 |pmc=2106759 |doi=10.1083/jcb.27.3.531}}</ref>

Platelets release [[platelet-derived growth factor]] (PDGF), a potent [[chemotaxis|chemotactic]] agent; and [[TGF beta]], which stimulates the deposition of [[extracellular matrix]]; [[fibroblast growth factor]], [[insulin-like growth factor 1]], platelet-derived [[epidermal growth factor]], and [[vascular endothelial growth factor]]. Local application of these factors in increased concentrations through [[platelet-rich plasma]] (PRP) is used as an adjunct in wound healing.<ref>{{cite journal |vauthors=Gawaz M, Vogel S |title=Platelets in tissue repair: control of apoptosis and interactions with regenerative cells |journal=Blood |volume=122 |issue=15 |pages=2550–4 |date=October 2013 |pmid=23963043 |doi=10.1182/blood-2013-05-468694 |doi-access=free}}</ref>

==Non-mammals==
Instead of platelets, non-mammalian vertebrates have nucleated thrombocytes, which resemble [[B lymphocyte]]s in morphology. They aggregate in response to thrombin, but not to ADP, serotonin, nor adrenaline, as platelets do.<ref>{{cite journal |vauthors=Schmaier AA, Stalker TJ, Runge JJ, Lee D, Nagaswami C, Mericko P, Chen M, Cliché S, Gariépy C, Brass LF, Hammer DA, Weisel JW, Rosenthal K, Kahn ML |title=Occlusive thrombi arise in mammals but not birds in response to arterial injury: evolutionary insight into human cardiovascular disease |journal=Blood |volume=118 |issue=13 |pages=3661–9 |date=September 2011 |pmid=21816834 |pmc=3186337 |doi=10.1182/blood-2011-02-338244}}</ref><ref>{{cite journal |vauthors=Belamarich FA, Shepro D, Kien M |title=ADP is not involved in thrombin-induced aggregation of thrombocytes of a non-mammalian vertebrate |journal=Nature |volume=220 |issue=5166 |pages=509–510 |date=November 1968 |pmid=5686175 |doi=10.1038/220509a0 |bibcode=1968Natur.220..509B |s2cid=4269208}}</ref>

==History==
* [[George Gulliver]] in 1841 drew pictures of platelets<ref>Lancet, 1882, ii. 916; Notes of Gulliver's Researches in Anatomy, Physiology, Pathology, and Botany, 1880; Carpenter's Physiology, ed. Power, 9th ed., see Index under 'Gulliver.'</ref> using the twin lens (compound) microscope invented in 1830 by [[Joseph Jackson Lister]].<ref>
{{cite book |last=Godlee |first=Sir Rickman |date=1917 |title=Lord Lister |url=https://archive.org/details/lordlister00godlgoog |location=London |publisher=Macmillan & Co.}}</ref> This microscope improved resolution sufficiently to make it possible to see platelets for the first time.
* [[William Addison (physician)|William Addison]] in 1842 drew pictures of a platelet-fibrin clot.<ref>{{cite journal |vauthors=Robb-Smith AH |title=Why the platelets were discovered |journal=British Journal of Haematology |volume=13 |issue=4 |pages=618–637 |date=July 1967 |pmid=6029960 |doi=10.1111/j.1365-2141.1967.tb00769.x |s2cid=5742616}}</ref>
* [[Lionel Beale]] in 1864 was the first to publish a drawing showing platelets.<ref>{{cite journal |vauthors=Beale LS |title=On the Germinal Matter of the Blood, with Remarks upon the Formation of Fibrin |journal=Transactions of the Microscopical Society & Journal |volume=12 |pages=47–63 |date=1864 |doi=10.1111/j.1365-2818.1864.tb01625.x}}</ref>
* [[Max Johann Sigismund Schultze|Max Schultze]] in 1865 described what he called "spherules", which he noted were much smaller than red blood cells, occasionally clumped, and were sometimes found in collections of fibrin material.<ref name=Schultze>{{cite journal |doi=10.1007/BF02961404 |author=Schultze M |title=Ein heizbarer Objecttisch und seine Verwendung bei Untersuchungen des Blutes |journal=Arch Mikrosk Anat |volume=1 |issue=1 |pages=1–42 |date=1865 |s2cid=84919090 |url=https://zenodo.org/record/1428432}}</ref>
* [[Giulio Bizzozero]] in 1882 studied the blood of amphibians microscopically ''[[in vivo]]''. He named Schultze's spherules (It.) ''piastrine'': little plates.<ref name=Bizzozero>{{cite journal |doi=10.1007/BF01931360 |author=Bizzozero, J. |date=1882 |title=Über einen neuen Forrnbestandteil des Blutes und dessen Rolle bei der Thrombose und Blutgerinnung |journal=Arch Pathol Anat Phys Klin Med |volume=90 |issue=2 |pages=261–332 |s2cid=37267098 |url=https://zenodo.org/record/2466112}}</ref><ref name=Brewer>{{cite journal |vauthors=Brewer DB |title=Max Schultze (1865), G. Bizzozero (1882) and the discovery of the platelet |journal=British Journal of Haematology |volume=133 |issue=3 |pages=251–8 |date=May 2006 |pmid=16643426 |doi=10.1111/j.1365-2141.2006.06036.x |doi-access=free}}</ref> Bizzozero possibly proposed the name Blutplattchen.<ref>{{cite book |url=https://books.google.com/books?id=zoE9AQAAIAAJ&q=carbonic+oxide |title=Scientific American |date=1882 |publisher=Munn & Company |page=105 |language=en}}</ref>
* [[William Osler]] observed platelets and, in published lectures in 1886, called them a ''third corpuscle'' and a blood ''plaque''; and described them as "a colorless protoplasmic disc".<ref>{{cite journal |vauthors=Osler W |title=On certain problems in the physiology of the blood corpuscles |journal=The Medical News |date=1886 |volume=48 |pages=421–5}}</ref>
* [[James Homer Wright|James Wright]] examined blood smears using the stain named for him, and used the term ''plates'' in his 1906 publication,<ref>{{cite journal |vauthors=Wright JH |title=The Origin and Nature of the Blood Plates |journal=The Boston Medical and Surgical Journal |volume=154 |issue=23 |pages=643–5 |date=1906 |doi=10.1056/NEJM190606071542301 |url=https://zenodo.org/record/1813919}}</ref> changing to platelets in his 1910 publication.<ref>{{cite journal |vauthors=Wright JH |title=The histogenesis of blood platelets |journal=Journal of Morphology |year=1910 |volume=21 |issue=2 |pages=263–278 |doi=10.1002/jmor.1050210204 |bibcode=1910JMorp..21..263W |hdl=2027/hvd.32044107223588 |s2cid=84877594 |url=https://babel.hathitrust.org/cgi/imgsrv/download/pdf?id=hvd.32044107223588;orient=0;size=100;seq=3;attachment=0 |hdl-access=free|url-access=subscription }}</ref>

==See also==
{{Commons category|Platelets|platelets}}
* [[List of distinct cell types in the adult human body]]

==References==
{{Reflist}}

{{Myeloid blood cells and plasma}}
{{Coagulation proteins}}
{{Transfusion medicine}}
{{Authority control}}

[[Category:Blood products]]
[[Category:Coagulation system]]
[[Category:Cell biology]]
[[Category:Human cells]]
[[Category:Transfusion medicine]]
[[Category:1842 in science]]