Editing Platelet (section)

====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>