Editing Platelet (section)

==Structure==
Structurally the platelet can be divided into four zones, from peripheral to innermost:{{citation needed|date=December 2021}}
* 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 |pages=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 |pages=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.