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Encyclopedia > Platelet
A 250 ml bag of newly collected platelets.

Platelets or thrombocytes are the cell fragments circulating in the blood that are involved in the cellular mechanisms of primary hemostasis leading to the formation of blood clots. Dysfunction or low levels of platelets predisposes to bleeding, while high levels, although usually asymptomatic, may increase the risk of thrombosis. Download high resolution version (480x640, 60 KB)A platelet blood bag, taken by my cellphone camera. ... Download high resolution version (480x640, 60 KB)A platelet blood bag, taken by my cellphone camera. ... Image File history File links Thrombocytes. ... Image File history File links Thrombocytes. ... Drawing of the structure of cork as it appeared under the microscope to Robert Hooke from Micrographia which is the origin of the word cell being used to describe the smallest unit of a living organism Cells in culture, stained for keratin (red) and DNA (green) The cell is the... Human blood smear: a - erythrocytes; b - neutrophil; c - eosinophil; d - lymphocyte. ... Hemostasis refers to a process whereby bleeding is halted in most animals with a closed circulatory system. ... Blood clot diagram. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Thrombosis is the formation of a clot or thrombus inside a blood vessel, obstructing the flow of blood through the circulatory system. ...



Like red blood cells, platelets are anuclear (no cell nucleus) and discoid (disc shaped); they measure 1.5–3.0 μm in diameter. The body has a very limited reserve of platelets, so they can be rapidly depleted. They contain RNA, mitochondria, a canalicular system, and several different types of granules; lysosomes (containing acid hydrolases), dense bodies (containing ADP, ATP, serotonin, histamine, and calcium) and alpha granules (containing fibrinogen, factor V, vitronectin, thrombospondin and von Willebrand factor), the contents of which are released upon activation of the platelet. Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ... HeLa cells stained for DNA with the Blue Hoechst dye. ... A disk or disc is anything that resembles a flattened cylinder in shape. ... A micrometre (American spelling: micrometer, symbol µm) is an SI unit of length equal to one millionth of a metre, or about a tenth of the diameter of a droplet of mist or fog. ... Ribonucleic acid or RNA is a nucleic acid polymer consisting of nucleotide monomers that plays several important roles in the processes that translate genetic information from deoxyribonucleic acid (DNA) into protein products; RNA acts as a messenger between DNA and the protein synthesis complexes known as ribosomes, forms vital portions... In cell biology, a mitochondrion is an organelle found in the cells of most eukaryotes. ... Canaliculi are small, microscopic canals between the various lacunae of ocified bone. ... Lysosomes are organelles in eukaryotic cells that contain digestive enzymes to digest macromolecules. ... An acid hydrolase is an enzyme that works best at acidic pHs. ... Dense bodies can refer to: granules in blood platelets electron-dense portions of muscle Dorlands/Elsevier b_17zPzhtm#12190824/{{{2}}} Dictionary at eMedicine dense+bodies Category: ... Adenosine diphosphate, abbreviated ADP, is a nucleotide. ... Adenosine 5-triphosphate (ATP) is a multifunctional nucleotide that is most important as a molecular currency of intracellular energy transfer. ... Serotonin (pronounced ) (5-hydroxytryptamine, or 5-HT) is a monoamine neurotransmitter synthesized in serotonergic neurons in the central nervous system (CNS) and enterochromaffin cells in the gastrointestinal tract of animals including humans. ... This article or section does not cite any references or sources. ... General Name, Symbol, Number calcium, Ca, 20 Chemical series alkaline earth metals Group, Period, Block 2, 4, s Appearance silvery white Standard atomic weight 40. ... In platelets, the term alpha granules is used to describe granules containing several growth factors (such as insulin-like growth factor 1 and platelet-derived growth factor ) and other clotting proteins (such as thrombospondin and fibronectin. ... Fibrin is a protein involved in the clotting of blood. ... Factor V is a protein of the coagulation system, rarely referred to as proaccelerin or labile factor. ... Vitronectin is an abundant glycoprotein found in blood plasma and the extracellular matrix. ... Thrombospondins (TSP) are secreted proteins with the ability to inhibit angiogenesis. ... Von Willebrand factor is a blood glycoprotein of the coagulation system. ...


Functions of Platelets can be generalised into a number of categories:

  • Adhesion
  • Aggregation
  • Clot retraction
  • Pro-Coagulation
  • Cytokine signalling
  • Phagocytosis[1]

Platelets are activated when brought into contact with collagen (which is exposed when the endothelial blood vessel lining is damaged), thrombin (primarily through PAR-1), ADP receptors (P2Y1 and P2Y12) expressed on platelets, a negatively charged surface (e.g., glass), or several other activating factors. Once activated, they release a number of different coagulation factors and platelet activating factors. Platelet activation further results in the scramblase-mediated transport of negatively charged phospholipids to the platelet surface. These phospholipids provide a catalytic surface (with the charge provided by phosphatidylserine and phosphatidylethanolamine) for the tenase and prothrombinase complexes. The platelets adhere to each other via adhesion receptors or integrins, and to the endothelial cells in the wall of the blood vessel forming a haemostatic plug in conjunction with fibrin. The high concentration of myosin and actin filaments in platelets are stimulated to contract during aggregation, further reinforcing the plug. The most abundant platelet adhesion receptor is glycoprotein (GP) IIb/IIIa; this is a calcium-dependent receptor for fibrinogen, fibronectin, vitronectin, thrombospondin and von Willebrand factor (vWF). Other receptors include GPIb-V-IX complex (vWF) and GPVI (collagen). Besides being the chief cellular effector of hemostasis, platelets are rapidly deployed to sites of injury or infection and potentially modulate inflammatory processes by interacting with leukocytes and by secreting cytokines, chemokines and other inflammatory mediators[2] [3] [4] [5] . Tropocollagen triple helix. ... The endothelium is the layer of thin, flat cells that lines the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. ... Thrombin (activated Factor II) is a coagulation protein that has many effects in the coagulation cascade. ... A protease-activated receptor is a cell receptor that is activated by cleavage of part of its extracellular domain. ... Adenosine diphosphate, abbreviated ADP, is a nucleotide. ... Coagulation is the thickening or congealing of any liquid into solid clots. ... To meet Wikipedias quality standards, this article or section may require cleanup. ... Phosphatidylserine is a phospholipid nutrient found in fish, green leafy vegetables, soybeans and rice, and is essential for the normal functioning of neuronal cell membranes. ... Phosphatidylethanolamine is a lipid found in biological membranes. ... The tenase complex is formed by the activated forms of the blood coagulation factors factor VIII and factor IX. It forms on a phospholipid surface in the presence of calcium and is responsible for the activation of factor X. ... Prothrombinase is the protein complex that converts prothrombin to thrombin. ... An integrin, or integrin receptor, is an integral membrane protein in the plasma membrane of cells. ... Fibrin is a protein involved in the clotting of blood. ... Myosin is a motor protein filament found in muscle tissue. ... G-Actin (PDB code: 1j6z). ... A glycoprotein is a macromolecule composed of a protein and a carbohydrate (an oligosaccharide). ... Fibronectin is a high-molecular-weight glycoprotein containing about 5% carbohydrate that binds to receptor proteins that span the cells membrane, called integrins. ... Tropocollagen triple helix. ... Hemostasis refers to a process whereby bleeding is halted in most animals with a closed circulatory system. ... White Blood Cells is also the name of a White Stripes album. ... Cytokines are small protein molecules that are the core of communication between immune system cells, and even between immune system cells and cells belonging to other tissue types. ... Chemokines are a class of chemotactic cytokines, or small secreted protein signals. ...

Role in disease

High and low counts

A normal platelet count in a healthy person is between 150,000 and 400,000 per mm³ of blood (150–400 x 106/L). 95% of healthy people will have platelet counts in this range. Some will have statistically abnormal platelet counts while having no abnormality, although the likelihood increases if the platelet count is either very low or very high. The litre or liter (see spelling differences) is a unit of volume. ...

Both thrombocytopenia (or thrombopenia) and thrombocytosis may present with coagulation problems. Generally, low platelet counts increase bleeding risks (although there are exceptions, e.g. immune heparin-induced thrombocytopenia) and thrombocytosis (high counts) may lead to thrombosis (although this is mainly when the elevated count is due to myeloproliferative disorder). Thrombocytopenia (or -paenia, or thrombopenia in short) is the presence of relatively few platelets in blood. ... Thrombocytosis is the presence of high platelet counts in the blood, and can be either reactive or primary (also termed essential and caused by a myeloproliferative disease). ... Heparin-induced thrombocytopenia (HIT) with or without thrombosis (HITT) is thrombocytopenia (low platelet counts) due to the administration of heparin. ... Thrombocytosis is the presence of high platelet counts in the blood, and can be either reactive or primary (also termed essential and caused by a myeloproliferative disease). ...

Low platelet counts are generally not corrected by transfusion unless the patient is bleeding or the count has fallen below 5 x 109/L; it is contraindicated in thrombotic thrombocytopenic purpura (TTP) as it fuels the coagulopathy. In patients having surgery, a level below 50 x 109/L) is associated with abnormal surgical bleeding, and regional anaesthetic procedures such as epidurals are avoided for levels below 80-100. An epidural catheter after insertion. ...

Normal platelet counts are not a guarantee of adequate function. In some states the platelets, while being adequate in number, are dysfunctional. For instance, aspirin irreversibly disrupts platelet function by inhibiting cyclooxygenase-1 (COX1), and hence normal hemostasis; normal platelet function may not return until the aspirin has ceased and all the affected platelets have been replaced by new ones, which can take over a week. Similarly, uremia (a consequence of renal failure) leads to platelet dysfunction that may be ameliorated by the administration of desmopressin. Aspirin, or acetylsalicylic acid (IPA: ), (acetosal) is a drug in the family of salicylates, often used as an analgesic (to relieve minor aches and pains), antipyretic (to reduce fever), and as an anti-inflammatory. ... Cyclooxygenase (COX) is an enzyme (EC 1. ... Uremia is a toxic condition resulting from renal failure, when kidney function is compromised and urea, a waste product normally excreted in the urine, is retained in the blood. ... Renal failure is the condition in which the kidneys fail to function properly. ... Desmopressin (DDAVP®, Stimate®, Minirin®) is a synthetic drug that mimics the action of antidiuretic hormone, also known as arginine vasopressin. ...


Disorders leading to a reduced platelet count:

Alloimmune disorders Thrombocytopenia (or -paenia, or thrombopenia in short) is the presence of relatively few platelets in blood. ... Idiopathic thrombocytopenic purpura (ITP) is the condition of having a low platelet count (thrombocytopenia) of no known cause (idiopathic). ... Thrombotic thrombocytopenic purpura (TTP or Moschcowitz disease) is a rare disorder of the blood coagulation system that in most cases arises from the deficiency or inhibition of the enzyme ADAMTS13, which is responsible for cleaving large multimers of von Willebrand factor. ... Heparin-induced thrombocytopenia (HIT) with or without thrombosis (HITT) is thrombocytopenia (low platelet counts) due to the administration of heparin. ... Gauchers disease (pronounced ) is the most common of the lipid storage diseases. ... Aplastic anemia is a condition where bone marrow does not produce sufficient new cells to replenish blood cells. ... ...

Disorders leading to platelet dysfunction or reduced count: In fetomaternal alloimmune thrombocytopenia (also known as Neonatal Alloimmune Thrombocytopenia or NAIT) maternal IgG antibodies (specific for platelet antigens) pass through the placenta and attack platelets in the fetal circulation. ...

Disorders featuring an elevated count: HELLP syndrome is a life-threatening obstetric complication considered by many to be a variant of pre-eclampsia. ... In medicine, Hemolytic-uremic syndrome (or haemolytic-uraemic syndrome, abbreviated HUS) is a disease characterised by microangiopathic hemolytic anemia, acute renal failure and a low platelet count (thrombocytopenia). ... Chemotherapy is the use of chemical substances to treat disease. ... For music group see Dengue Fever (rock band) Dengue and dengue hemorrhagic fever (DHF) are acute febrile diseases, found in the tropics, with a geographical spread similar to malaria. ...

Disorders of platelet adhesion or aggregation: Thrombocytosis is the presence of high platelet counts in the blood, and can be either reactive or primary (also termed essential and caused by a myeloproliferative disease). ... The myeloproliferative diseases (MPDs) are a group of diseases of the bone marrow in which excess cells are produced. ...

Disorders of platelet metabolism Bernard-Soulier syndrome (BSS, named after Jean Bernard and Jean Pierre Soulier) is a rare congenital bleeding disorder characterized by thrombocytopenia and large platelets and was first described in 1948. ... Glanzmanns thrombasthenia is an extremely rare, autosomal recessive disorder of the blood, in which the platelets lack GPIIb/IIIa. ... Von Willebrand disease (vWD) is the most common hereditary coagulation abnormality described in humans, although it can also be acquired as a result of other medical conditions. ... Hermansky-Pudlak Syndrome (HPS) is an inherited disease which results in decreased pigmentation (oculocutaneous albinism), bleeding problems due to a platelet abnormality (platelet storage pool defect), and storage of an abnormal fat-protein compound (lysosomal accumulation of ceroid lipofuscin). ...

  • Decreased cyclooxygenase activity, induced or congenital
  • Storage pool defects, acquired or congenital

Disorders that compromise platelet function: Cyclooxygenase (COX) is an enzyme (EC 1. ...

This article does not cite any references or sources. ...


Brewer[6] traced the history of the discovery of the platelet. Although red blood cells had been known since van Leeuwenhoek, it was the German anatomist Max Schultze (1825-1874) who first offered a description of the platelet in his newly founded journal Archiv für mikroscopische Anatomie[7]. He describes "spherules" much smaller than red blood cells that are occasionally clumped and may participate in collections of fibrous material. He recommends further study of the findings. Anton van Leeuwenhoek Anton van Leeuwenhoek (October 24, 1632 - August 30, 1723, full name Thonius Philips van Leeuwenhoek (pronounced Layewenhook) was a Dutch tradesman and scientist from Delft, Netherlands. ... Max Johann Sigismund Schultze (March 25, 1825 - January 16, 1874), German microscopic anatomist, was born at Freiburg in Breisgau (Baden). ... Fibrin is a protein involved in the clotting of blood. ...

Giulio Bizzozero (1846-1901), building on Schultze's findings, used "living circulation" to study blood cells of amphibians microscopically in vivo. One of his findings was the fact that platelets clump at the site of blood vessel injury, which precedes the formation of a blood clot. This observation confirmed the role of platelets in coagulation[8]. Giulio Bizzozero (1846-1901) was an Italian doctor and medical researcher. ... In vivo (Latin for (with)in the living). ... Blood clot diagram. ... Coagulation is a complex process by which blood forms solid clots. ...

Additional images

In transfusion medicine

Platelets are either isolated from collected units of Whole Blood and pooled to make a therapeutic dose or collected by Apheresis, sometimes concurrently with Plasma or Red Blood Cells. The industry standard is for platelets to be tested for bacteria before transfusion to avoid septic reactions, which can be fatal. Whole blood enters the centrifuge on the left and separates into layers so that selected components can be drawn off on the right. ... Look up plasma in Wiktionary, the free dictionary. ... Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ... Phyla Actinobacteria Aquificae Chlamydiae Bacteroidetes/Chlorobi Chloroflexi Chrysiogenetes Cyanobacteria Deferribacteres Deinococcus-Thermus Dictyoglomi Fibrobacteres/Acidobacteria Firmicutes Fusobacteria Gemmatimonadetes Lentisphaerae Nitrospirae Planctomycetes Proteobacteria Spirochaetes Thermodesulfobacteria Thermomicrobia Thermotogae Verrucomicrobia Bacteria (singular: bacterium) are unicellular microorganisms. ...

Pooled Whole Blood Platelets, sometimes called "random" platelets, are made by taking a unit of Whole Blood from a donor that has not been cooled and placing it into a large centrifuge in what is referred to as a "soft spin." This splits the blood into three layers: the plasma, a "buffy coat" layer which includes the platelets, and the red blood cells. These are expressed into different bags for storage. From four to six of these are typically pooled into a single bag for a therapeutic dose, though individual components can also be used.

Apheresis Platelets are collected using a device which 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 not exposed to as many different 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 will receive repeated donations from a specific donor to further minimize the risk. Cancer is a class of diseases or disorders characterized by uncontrolled division of cells and the ability of these to spread, either by direct growth into adjacent tissue through invasion, or by implantation into distant sites by metastasis (where cancer cells are transported through the bloodstream or lymphatic system). ...

Platelets are not crossmatched unless they contain a significant amount of RBCs, which results in a reddish-orange color to the product. This is usually associated with whole blood platelets, as apheresis methods are more efficient than "soft spin" centrifugation at isolating the specific components of blood. An effort is usually made to issue type specific platelets, but this is not as critical as it is with Red Blood Cells. Human red blood cells Red blood cells are the most common type of blood cell and are the vertebrate bodys principal means of delivering oxygen to body tissues via the blood. ...

Platelets collected by either method have a very short shelf life, typically five or seven days depending on the system used. This results in frequent problems with short supply, as testing the donations often uses up a full day of this time. Since there are no effective preservative solutions for platelets, they lose potency quickly and are best when fresh.


  1. ^ Movat H.Zet al (1965). "Platelet Phagocytosis and Aggregation". Journal of Cell Biology 27: 531-543. 
  2. ^ Weyrich A.S. et al (2004). "Platelets: signaling cells in the immune continuum.". Trends Immunol 25: 489-495. 
  3. ^ Wagner D.D. et al (2003). "Platelets in inflammation and thrombosis.". Thromb Vasc Biol 23: 2131-2137. 
  4. ^ Diacovo T.G. et al (1996). "Platelet-mediated lymphocyte delivery to high endothelial venules.". Science 273: 252-255. 
  5. ^ Iannacone M. et al (2005). "Platelets mediate cytotoxic T lymphocyte-induced liver damage". Nat Med 11: 1167-1169. 
  6. ^ Brewer DB. Max Schultze (1865), G. Bizzozero (1882) and the discovery of the platelet. Br J Haematol 2006;133:251-8. PMID 16643426.
  7. ^ Schultze M. Ein heizbarer Objecttisch und seine Verwendung bei Untersuchungen des Blutes. Arch Mikrosc Anat 1865;1:1-42.
  8. ^ Bizzozero J. Über einen neuen Forrnbestandteil des Blutes und dessen Rolle bei der Thrombose und Blutgerinnung. Arch Pathol Anat Phys Klin Med 1882;90:261-332.

See also

  Results from FactBites:
American Red Cross (507 words)
Platelet donors have the satisfaction of knowing that their donation will be saving a life within just a few days.
Until recently, the only way to collect enough platelets for a single transfusion was to take blood donations from 5 to 10 donors, separate the platelets from the other blood cells, and combine the platelets to obtain a large enough quantity for transfusion.
By appropriately adjusting the instrument, a selected portion of the blood, such as the platelets, can be recovered, while the rest of the blood is returned to the donor either into the same vein or into a vein in the other arm.
Platelet Immunology Laboratory | About This Lab (483 words)
The Platelet Immunology Laboratory was established over 20 years ago in response to the need to identify and characterize antibodies in alloimmunized thrombocytopenic patients with the goal of identifying compatible donors.
The development of a platelet antibody laboratory was a logical extension of the research studies on platelet transfusion therapy that were initiated by Dr. Sherrill Slichter when she joined the Puget Sound Blood Center in 1970.
With the availability of platelets for transfusion and their rapidly increasing use, the development of alloimmune platelet refractoriness became a recognized consequence of repeated platelet transfusions in chronically-thrombocytopenic patients.
  More results at FactBites »



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