Thrombocytes or thrombocytes ( singular of the thrombocyte ; from ancient Greek θρόμβος thrómbos "lump" and ancient Greek κύτος kýtos "cavity", "vessel", "envelope") or platelets are the smallest cells in the blood . They play an important role in blood clotting by attaching themselves to the surrounding tissue (" platelet adhesion ") or sticking together (" platelet aggregation ") when a blood vessel is injured , so that the injury is closed. In addition, they release substances that promote coagulation.
Structure and function
Platelets are created in the bone marrow, where they are pinched off by megakaryocytes, huge platelet-forming cells. Up to 8000 platelets can be cut off from such a bone marrow cell in the course of a lifetime. This process is called thrombopoiesis and is supported by the hormone thrombopoietin .
Platelets have a diameter of 1.5 µm to 3.0 µm and are flat like a disk ( discoid ). During blood clotting, platelets change their shape due to activators such as ADP , collagen , thromboxane and thrombin . The protuberance of pseudopodia takes place , which is accompanied by a multiple increase in surface area. This promotes thrombus formation through fibrin-mediated binding with other platelets. This process describes the function of platelets and is called platelet aggregation .
In healthy people, a platelet count between 150,000 and 450,000 per µl of blood is considered normal. The platelet concentration is genetically determined and varies little over the lifetime of a healthy adult. In a healthy population, the platelet concentration follows a normal distribution .
The average lifespan of platelets is eight to twelve days. The breakdown takes place mainly in the spleen , in which a third of the platelets are also temporarily stored, as well as in the lungs and, to a lesser extent, in the liver .
Despite the lack of a cell nucleus, mRNA from megakaryocytes was found in platelets . They are therefore capable , albeit to a limited extent, of the new synthesis of proteins . Like other cells , they have mitochondria , but a special form of the rough endoplasmic reticulum (rER), the canalicular system. It serves as a calcium ion store, the rapid emptying of which into the cytosol is an essential prerequisite for physiological platelet aggregation . The storage granules are important as further compartments in platelets : They are divided into α-granules, electron-dense granules and lysosomes and contain substances and proteins that promote aggregation , the secretion (release) of which is necessary for platelet function. The plasma membrane of platelets contains among other things the protein tissue factor (engl. Tissue factor ). It is assumed that vesicles with this protein constrict themselves and can now be found as circulating microparticles in the blood. This ability, which they have in common with monocytes , helps blood to clot .
Enlargement of the spleen ( splenomegaly ) or immune thrombocytopenia lead to a reduction in the average platelet life, which leads to an overall low platelet concentration with a constant production rate. Conversely, after removal of the spleen ( splenectomy ) there is a sharp increase in the number of platelets due to the lack of spleen-mediated breakdown.
There are three types of disturbance or abnormality in platelets. A distinction is made between primary (mostly genetic) deviations, which are sometimes also called idiopathic or essential, and secondary deviations due to another underlying disease or disorder. The three types of deviation are:
Thrombocytopenia : Decreased concentration of platelets in the blood with values below 150,000 / µl. Below 80,000 / µl an increased tendency to bleeding is to be expected, below 50,000 / µl it can lead to spontaneous bleeding ( nosebleeds , bruises ). Concentrations below 10,000 / µl must be closely monitored if medically necessary and, if necessary,treatedwith platelet concentrates .
Causes can be a reduced formation, an increased breakdown or consumption. Primary, genetic causes of thrombocytopenia are e.g. b. TAR syndrome , Wiskott-Aldrich syndrome , Jacobsen syndrome or Gaucher's disease . Thrombocytopenia can occasionally occur even with drug therapy, and heparin-induced thrombocytopenia causedby heparin or other low-molecular-weight heparinsis considered very rare but very dangerous. During pregnancy , the number of platelets in the blood is often low. About 5–10% of pregnant women have fewer than 150,000 platelets / µl. If values are below 100,000 / µl, causes other than pregnancy or a pregnancy complication should be considered.
Thrombocytosis : increased concentration of platelets in the blood with values above 450,000 / µl, which can be associated with a risk of arterial thrombosis .
A primary cause can be essential thrombocythemia , a myeloproliferative neoplasia .
However, secondary thrombocytosis, also known as "reactive", is more common, for example after a spleen has been removed ( splenectomy ), in the case of malignant tumors or chronic inflammatory diseases (e.g. Crohn's disease , rheumatic diseases ), but also, for example, in the case of chronic iron deficiency or acute Blood loss . An increased platelet concentration can also be a general indication of systemic inflammation associated with activation of the proinflammatory mediator interleukin-6 .
Thrombocytopathy : dysfunction of the blood platelets with little or no change in the concentration in the blood, which is associated with an increased tendency to bleed. Disorders affect the function of the platelets, i.e. above all the adhesion and aggregation .
Primary, mostly congenital causes can be the Glanzmann thrombasthenia with a lack of aggregation of the platelets and a strongly increased bleeding tendency, or the Bernard-Soulier syndrome , a platelet dystrophy with considerably enlarged "giant platelets".
The von Willebrand syndrome is a coagulopathy with a decrease or dysfunction of the von Willebrand factor , whereby the platelet adhesion is secondarily disturbed and there is an increased tendency to bleeding (also called hemorrhagic diathesis ). In addition, however, secondary thrombocytopathies due to exogenous causes are much more common, and in some cases they are also therapeutic. The group of drugs called platelet aggregation inhibitors , to which z. B. Clopidogrel , and above all acetylsalicylic acid counts, is very often used for the prevention and treatment of strokes , heart attacks and other circulatory disorders . But penicillin and anti-inflammatory drugs such as diclofenac as well as some chemotherapeutic agents in cancer therapy can also lead to dysfunction.
Presumably, thrombocytes were described for the first time in 1844 by Alfred Donné (1801–1878) as “globules” in blood plasma (De l'órigine d. Glob. Du sang etc. Comptes rend. Soc. Biolog. 1844, t. XIV). In 1865, Max Schultze described them much more precisely in the archive for microscopic anatomy , but did not recognize them as a separate type of cell, but instead considered them to be degradation products. Giulio Bizzozzero corrected this mistake in 1883 and was the first to recognize the role of platelets in blood clotting.
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