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| '''Platelets''' |
'''Platelets''' or '''thrombocytes''' are the ] ]s that are involved in the cellular mechanisms that lead to the formation of ]s. Low levels or dysfunction predisposes for ], while high levels - although usually asymptomatic - may increase the risk of ]. | ||
| ==Anatomy== | |||
| ⚫ | |||
| Like ], platelets are anuclear and discoid; they measure 1.5-3.0 ] in size. The body has a very limited reserve of platelets and so they can be rapidly depleted. They contain ] a ] system and several different types of granules; ], dense bodies and alpha granules (containing ]), the contents of which are released upon activation of the platelet. These granule contents play an important role in both ] and in the inflammatory response. | |||
| ==Physiology== | |||
| ⚫ | Platelets are separated from ] using an ] blood seperator |
||
| ===Production=== | |||
| Platelets are produced in the ]; the progenitor cell for platelets is the ]. This large, multinucleated cell sheds platelets into the circulation. ] (''c-mpl ligand'') is a hormone, mainly produced by the ], that stimulates platelet production. It is bound to circulating platelets; if platelet levels are adequate, serum levels remain low. If the platelet count is decreased, more thrombopoeitin circulates freely and increases marrow production. | |||
| ===Circulation=== | |||
| A normal platelet count in a healthy person is between 150 and 400 (x 10<sup>9</sup>/] of blood). People can live independently with a count as low as 20. People can live in hospital with a count as low as 5, but spontaneous bleeding gets to be a problem. Platelets can be ] if a patient's platelet count falls too low. A low platelet count is called ], having too many platelets is called ]. | |||
| The circulating life of a platelet is 9-10 days. After this it is sequestered in the ]. ] (or absence) of the spleen may increase platelet counts, while ] (overactivity of the spleen, e.g. in ] or ]) may lead to increased elimination and hence low platelet counts. | |||
| ===Function=== | |||
| Thrombocytopenia may be caused by the drugs that stimulate the production of antibodies against platelets. This condition is called "Drug-induced immune thrombocytopenia" | |||
| ⚫ | Platelets are activated when brought into contact with ], which is exposed when the ] blood vessel lining is damaged, or with receptors expressed on ] or the endothelial cells of the blood vessels. Once activated, they release a number of different ]s and platelet activating factors. This causes the platelets to adhere to each other and to the endothelial cells in the wall of the blood vessel, forming a clot in conjunction with ]. | ||
| == |
==Role in disease== | ||
| ===High and low counts=== | |||
| ⚫ | |||
| A normal platelet count in a healthy person is between 150 and 400 (x 10<sup>9</sup>/] of blood). | |||
| ⚫ | |||
| ⚫ | |||
| Both ] (or thrombopenia) and ] may present with coagulation problems. Generally, low platelet counts increase bleeding risks (although there are exceptions, e.g. ]) and ] (high counts) may lead to thrombosis (although this is mainly when the elevated count is due to myeloproliferative disorder). | |||
| ⚫ | |||
| **] | |||
| Low platelet counts are generally not corrected by transfusion unless the patient is bleeding or the count has fallen below 5. | |||
| ⚫ | |||
| *disorders of platelet aggregation | |||
| ===Diseases=== | |||
| ⚫ | |||
| Disorders of platelet number: | |||
| ⚫ | *] | ||
| ⚫ | |||
| ⚫ | |||
| *disorders of platelet coagulant activity | |||
| ⚫ | |||
| *related disorders | |||
| **] | **] | ||
| **] | **] | ||
| **] | |||
| ⚫ | *], including ] | ||
| *Diseases where platelet counts are important: | |||
| ** ] | |||
| ** ] | |||
| ⚫ | Disorders of platelet adhesion or aggregation: | ||
| ⚫ | *] | ||
| ⚫ | *] | ||
| ⚫ | *] | ||
| ⚫ | Disorders of platelet release | ||
| ⚫ | *decreased ] activity, induced or congenital | ||
| ⚫ | *granule storage pool defects, acquired or congenital | ||
| ==Transfusion== | |||
| ⚫ | Platelets are separated from ] using an ] blood seperator. This is necessary because platelets will not survive at the low temperatures used to store ]s, so they must be stored separately using porous storage bags that allow ] to flow in. People with few platelets or platelets that are dysfunctional may benefit from a platelet transfusion, however patients with autoimmune disorders that affect platelets may not. | ||
| ==See also== | |||
| *] | *] | ||
Revision as of 15:08, 3 November 2004
Platelets or thrombocytes are the blood cells that are involved in the cellular mechanisms that lead to the formation of blood clots. Low levels or dysfunction predisposes for bleeding, while high levels - although usually asymptomatic - may increase the risk of thrombosis.
Anatomy
Like red blood cells, platelets are anuclear and discoid; they measure 1.5-3.0 u in size. The body has a very limited reserve of platelets and so they can be rapidly depleted. They contain RNA a canalicular system and several different types of granules; lysosomes, dense bodies and alpha granules (containing von Willebrand factor), the contents of which are released upon activation of the platelet. These granule contents play an important role in both hemostasis and in the inflammatory response.
Physiology
Production
Platelets are produced in the bone marrow; the progenitor cell for platelets is the megakaryocyte. This large, multinucleated cell sheds platelets into the circulation. Thrombopoietin (c-mpl ligand) is a hormone, mainly produced by the liver, that stimulates platelet production. It is bound to circulating platelets; if platelet levels are adequate, serum levels remain low. If the platelet count is decreased, more thrombopoeitin circulates freely and increases marrow production.
Circulation
The circulating life of a platelet is 9-10 days. After this it is sequestered in the spleen. Decreased function (or absence) of the spleen may increase platelet counts, while hypersplenism (overactivity of the spleen, e.g. in Gaucher's disease or leukemia) may lead to increased elimination and hence low platelet counts.
Function
Platelets are activated when brought into contact with collagen, which is exposed when the endothelial blood vessel lining is damaged, or with receptors expressed on white blood cells or the endothelial cells of the blood vessels. Once activated, they release a number of different coagulation factors and platelet activating factors. This causes the platelets to adhere to each other and to the endothelial cells in the wall of the blood vessel, forming a clot in conjunction with fibrin.
Role in disease
High and low counts
A normal platelet count in a healthy person is between 150 and 400 (x 10/L of blood).
Both thrombocytopenia (or thrombopenia) and thrombocytosis may present with coagulation problems. Generally, low platelet counts increase bleeding risks (although there are exceptions, e.g. heparin-induced thrombocytopenia) and thrombocytosis (high counts) may lead to thrombosis (although this is mainly when the elevated count is due to myeloproliferative disorder).
Low platelet counts are generally not corrected by transfusion unless the patient is bleeding or the count has fallen below 5.
Diseases
Disorders of platelet number:
- thrombocytopenia
- thrombocytosis, including benign essential thrombocytosis
- Diseases where platelet counts are important:
Disorders of platelet adhesion or aggregation:
Disorders of platelet release
- decreased cyclooxygenase activity, induced or congenital
- granule storage pool defects, acquired or congenital
Transfusion
Platelets are separated from donated blood using an apheresis blood seperator. This is necessary because platelets will not survive at the low temperatures used to store red blood cells, so they must be stored separately using porous storage bags that allow oxygen to flow in. People with few platelets or platelets that are dysfunctional may benefit from a platelet transfusion, however patients with autoimmune disorders that affect platelets may not.
See also
| Myeloid blood cells and plasma | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Hematopoiesis |
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| Myeloid tissue |
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| Other | |||||||||||||||||