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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 (containing ADP, ATP serotonin 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. 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 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 most common 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)

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
  • idiopathic thrombocytopenic purpura
  • thrombotic thrombocytopenic purpura
  • drug-induced thrombocytopenia
• thrombocytosis, including benign essential thrombocytosis
• Diseases where platelet counts are important:
  • HELLP syndrome
  • Hemolytic-uremic syndrome

Disorders of platelet adhesion or aggregation:

• Bernard-Soulier syndrome
• Glanzmann's thrombasthenia
• Scott's syndrome

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

• hemostasis

• Blood and immune system cells
• Coagulation system