Platelets and Its Functions

Platelets are irregularly shaped cell fragments present in the blood. They are not actually true cells but merely the circulating fragments of cells. They are smaller and lighter than the other blood cells. They are colorless bodies with small nonnucleated protoplasm. These are cell fragments, which are derived from the fragmentation of precursor megakaryocytes. The size of the platelet is about 2 to 3 micrometer in diameter. The normal platelets count is 1.5 to 4 hundred thousand per microliter in peripheral blood. The average lifespan of a platelet is 5 to 9 days.

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Formation

Platelets are produced by thrombopoiesis in the bone marrow. Platelets are produced from very large bone marrow cells called megakaryocytes. When megakaryocytes develop itself into giant cells, they undergo a process of fragmentation which results in the release of platelets. Between 5,000 and 10,000 platelets per megakaryocyte are produced by fragmentation or budding. Megakaryocyte and platelet production is regulated by thrombopoietin, a hormone produced by the liver and kidneys. When the platelets gets older, they are destroyed by a process called phagocytosis in the spleen and by Kupffer cells in the liver. The reserve platelets are stored in the spleen and are released when needed by splenic contraction.

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Functions

Hemostatic Function: Platelets are involved with hemostasis, leading to the formation of blood clots. When damage to the endothelium of blood vessels occurs, they form thrombi and prevent further bleeding. They contain many structures that are important for stopping the bleeding. Platelets contain proteins on their surface that allow them to stick to breaks in the blood vessel wall and also to stick with each other. They contain granules that secrete proteins required for creating a firm plug that seal the blood vessel breaks. Platelets also contain proteins similar to the muscle proteins that allow them to change shape when they become sticky. When the platelets are stimulated by a break in the blood vessel wall, they changes their shape. They become round and extends long filaments or pseudopods. They extend and reach out to make contact with the broken blood vessel wall and other platelets. With these filaments platelets form a plug and seals the broken blood vessel thus stopping the bleeding.

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Platelet Action: In normal blood flow, the platelets roll along the surface of the vessel wall, which is lined by endothelial cells. Endothelial cells produce a protein called the von Willebrand factor or vWF, a cell adhesion agent, which helps endothelial cells adhere to collagen of the basement membrane. When there is a cut or an injury, the collagen, vWF, and the tissue factor from the subendothelium is exposed to the bloodstream. When the platelets contact collagen, vWF or thrombin, they are activated and clump together. The tough fibers surrounding the vessel wall act like an envelop and attract platelets. These platelet changes in shape and then clump onto these fibers thus providing an initial seal preventing any further bleeding.

Wound Healing Function: Platelets release growth factors such as platelet-derived growth factor which is a chemotactic agent and TGF beta which stimulates deposition of extracellular matrix. These growth factors are involved with the repair and regeneration of connective tissues. Other growth factors produced by the platelets include basic fibroblast growth factor, insulin-like growth factor 1, platelet-derived epidermal growth factor, and vascular endothelial growth factor. These growth factors have been involved with the healing of wounds. Local application of these growth factors in high concentration in the form of platelet-rich plasma has been used for wound healing.

​Other functions of platelets include clot retraction, pro-coagulation, inflammation, cytokine signalling, and phagocytosis.

Platelet Disorders:

High Platelet Count: The condition is also called thrombocytosis. When the platelet count becomes too high than normal range, there is increased risk for formation of blood clots or thrombosis. Clot formation due to high number of leukocytes can cause obstruction of blood vessels and result in events such as stroke, myocardial infarction, pulmonary embolism, circulatory block etc.  The elevated platelet count is due to myeloproliferative disorder. Disorders due to an elevated count includes essential thrombocytosis. Elevated counts either reactive or as an expression of myeloproliferative disease may produce dysfunctional platelets.

Low Platelet Count: The condition is also called thrombocytopenia. When the platelet count becomes low than normal range, excessive bleeding occurs. This condition may be caused by the failure of bone marrow to produce normal amount of platelets. Thrombocytopenia is also produced by increased destruction of platelets after they have been formed and released into the blood. Disorders due to a reduced platelet count include thrombocytopenia, idiopathic thrombocytopenic purpura, thrombotic thrombocytopenic purpura, drug-induced thrombocytopenic purpura, Gaucher's disease, and aplastic anemia​

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