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Platelets, also called thrombocytes, are a component of blood whose function is to react to bleeding from blood vessel injury by clumping, thereby initiating a blood clot. Platelets have no cell nucleus: they are fragments of cytoplasm that are derived from the megakaryocytes of the bone marrow, which then enter the circulation. Circulating unactivated platelets are biconvex discoid (lens-shaped) structures, 2–3 µm in greatest diameter. Activated platelets have cell membrane projections covering their surface. Platelets are found only in mammals, whereas in other vertebrates thrombocytes circulate as intact mononuclear cells.
Coagulation, also known as clotting, is the process by which blood changes from a liquid to a gel, forming a blood clot. It potentially results in hemostasis, the cessation of blood loss from a damaged vessel, followed by repair. The mechanism of coagulation involves activation, adhesion and aggregation of platelets, as well as deposition and maturation of fibrin.
Von Willebrand disease (vWD) is the most common hereditary blood-clotting disorder in humans. An acquired form can sometimes result from other medical conditions. It arises from a deficiency in the quality or quantity of von Willebrand factor (vWF), a multimeric protein that is required for platelet adhesion. As well as humans, it is known to affect several breeds of dogs. The three forms of vWD are: hereditary, acquired, and pseudo or platelet type. The three types of hereditary vWD are: vWD type 1, vWD type 2, and vWD type 3. Type 2 contains various subtypes. Platelet type vWD is also an inherited condition. In 2008 a new diagnostic category of "Low VWF" was proposed to include those individuals whose Von Willebrand Factor levels were below the normal reference range but not low enough to be Von Willebrand Disease.
Immune thrombocytopenia purpura (ITP), also known as idiopathic thrombocytopenic purpura, is a type of thrombocytopenic purpura defined as an isolated low platelet count with a normal bone marrow in the absence of other causes of low platelets. It causes a characteristic red or purple bruise-like rash and an increased tendency to bleed. Two distinct clinical syndromes manifest as an acute condition in children and a chronic condition in adults. The acute form often follows an infection and spontaneously resolves within two months. Chronic immune thrombocytopenia persists longer than six months with a specific cause being unknown.
Thrombotic thrombocytopenic purpura (TTP) is a blood disorder that results in blood clots forming in small blood vessels throughout the body. This results in a low platelet count, low red blood cells due to their breakdown, and often kidneys, heart, and brain dysfunction. Symptoms may include large bruises, fever, weakness, shortness of breath, confusion, and headache. Repeated episodes may occur.
von Willebrand factor (VWF) is a blood glycoprotein involved in hemostasis. It is deficient and/or defective in von Willebrand disease and is involved in many other diseases, including thrombotic thrombocytopenic purpura, Heyde's syndrome, and possibly hemolytic-uremic syndrome. Increased plasma levels in many cardiovascular, neoplastic, and connective tissue diseases are presumed to arise from adverse changes to the endothelium, and may predict an increased risk of thrombosis.
ADAMTS13 —also known as von Willebrand factor-cleaving protease (VWFCP)—is a zinc-containing metalloprotease enzyme that cleaves von Willebrand factor (vWf), a large protein involved in blood clotting. It is secreted into the blood and degrades large vWf multimers, decreasing their activity.
Bernard–Soulier syndrome (BSS), is a rare autosomal recessive bleeding disorder that is caused by a deficiency of glycoprotein Ib (GpIb), the receptor for von Willebrand factor. The incidence of BSS is estimated to be less than 1 case per million persons, based on cases reported from Europe, North America, and Japan. BSS is a giant platelet disorder, meaning that it is characterized by abnormally large platelets.
In medicine, glycoprotein IIb/IIIa is an integrin complex found on platelets. It is a receptor for fibrinogen and von Willebrand factor and aids platelet activation. The complex is formed via calcium-dependent association of gpIIb and gpIIIa, a required step in normal platelet aggregation and endothelial adherence. Platelet activation by ADP leads to the aforementioned conformational change in platelet gpIIb/IIIa receptors that induces binding to fibrinogen. The gpIIb/IIIa receptor is a target of several drugs including abciximab, eptifibatide, and tirofiban.
Ristocetin is an antibiotic, obtained from Amycolatopsis lurida, previously used to treat staphylococcal infections. It is no longer used clinically because it caused thrombocytopenia and platelet agglutination. It is now used solely to assay those functions in vitro in the diagnosis of conditions such as von Willebrand disease (vWD) and Bernard-Soulier syndrome. Platelet agglutination caused by ristocetin can occur only in the presence of von Willebrand factor multimers, so if ristocetin is added to blood lacking the factor, the platelets will not clump.
Platelet membrane glycoproteins are surface glycoproteins found on platelets (thrombocytes) which play a key role in hemostasis. When the blood vessel wall is damaged, platelet membrane glycoproteins interact with the extracellular matrix.
Platelet glycoprotein Ib alpha chain also known as glycoprotein Ib (platelet), alpha polypeptide or CD42b, is a protein that in humans is encoded by the GP1BA gene.
The Harrington–Hollingsworth experiment was an experiment that established the autoimmune nature of the blood disorder immune thrombocytopenic purpura. It was performed in 1950 by the academic staff of Barnes-Jewish Hospital in St. Louis, Missouri.
Glycoprotein Ib (platelet), beta polypeptide (GP1BB) also known as CD42c, is a protein that in humans is encoded by the GP1BB gene.
Glycoprotein IX (platelet) (GP9) also known as CD42a, is a human gene.
Glycoprotein V (platelet) (GP5) also known as CD42d, is a human gene.
The ristocetin-induced platelet aggregation (RIPA) is an ex vivo assay for live platelet function. It measures platelet aggregation with the help of von Willebrand factor (vWF) and exogenous antibiotic ristocetin added in a graded fashion. It is similar to the ristocetin cofactor assay but has the added benefit in that it helps in the diagnosis of type 2B/pseudo von Willebrand disease (vWD) and Bernard–Soulier syndrome because it uses patient's live endogenous platelets, whereas ristocetin cofactor assay tests the function of only the vWF and not the platelets. Ristocetin cofactor assay uses the patient's platelet poor plasma and adds ristocetin and exogenous formalin-fixed platelets which can passively agglutinate. Formalin does not allow the extrinsic platelets to secrete the vWF of their α-granules, and thus only the activity of the intrinsic vWF is tested.
The GPIb-IX-V complex is a profuse membrane receptor complex originating in megakaryocytes and exclusively functional on the surface of platelets. It primarily functions to mediate the first critical step in platelet adhesion, by facilitating binding to von Willebrand factor (VWF) on damaged sub-endothelium under conditions of high fluid shear stress. Although the primary ligand for the GPIb-V-IX receptor is VWF, it can also bind to a number of other ligands in the circulation such as thrombin, P-selectin, factor XI, factor XII, high molecular weight kininogen as well as bacteria. GPIb-IX-V offers a critical role in thrombosis, metastasis, and the life cycle of platelets, and is implicated in a number of thrombotic pathological processes such as stroke or myocardial infarction.
Upshaw–Schulman syndrome (USS) is the recessively inherited form of thrombotic thrombocytopenic purpura (TTP), a rare and complex blood coagulation disease. USS is caused by the absence of the ADAMTS13 protease resulting in the persistence of ultra large von Willebrand factor multimers (ULVWF), causing episodes of acute thrombotic microangiopathy with disseminated multiple small vessel obstructions. These obstructions deprive downstream tissues from blood and oxygen, which can result in tissue damage and death. The presentation of an acute USS episode is variable but usually associated with thrombocytopenia, microangiopathic hemolytic anemia (MAHA) with schistocytes on the peripheral blood smear, fever and signs of ischemic organ damage in the brain, kidney and heart.
Inosine triphosphate (ITP) is an intermediate in the purine metabolism pathway, seen in the synthesis of ATP and GTP. It comprises an inosine nucleotide containing three phosphate groups esterified to the sugar moiety.
References
- ↑ Bode AP, Read MS, Reddick RL (February 1999). "Activation and adherence of lyophilized human platelets on canine vessel strips in the Baumgartner perfusion chamber". J. Lab. Clin. Med. 133 (2): 200–11. doi:10.1016/S0022-2143(99)90013-6. PMID 9989772.
- ↑ McPherson & Pincus: Henry's Clinical Diagnosis and Management by Laboratory Methods, 21st ed., pp. 760–2 (W. B. Saunders, 2006).
- ↑ McMillan R (October 2007). "The pathogenesis of chronic immune thrombocytopenic purpura". Semin. Hematol. 44 (4 Suppl 5): S3–S11. doi:10.1053/j.seminhematol.2007.11.002. PMID 18096470.
