platelet

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platelet

 [plāt´let]
the smallest of the formed elements in blood, a disk-shaped, non-nucleated blood element with a fragile membrane, formed in the red bone marrow by fragmentation of megakaryocytes. Platelets tend to adhere to uneven or damaged surfaces, and there are an average of about 250,000 per mm3 of blood. The bone marrow produces from 30,000 to 50,000 platelets per mm3 of blood daily, which means that in any ten-day period all the platelets in the body are completely replaced. Called also thrombocyte.

The rate of platelet formation seems to be governed by the amount of oxygen in the blood and the presence of nucleic acid derivatives from injured tissue. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood. Their primary functions are related to coagulation of blood. Because of their adhesion and aggregation capabilities they can occlude small breaks in blood vessels and prevent escape of blood . They also are able to take up, store, transport, and release serotonin and platelet factor 3. Abnormally high numbers of platelets occur in the presence of malignancy, splenectomy, asphyxiation, polycythemia vera, and acute infections. A very low count can occur as a result of idiopathic thrombocytopenic purpura, pernicious anemia, and allergic conditions, and during cancer chemotherapy. Many drugs can cause a toxic decrease in the number of platelets.
Platelet response to vascular injury. From Malarkey and McMorrow, 2000.
platelet factors factors important in hemostasis which are contained in or attached to the platelets: platelet factor 1 is adsorbed coagulation factor V from the plasma; platelet factor 2 is an accelerator of the thrombin-fibrinogen reaction; platelet factor 3 is a lipoprotein with roles in the activation of both coagulation factor X and prothrombin; platelet factor 4 is capable of inhibiting the activity of heparin.

plate·let

(plāt'let),
An irregularly shaped, disclike cytoplasmic fragment of a megakaryocyte that is shed in the marrow sinus and subsequently found in the peripheral blood, where it functions in clotting. A platelet contains granules in its central part (granulomere) and, peripherally, clear protoplasm (hyalomere), but no nucleus, is about one third to one half the size of an erythrocyte, and contains no hemoglobin.
[see plate]

platelet

/plate·let/ (plāt´let) thrombocyte; a disk-shaped structure, 2 to 4 μm in diameter, found in the blood of mammals and important for its role in blood coagulation; platelets, which are formed by detachment of part of the cytoplasm of a megakaryocyte, lack a nucleus and DNA but contain active enzymes and mitochondria.

platelet

(plāt′lĭt)
n.
A minute, nonnucleated, disklike cytoplasmic body found in the blood plasma of mammals that is derived from a megakaryocyte and functions to promote blood clotting. Also called blood platelet, thrombocyte.

plate·let

(plāt'lĕt)
An irregularly shaped, disclike, cytoplasmic fragment of a megakaryocyte that is shed in the marrow sinus and subsequently found in the peripheral blood, where it functions in clotting. Contains granules in the central part (granulomere) and, peripherally, clear protoplasm (hyalomere), but no definite nucleus; is about one third to one half the size of an erythrocyte.
See also: plate
Synonym(s): blood disc, elementary particle (1) , thrombocyte, thromboplastid (1) .

platelet

(plat'let) [Gr. plate, flat]
Enlarge picture
PLATELET PLUG FORMATION AND CLOTTING
A round or oval disk, 2 to 4 µm in diameter, found in the blood of vertebrates. Platelets number 130,000 to 400,000/mm3. They are fragments of megakaryocytes, large cells found in the bone marrow. Synonym: thrombocyte See: illustration; blood for illus.; megakaryocyte for illus.; thrombopoietin

Function

Platelets contribute to chemical blood clotting and to other aspects of hemostasis. Platelet factors are the chemicals released by platelets to initiate the first stage of (intrinsic pathway) chemical clotting. When a capillary ruptures, platelets adhere to each other and to the cut edges of the vessel, forming a platelet plug. Blood clotting may be beneficial (e.g., in preventing blood loss from wounds) or may be harmful when it occurs within arteries or veins inside the body (e.g., during coronary thrombosis). Blood clotting is a positive feedback cascade that may continue and occlude an unbroken vessel.

Disorders

Thrombocytopenia (reduced platelet count) occurs in acute infections, anaphylactic shock, and certain hemorrhagic diseases and anemias. Thrombocytosis (increased platelet count) occurs after operations, esp. splenectomy, and after violent exercise and tissue injury.

illustration

platelet

A fragment of the CYTOPLASM of a MEGAKARYOCYTE 2–4 mm in diameter. Each megakaryocyte produces 1000 to 3000 platelets, which are present in large numbers in the blood-50,000 to 300,000 per cu. mm. Platelets survive for about 10 days and play an essential part in blood clotting. Platelet plasma membranes contain a range of glycoproteins by means of which they bind to different materials including collagen, fibrinogen and von Willebrand factor. Platelets are by no means the passive tissue fragments they were formerly thought to be. They carry many granules and a canalicular system by which the granules are released. They also have a dense tubular membrane system in which prostaglandins and thromboxanes are synthesized. Platelet granules contain heparin-neutralizing factor, von Willebrand factor, smooth muscle growth factor and fibrinogen. Deficiency of platelets is known as thrombocytopenia.

platelet

see BLOOD PLATELETS.

Platelet

An irregularly shaped cell-like particle in the blood that is an important part of blood clotting. Platelets are activated when an injury causes a blood vessel to break. They change shape from round to spiny, "sticking" to the broken vessel wall and to each other to begin the clotting process.

Deetjen,

Hermann, German physician, 1867-1915.
Deetjen bodies - a disklike cytoplasmic fragment found in the peripheral blood where it functions in clotting. Synonym(s): platelet

Bizzozero,

Giulio, Italian physician, 1846-1901.
Bizzozero corpuscle - an irregularly shaped disklike cytoplasmic fragment of a megakaryocyte found in the peripheral blood; functions in clotting. Synonym(s): platelet

Hayem,

Georges, French physician, 1841-1933.
Hayem hematoblast - an irregularly shaped, disklike cytoplasmic fragment of a megakaryocyte found in the peripheral blood where it functions in clotting. Synonym(s): platelet
Hayem solution - a blood diluent used prior to counting red blood cells.
Hayem-Widal syndrome - obsolete term for acquired hemolytic icterus. Synonym(s): Widal syndrome

Zimmermann,

Karl W., German histologist, 1861-1935.
polkissen of Zimmermann - mesangial cells that fill the triangular space between the macula densa and the afferent and efferent arterioles of the juxtaglomerular apparatus. Synonym(s): extraglomerular mesangium
Zimmermann corpuscle - Synonym(s): Zimmermann elementary particle
Zimmermann elementary particle - a disklike cytoplasmic fragment found in the peripheral blood where it functions in clotting. Synonym(s): platelet; Zimmermann corpuscle; Zimmermann granule
Zimmermann granule - Synonym(s): Zimmermann elementary particle

platelet

non-nucleated, granular, cell fragment (derived from bone marrow megakaryocytes) within blood (~400 M/mL); they trigger inflammation (by releasing vasoactive amines), facilitate healing (by releasing a number of growth factors) and become enmeshed in fibrin mesh (to form a clot)

plate·let

(plāt'lĕt)
An irregularly shaped, disclike, cytoplasmic fragment of a megakaryocyte that is shed in the marrow sinus and subsequently found in the peripheral blood, where it functions in clotting.
Synonym(s): blood disc, elementary particle (1) , thrombocyte, thromboplastid (1) .

platelet (plāt´let),

n a disc found in the blood of mammals that is involved in the coagulation and clotting of blood.
platelet ac-globulin,
platelet activating factor,
n 1-0-alkyl-2-sn-glycero-3-phosphocholine. A phospholipid derivative formed by platelets, basophils, neutrophils, monocytes, and macrophages. It is a potent platelet-aggregating agent and inducer of systemic anaphylactic symptoms, including hypotension, thrombocytopenia, neutropenia, and bronchoconstriction.
platelet aggregation,
n a clumping together of platelets in vitro, and likely in vivo, by a number of agents, such as adenosine diphosphate, thrombin, and collagen, as part of a sequential mechanism leading to the initiation and formation of a thrombus or hemostatic plug.
platelet aggregation inhibitors,
n.pl the drugs or agents that antagonize or impair any mechanism leading to blood platelet aggregation.
platelet cofactor I,
platelet cofactor II,
platelet count,
n the number of platelets found in 1 mm3 of blood; the normal range is between 200,000 and 300,000 platelets.
platelet-derived growth factors (PDGF),
n a type of protein released by platelets of the blood that aid in the repair and regeneration of connective tissue.
platelet disorder,
n See disorder, platelet.
platelet-rich plasma (PRP),
platelet transfusion,
n 1. the transfer of blood platelets from a donor to a recipient or reinfusion to the donor.
n 2. a treatment modality used in treating hemophilia and other conditions of impaired blood coagulation.

platelet

a small disk or platelike structure, the smallest of the formed elements in blood. Blood platelets (called also thrombocytes) are disk-shaped, non-nucleated blood elements with a very fragile membrane; they tend to adhere to uneven or damaged surfaces. They average about 250,000 per cubic millimeter of blood and are formed in the red bone marrow by fragmentation of megakaryocytes, the largest of the bone marrow cells. Platelet production is controlled by a hormone, thrombopoietin, and regulatory lymphocytes acting at the stem cell level. At any given time about one-third of the total blood platelets can be found in the spleen; the remaining two-thirds are in the circulating blood.
The functions of platelets are related to the clotting of blood. Because of their adhesion and aggregation capabilities platelets can occlude small breaks in blood vessels and prevent the escape of blood. Platelets which have adhered to exposed collagen in damaged vessels release ADP in milliseconds which in turn initiates the synthesis of thromboxane A2, a very potent prostaglandin which causes platelet aggregation and localized vasoconstriction. Fibrinogen, factors V and VIII, calcium ions, platelet phospholipid (PF-3), associated with the platelet membrane are also released. Substances contained within the platelet granules such as thromboglobulin, heparin neutralizing activity (PF-4) mitogens such as platelet derived growth factor, thrombospondin, ADP, serotonin and calcium ions are also released by aggregated platelets.

platelet-activating factor (PAF)
see platelet-activating factor.
platelet adhesion
the adherence of platelets to any area with damaged blood vessels; an important component of hemostasis.
platelet aggregation
the progressive accumulation of platelets, attracted by other platelets once adhesion begins. Thromboxane A2 causes irreversible platelet aggregation.
platelet aggregation test
a known platelet aggregating factor such as collagen, ADP or thrombin is added to a suspension of the platelets under test and the degree of aggregation measured by decrease in turbidity of the suspension.
platelet count
may be performed directly (in a hemocytometer chamber) or indirectly (estimating from the stained blood smear by number per field or in comparison to the number of white blood cells), expressed as number of cells per liter of blood.
platelet-derived growth factor
one of three growth factors released by platelets which undergo the release reaction; the growth factors stimulate endothelial cell proliferation.
platelet distribution width (PDW)
an indication of variation in platelet size which can be a sign of active platelet release.
platelet factor 3 (PF-3) test, platelet release test
test the antiplatelet activity of serum; used to detect circulating antiplatelet antibodies. Antibody-antigen reactions involving platelets cause the release of PF-3 from platelets which in turn shortens the contact-activated clotting time of platelet-rich plasma (PRP).
platelet factors
factors important in hemostasis which are contained in or attached to the platelets: platelet factor 1 is adsorbed clotting factor V from the plasma; platelet factor 2 is an accelerator of the thrombin-fibrinogen reaction; platelet factor 3 is a phospholipid with potent procoagulant activity; platelet factor 4 is capable of inhibiting the activity of heparin (heparin neutralizing activity).
mean platelet volume (MPV)
elevated level is an indication of increased megakaryocyte shedding of platelets and decreased level is seen in thrombocytopenia.
platelet plug formation
see platelet aggregation (above).
platelet release reaction
measured by the degree of secondary ADP-mediated aggregation that occurs. This is assessed by the amount of PF-4, PF-3 or serotonin, etc. released.
platelet retention
tested by testing the adhesiveness of a suspension of the subject platelets to a glass bead column or standard size filter.
platelet rich plasma
plasma prepared by centrifugation to separate out red blood cells but not platelets for transfusion.
platelet storage-pool disease
an inherited autosomal thrombopathia in American foxhounds and cats characterized by a deficiency of platelet storage granules.
platelet transfusion
transfusion of fresh, nonchilled whole blood is the usual method of transfusing platelets to an animal with thrombocytopenia.