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.
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.
platelet (plat'let) [Gr. plate, flat]
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
for illus.; megakaryocyte
for illus.; thrombopoietin
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.
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.
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.
- 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
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
the adherence of platelets to any area with damaged blood vessels; an important component of hemostasis.
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.
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).
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.
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.
transfusion of fresh, nonchilled whole blood is the usual method of transfusing platelets to an animal with thrombocytopenia.