Platelets

Platelets are small anucleate cells produced by cytoplasmic budding from bone marrow megakaryocytes ( Fig. 16.1 ). Megakaryocyte growth and platelet production are largely controlled by thrombopoietin (TPO) , a growth factor produced by the liver. The TPO level is constant and does not vary with platelet count. The TPO receptor ( c-mpl gene) is expressed on both megakaryocytes and platelets. Because under normal conditions the platelet mass greatly exceeds the megakaryocyte mass, the amount of TPO available for binding to megakaryocytes is limited. In situations in which the platelet mass is reduced, such as thrombocytopenia, more TPO is available for binding to megakaryocytes, which stimulates megakaryocyte growth and increased platelet production. Increased platelet production is marked by release into blood of numerous young platelets that are relatively large with abundant cytoplasmic RNA. These platelets, known as reticulated platelets, have an increased mean platelet volume and can be enumerated by automated complete blood count instruments after staining with the fluorescent RNA-binding dye thiazole orange.

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Large megakaryocyte (center) with a multilobated nucleus and irregular cytoplasmic borders (marrow aspirate smear and Wright-Giemsa stain).

On Wright-stained peripheral smears, platelets appear as small, blue-gray particles with purple-red granules about one-tenth the size of red blood cells (RBCs; Fig. 16.2 ). The normal platelet count ranges from 150,000 to 400,000/μL, yielding a ratio of platelets to RBCs on a peripheral smear of about 1 to 20. About one-third of all platelets are sequestered in the spleen. Platelets in the splenic pool freely exchange with circulating platelets and can be immediately released to the blood in response to epinephrine (from stress). Splenomegaly of any cause is often accompanied by thrombocytopenia because of increased size of the splenic pool. The normal platelet lifespan is 8 to 10 days. Aged or damaged platelets are removed primarily by the spleen, with contributions by the liver and bone marrow. The age-associated loss of terminal sialic acid residues on platelet surface glycoproteins exposes mannose residues that are recognized by mannose receptor bearing macrophages, leading to phagocytosis. In addition, nonimmune immunoglobulin (Ig) G bound to aged or damaged platelets leads to ingestion by IgG (Fc) receptor-bearing macrophages.

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Peripheral blood smear with several small, blue platelets and many larger red blood cells (Wright stain).

Clotting is initiated at the site of vascular injury not only by factor VII surface activation on damaged endothelial cells by tissue factor but also by platelet activation . Circulating platelets bind to subendothelial collagen and von Willebrand factor (VWF) exposed by loss of endothelial cells via platelet surface glycoprotein receptors GPIa/IIa and GPIb/IX , respectively, in a process termed platelet adhesion ( Fig. 16.3 ). Exposure to collagen, adenosine diphosphate (ADP) , and thrombin induces shape changes in adherent platelets with release of platelet granule contents and expression of the high-affinity fibrinogen–VWF receptor GPIIb/IIIa . The platelet shape change results from calcium-dependent contraction of actin–myosin bundles within the platelet cytoplasm.

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Initiation of platelet adhesion. ADP, Adenosine diphosphate; TXA2, thromboxane A2; VWF, von Willebrand factor.

Nearby platelets bind to adherent platelets via fibrinogen and VWF bridges in a process termed platelet aggregation ( Fig. 16.4 ). Phosphatidyl serine residues exposed on the surface of activated platelets serve as sites for calcium-dependent binding of the vitamin K–dependent coagulation factors (II, VII, IX, and X). Aggregated platelets at the site of a clot further contribute to thrombosis by binding circulating tissue factor-containing vesicles; releasing ADP, factor V, and fibrinogen; and providing a phospholipid surface receptive to binding of coagulation cofactors Va and VIIIa and subsequent production of factor Xa and thrombin by membrane-bound IXa-VIIIa and Xa-Va complexes, respectively.

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Platelet adhesion and aggregation. VWF, Von Willebrand factor.

Platelets contain several granules that are released upon platelet activation ( Fig. 16.5 ). Alpha granules contain procoagulants (factor V, fibrinogen, VWF, and PF4) and growth factors ( platelet-derived growth factor [PDGF] and transforming growth factor β [TGF-β]). Platelet factor 4 (PF4) contributes to clot formation by inactivating endogenous heparan sulfate at the site of endothelial cell injury. (PDGF) enhances vascular repair by stimulating fibroblast and smooth muscle cell growth. Both platelet-derived TGF-β and vascular endothelial growth factor induce vessel wall repair by stimulating endothelial cell and myofibroblast proliferation. Dense granules contain ADP, serotonin, and calcium. ADP is important in the recruitment and activation of platelets at the site of platelet adhesion. Serotonin minimizes bleeding at sites of injury by inducing vasoconstriction. Calcium (Ca ²⁺ ) induces platelet shape change by stimulating contraction of the actin–myosin cytoskeleton. Calcium also serves as an essential co-factor in the coagulation cascade by bridging vitamin K–dependent factors with platelet membrane phospholipid and as co-factor for phospholipase C , leading to the production of platelet thromboxane A2 , a potent inducer of both platelet aggregation and vasoconstriction. Arachidonic acid is formed from platelet membrane phospholipid by phospholipase C. Arachidonic acid is converted to endoperoxides by the platelet enzyme cyclooxygenase , which are then converted to thromboxane A2 by thromboxane synthase. Aspirin (acetylsalicylic acid) inhibits platelet activation by irreversibly acetylating cyclooxygenase, thus preventing formation of thromboxane A2 by platelets. Endothelial cells inhibit platelet activation (and vasoconstriction) by converting endoperoxides to the potent platelet inhibitor (and vasodilator) prostacyclin through the action of the enzyme prostacyclin synthase . Endothelial cell prostacyclin activates platelet adenylate cyclase , increasing the intracellular concentration of cyclic adenosine monophosphate (cAMP) , which, by reducing intracellular calcium, inhibits platelet activation. Similarly, the antiplatelet drug dipyridamole , a phosphodiesterase inhibitor, inhibits platelet activation; it increases the level of platelet cAMP by inhibiting the cAMP-degradative action of phosphodiesterase. Lysosomes contain several proteolytic enzymes, including acid phosphatase, collagenase, and elastase. Microperoxisomes contain catalase , an enzyme that neutralizes toxic hydrogen peroxide released by leukocytes.

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Platelet structure and function. ADP, Adenosine diphosphate; PDGF, platelet-derived growth factor A2; VWF, von Willebrand factor.