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Essential Thrombocythaemia thrombosis (see also section below).69 Increased expression of


P-selectin, thrombospondin and the activated fibrinogen receptor GPIIb/IIIa, have also been demonstrated in ET and show variable correlation with thrombosis.


Currently, the exact pathogenesis of platelet activation in ET, and the other MPDs, is unknown. A large proportion of patients have a deficiency of lipoxygenase, which could increase the availability of endoperoxides to produce TXA2.70


not increased.73


This could be explained by most ET patients receiving antithrombotic drugs at the time of blood sampling, which may affect platelet activation. For example, aspirin inhibits the expression of CD62P and CD63 on platelets.


However, patients with ET can show an increase in CD62E-positive microparticles.74


However, the same patients have a


tendency for haemorrhagic rather than thrombotic diathesis. Alternative explanations for increased platelet activation include an effect of the janus kinase 2 (JAK2)-activating mutation (found in approximately half of patients with ET), interaction of abnormal haematocrit, activated white cells, turbulent flow or an increase in the known priming effect of thrombopoietin due to elevated thrombopoietin levels.71


There is also a suggestion that JAK2 affects


cMPL cell surface localisation and stability, which may have implications for the pathogenesis of platelet activation.72


Platelet Microparticles in Essential Thrombocythaemia Like patients with other thromboembolic diseases, ET patients show higher levels of platelet-derived microparticles than healthy subjects. However, this is not necessarily a consequence of increased platelet number in ET because microparticle numbers do not correlate with platelet numbers in either ET patients or controls. This suggests that microparticle formation may be a regulated rather than a constitutive process. Despite the large proportion of platelet- derived microparticles in ET patients, the actual number of these microparticles with markers of platelet activation (CD62P and CD63) is


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Summary


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