Background. Essential thrombocythaemia (ET) is amyeloproliferative disorder characterized by augmented platelet counts. In ET major bleedings occur infrequently but they are more common than in healthy objects. Platelet density (kg/L) depends mainly on the cell organelle content and ‘‘more dense’’ platelets are believed to be ‘‘more reactive’’. We examined biochemical characteristics of platelet density subpopulations in ET (n=2) and healthy blood donors (n=2) served as controls.

Methods. A linear 50 ml isotonic PercollTM gradient was employed to separate platelets obtained from venous whole blood according to density. The gradient contains prostaglandin E 1 to prevent in vitro platelet activity. Subsequently, the platelet population was divided into 16 or 17density-dependent subpopulations. In each fraction determination of platelet counts and intracellular concentrations of CD40 ligand and P-selectin, as measures of platelet -granule content was carried out. To estimate the degree of in vivo of platelet activity, platelet bound fibrinogen was analysed in all fractions using a flow cytometrytechnique.

Results. All subjects demonstrated platelet populations with one density peak. Both ET and controls displayed measurable high-density subpopulations.These platelets circulate in an activated state. As a consequence they have more platelet bounding fibrinogen and lower intracellular concentrations ofCD40-ligand and P-selectin. Peak density platelets displayed lower -granule content and their in vivo activation status was lower than the neighbouring subpopulations having higher and lower platelet density, respectively. In all subjects the lightest platelets showed elevated -granule content and they circulated in an activated form.

Conclusion. ET patients resemble healthy blood donors with respect to the platelet density distribution. The study indicates that the normal thrombo-cytopoiesis exists in parallel with the malignant platelet production. However, alternative explanations exist such as the possibility that malignant megakaryocytes produce platelets of different density. The normal platelet density distribution may explain why major bleedings do not occur more frequently in ET. The finding warrants further research.