Platelet density distribution in essential thrombocythemia.

Essential thrombocythemia (ET) is characterized by high platelet counts and a slightly increased bleeding risk. Why severe hemorrhage does not occur more frequently is not known. Variations of platelet density (kg/l) depend mainly on cell organelle content in that high-density platelets contain more α and dense granules. This study compares ET patients (n = 2) and healthy volunteers (n = 2) with respect to platelet density subpopulations. A linear Percoll™ gradient containing prostaglandin E(1) was employed to separate platelets according to density. The platelet population was subsequently divided by density into 16 or 17 subpopulations. Determination of platelet counts was carried out. In each density fraction, platelet in vivo activity, i.e. platelet-bound fibrinogen, was measured using a flow cytometer. To further characterize platelet subpopulations, we determined intracellular concentrations of CD40 ligand (CD40L) and P-selectin in all fractions. Patients and controls demonstrated similar density distributions, i.e. 1 density peak. High-density platelets had more surface-bound fibrinogen in conjunction with signs of platelet release reactions, i.e. with few exceptions they contained less CD40L and P-selectin. Peak density platelets showed less surface-bound fibrinogen. These platelets contained less CD40L and P-selectin than nearby denser populations. The light platelets had more surface-bound fibrinogen than peak platelets together with elevated concentrations of CD40L. In ET, the malignant platelet production could exist together with platelets originating from normal megakaryocytes. It is also possible that clonal megakaryocytes produce platelets covering the entire density span. The 'normal' density distribution offers a tenable explanation as to why serious bleedings do not occur more frequently.