ABSTRACT
Essential thrombocythemia (ET) is a relatively indolent and often asymptomatic clonal myeloproliferative neoplasm that involves predominantly megakaryocytic lineage characterized primarily by a sustained elevation in platelets, megakaryocytosis, and minimal to absent bone marrow (BM) fibrosis. The blood shows an increase in the number of platelets, which may display atypia and anisocytosis. The BM aspirate is normocellular to slightly hyper-cellular with increased megakaryocytes, which often cluster within spicules. Megakaryocytes are large and occasionally giant with mature cytoplasm and irregular nuclei. Chromosomal abnormalities in ET are very rare and most of the patients studied either were in leukemic transformation or had received treatment with cytotoxic agents. The histologic features characteristic of ET includes the presence of staghorn megakaryocytes, normal overall cellularity, and loose megakaryocyte clustering. In contrast, prefibrotic myelofibrosis is characterized by the presence of thighs megakaryocyte clusters; cloudlike, dysplastic, or pyknotic megakaryocytes; and abnormal cellularity.
