Abstract
A histological picture in pretreatment bone marrow trephine biopsy is an essential part of Ph-negative myeloproliferative neoplasm diagnosis according to WHO classification. Polycythaemia vera is histologically defined as a hypercellular trilinear myeloproliferation.
Hypercellular haematopoiesis with granulocytic and megakaryocytic proliferation is typical for primary myelofibrosis. In essential thrombocythaemia the haematopoiesis is normocellular with proliferation of megakaryocytes only.
The most important differential diagnostic features are morphology and distribution of megakaryocytes, and presence of fibrosis. In primary myelofibrosis there are typically "dysplastic" megakaryocytes forming tight (dense) clusters, and variable extent of fibrosis, while mature megakaryocytes forming loose clusters and no fibrosis are found in essential thrombocythaemia.
In reactive thrombocytosis and erythrocytosis the number of normally appearing megakaryocytes is not increased and they are not forming clusters. Prodromal (latent) phases of myeloproliferative neoplasms often unrecognized by recent WHO classification criteria are discussed as well as a differential diagnosis of myeloproliferative disorders associated with thrombocytosis.