ET is usually characterized by hypercellular bone marrow, including increased megakaryocytes. Other features include giant megakaryocytes, clusters of megakaryocytes, or other dysplastic megakaryocytic changes. Bone marrow reticulin is usually increased, although collagen fibrosis is rare. The image is a bone marrow biopsy showing megakaryocytic hyperplasia and clustering. These features are consistent with ET. The arrows in the image point to a cluster of megakaryocytes and an abnormal, multi-lobulated megakaryocyte.
Secondary or reactive thrombocytosis, polycythemia vera, and other disorders can have similar features. Therefore, additional criteria must be considered along with bone marrow findings when diagnosing ET.
