ET is usually characterized by a hypercellular bone marrow, including increased numbers of 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 shown below 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 and polycythemia vera and other disorders can have similar features. Therefore, other criteria must be considered along with bone marrow findings when making a diagnosis of ET.
