Introduction Neoplasia of mature myeloid cells most notably megakaryocytes Most commonly seen in older adults (5th/6th decade) Result of a JAK2 kinase mutation Megakaryocyte hyperplasia results in fibrotic obliteration of the bone marrow result of increased production of PDGF Presentation Symptoms splenomegaly secondary to extramedullary hematopoiesis bleeding megakaryocytes are dysfunctional thrombosis platelets can rupture which stimulates coagulation cascade Evaluation Peripheral blood smear tear-drop RBC membrane is disrupted when RBC passed through fibrosis to leave bone marrow nucleated RBCs band granulocytes