megakaryocytes

Megakaryocytes arise from hematopoietic stem cells in the bone marrow via a process called megakaryopoiesis, regulated by thrombopoietin (TPO), which binds the c-Mpl receptor. Unlike most cells, megakaryocytes undergo endomitosis (repeated DNA replication without cell division), becoming polyploid (up to 64N) with a single multilobulated nucleus. As they mature, their cytoplasm develops an extensive demarcation membrane system, and they extend long proplatelet processes into bone marrow sinusoids, which fragment to release platelets into circulation. Platelets are essential for primary hemostasis, forming platelet plugs at sites of vascular injury. Clinically, megakaryocyte number and morphology are assessed on bone marrow biopsy to evaluate thrombocytopenia or thrombocytosis. Decreased or dysfunctional megakaryocytes are seen in conditions like aplastic anemia, myelodysplastic syndromes, and chemotherapy-induced marrow suppression. Increased megakaryocytes with normal/increased platelet destruction are characteristic of immune thrombocytopenic purpura (ITP), where megakaryocytes are normal/increased but peripheral platelet destruction leads to low platelet counts. Megakaryocytes are also central to myeloproliferative neoplasms—essential thrombocythemia features abnormal, large megakaryocytes with hyperlobulated nuclei, often associated with JAK2, CALR, or MPL mutations. Primary myelofibrosis also involves atypical megakaryocyte hyperplasia with release of fibrogenic cytokines (e.g., PDGF, TGF-β) causing bone marrow fibrosis. Congenital disorders such as thrombocytopenia with absent radius (TAR syndrome) and Wiskott-Aldrich syndrome also involve megakaryocyte abnormalities affecting platelet production.