megakaryocyte

Megakaryocytes arise from the common myeloid progenitor and undergo endomitosis (repeated DNA replication without cell division), resulting in large polyploid cells (up to 64N) with a single multilobulated nucleus. Thrombopoietin, produced primarily by the liver and kidney, is the main regulator of megakaryocyte maturation and platelet production by binding to the c-Mpl receptor. Mature megakaryocytes extend long cytoplasmic processes called proplatelets into bone marrow sinusoids, which fragment into individual platelets (thrombocytes) released into circulation. Each megakaryocyte can generate 1,000–3,000 platelets. Clinically, megakaryocyte number and morphology are assessed in bone marrow biopsies to differentiate causes of thrombocytopenia: decreased/absent megakaryocytes suggest marrow failure (e.g., aplastic anemia, chemotherapy effect), while increased megakaryocytes with normal or increased platelet destruction suggest peripheral causes (e.g., ITP, where megakaryocytes are increased but platelets are destroyed peripherally by autoantibodies). Megakaryocytes are also central to myeloproliferative neoplasms—particularly essential thrombocythemia, where megakaryocytes are increased and large with hyperlobulated nuclei, and primary myelofibrosis, where atypical megakaryocytes cluster and release cytokines (e.g., PDGF, TGF-β) promoting marrow fibrosis. Mutations in JAK2, CALR, or MPL genes drive these myeloproliferative disorders by activating megakaryocyte proliferation pathways independent of TPO regulation. Understanding megakaryocyte biology is essential for interpreting bone marrow findings in cytopenias and myeloproliferative neoplasms on Step 1 and Step 2.