Representative bone marrow biopsies from patients with MPNs. (A) ET: Normocellular marrow, proliferation of giant megakaryocytes with hyperlobulated nuclei, scattered or in loose clusters (hematoxylin and eosin [H&E], original magnification ×40). (B) PV: Hypercellular marrow with erythroid proliferation and scattered pleomorphic megakaryocytes (H&E, original magnification ×20). (C) PMF: Hypercellular marrow with granulocytic proliferation and large megakaryocytes with atypical bulbous nuclei (H&E, original magnification ×40). (D) Overt PMF: Hypercellular marrow, proliferation of atypical megakaryocytes forming dense clusters, and dilated vessels with intraluminal hematopoiesis (H&E, original magnification ×40). (E) Overt PMF (collagen fibrosis): Bands of collagen fibrosis within hematopoietic lacunae (Masson trichrome staining, original magnification ×40). Courtesy of Emanuela Boveri, Fondazione Istituto di Ricovero e Cura a Carattere Scientifico Policlinico San Matteo, Pavia, Italy.