Representative bone marrow biopsies from patients with MPNs. (A) ET: normocellular marrow with the proliferation of giant megakaryocytes with hyperlobulated nuclei scattered or in loose clusters (hematoxylin and eosin [H&E]). (B) PV: hypercellular marrow with erythroid proliferation and scattered pleomorphic megakaryocytes (H&E). (C) Prefibrotic PMF: hypercellular marrow with granulocytic proliferation and large megakaryocytes with atypical bulbous nuclei (H&E). (D) Overt PMF: hypercellular marrow, proliferation of atypical megakaryocytes forming dense clusters, and dilated vessels with intraluminal hematopoiesis (H&E). (E) Overt PMF (collagen fibrosis): bands of collagen fibrosis within hematopoietic lacunae (Masson trichrome staining). Original magnifications ×40 (A,C-E) and ×20 (B). Reproduced from Rumi and Cazzola.¹⁵