BM biopsy histology in early PMF in low-power (×170) or medium-power (×350) field. (A) Survey with hematoxylin and eosin-stained section shows increase in age-matched cellularity and conspicuous clusters of atypical megakaryocytes. (B) Chloroacetate esterase reaction discloses that increased cellularity is mostly the result of neutrophil granulopoiesis (red) and prominent clustering of large, abnormal megakaryocytes with paratrabecular dislocation. (C) PAS staining reveals not only loose and tight clustering but also striking abnormalities of megakaryocytes showing large hyperchromatic-hypolobulated, cloud-like nuclei. (D) Reticulin stain displays minimal increase with a fine network of fibers between the clustered abnormal megakaryocytes. Images were acquired using Zeiss Axioplan 2, 20×/0.50 and 40×/0.60 EC PlanNeofluar.