A) Case of PV showing hypercellular BM with trilineage proliferation of all hematopoietic lineages, including prominent megakaryocytes. (B) Case of PV by immunohistochemistry revealing red-stained neutrophil granulopoiesis and dark-blue nucleated erythropoietic precursors as well as loose clusters of small to large megakaryocytes (C) Case of post-PV myelofibrosis showing decrease in overall cellularity and only a few small and loose erythropoietic islets, abnormal megakaryocytes, and greatly extended stroma compartment (D) Case of post-PV myelofibrosis revealing effacement of hematopoiesis by dense bundles of (yellow-brownish) collagen and reticulin fibers associated with osteosclerosis enclosing few dispersed megakaryocytes