Staining with factor VIII of bone marrow from patients with (A) MM at relapse, (B) MM at plateau, (C) MGUS, and (D) a control subject (patient with pernicious anemia). Note in (A) numerous microvessels, whereas in (B), a microvessel and some rare endothelial cell clusters and in (C), (D) the lack of vessels in the presence of strongly stained megakaryocytes. Bar, (A) to (C) 40 μm; (D) 55 μm.