Megakaryocytopoiesis in Mpl^+/+ c-Myb^Plt4/Plt4 mice. (A) Reduced modal ploidy in megakaryocytes from Mpl^+/+ c-Myb^Plt4/Plt4 mice. The propidium iodide (PI) fluorescence intensity, representing cellular DNA content, in CD41⁺ bone marrow cells is shown. (B) Reduced size of megakaryocyte colonies in cultures of Mpl^+/+ c-Myb^Plt4/Plt4 bone marrow. The number of cells in each acetylcholinesterase-positive megakaryocyte colony was scored by microscopic examination of stained cultures.