Abnormally high numbers of megakaryocytes and areas of packed platelets in bone marrow and spleen of Yall;Mpl^−/− mice. (A) Sections of bone marrow and spleen show an increase in the number of megakaryocytes in Yall;Mpl^−/− (right) compared with wild-type (left) mice. Aggregates of small platelet-like particles (brown color) are only found in Yall;Mpl^−/− mice and are more prominent in the spleen. (B) Electron microscopy reveals densely packed platelets in spleen sections of Yall;Mpl^−/− mice. Large arrows indicate the boundaries of platelets; the small arrow points at a dense granule within a platelet. No such platelet clusters were found in wild-type mice (data not shown). Images were taken using an Axioscope 2a microscope with a Planar Neofluar objective (40×/0.75 NA) and an AxioCam HR digital camera (all from Carl Zeiss, Jena, Germany). The software for image acquisition was AxioVision Rel. 4.6.