Splenomegaly and extramedullary megakaryopoiesis. (A) Mean (± standard deviation) mass of the spleen freshly dissected from adult BUBR1^+/– and wild-type mice. The mean body mass for these animals is also shown. (B) The spleen from BUBR1^+/– and wild-type mice were fixed, embedded in paraffin, sectioned, stained with H&E, and examined by light microscopy at low (scale bar = 0.2 μm) and high magnification (scale bar = 5 nm) (upper and lower panels, respectively). (C) Spleen cells from BUBR1^+/– and wild-type mice stained with propidium iodide (PI) were subjected to analysis of DNA content by flow cytometry. (D) Sections of freshly frozen BUBR1^+/– spleen were stained in situ for AChE⁺ cells. (E) Spleen and peripheral blood cells from BUBR1^+/– and wild-type mice were subjected to flow cytometric analysis of CD41 and Ter119 expression, with an isotype-matched IgG used as a negative control. Data in all panels are representative of 3 independent experiments. These observations were confirmed in multiple pairs of BUBR1^+/– and wild-type littermates.