Defective megakaryocytopoiesis in Wdr1^rd/rd mice. (A,B) Hematoxylin and eosin stained spleen sections illustrating megakaryocytosis in the bone marrow (A) and spleen (B) of Wdr1^rd/rd mice. Arrows indicate examples of the fragments of megakaryocyte cytoplasm observed. (C) Anti-CD41 immunofluorescence of frozen sections, demonstrating the presence of many fragments of megakaryocyte cytoplasm in the spleen of Wdr1^rd/rd mice. (D) Serum thrombopoietin levels as measured by enzyme-linked immunosorbent assay in 12-week-old male mice. Horizontal bars indicate the mean. (E) Ploidy distribution of bone marrow megakaryocytes demonstrating a shift toward 32N in Wdr1^rd/rd mice; n equals 12 male mice per genotype. Solid bars represent the mean; errors bars, standard deviation. (F) Transmission electron microscopy studies of bone marrow megakaryocytes. A typical mature wild-type megakaryocyte is shown at left. In contrast, Wdr1^rd/rd megakaryocytes are smaller and exhibit gross abnormalities, particularly failure of the demarcation membrane system to develop, and large peripheral zones devoid of organelles and granules. The Wdr1^rd/rd megakaryocyte shown in the right panel contains no nucleus; this class of cell constituted approximately 25% of the more than 60 individual Wdr1^rd/rd megakaryocytes that were viewed. It may be representative of the abundant cytoplasmic fragments seen in the bone marrow and spleen of Wdr1^rd/rd mice.