Myeloproliferation and altered BM niches in aged Sipa1^−/− MPN mice. The PB, BM, and spleen from the age- and sex-matched aged (16-17 months old) Sipa1^+/+ and Sipa1^−/− mice were collected for analyzing both hematopoiesis and BM niches. Statistical analysis was performed by unpaired Mann-Whitney U test. (A) Total white blood cells (WBCs) in 16-month-old Sipa1^+/+ and Sipa1^−/− mice. (B) Myeloid cells in the PB of the Sipa1^+/+ and Sipa1^−/− male and female mice. (C) A representative splenomegaly (right) of the aged Sipa1^−/− mice. (D) Hematoxylin and eosin (H&E) staining of dysplastic MKs in the Sipa1^−/− mouse BM. Scale bars represent 50 μm. (E) The increased numbers of MKs in the Sipa1^−/− mouse BM. The data are expressed as numbers per squared millimeters. (F) FACS profiles for phenotypic analysis of BM stromal cells in the Sipa1^+/+ and Sipa1^−/− mice. The CD44⁻ cells were first gated within CD45⁻TER119⁻CD31⁻ cells and then subdivided into SCA1⁺CD51⁺ MSCs, SCA1⁻CD51⁺ MPCs, and SCA1⁻CD51⁻ mature stromal cells. (G) Altered stromal cell composition in the BM of 16-month-old Sipa1^−/− mice. Data are percent of the cells within total CD45⁻TER119⁻CD31⁻ cells from 8 independent experiments. (H) CFU-F frequencies in whole BM cells. The right panel shows the difference in the frequencies of CFU-Fs observed between the female or male Sipa1^+/+ and Sipa1^−/− mice. (I) Multilineage differentiation potentials of the Sipa1^−/− BM MSCs. Scale bars represent 250 μm (left), 500 μm (middle), and 100 μm (right). n = 3 independent sorting experiments. (J) μCT images of femurs in the aged Sipa1^+/+ and Sipa1^−/− female mice. Scale bars represent 1.0 mm. (K) Femoral bone volumes of Sipa1^+/+ and Sipa1^−/− mice. The statistical difference was determined by Mann-Whitney U test (A) or unpaired Student t test (B-E,G-H). See also supplemental Figure 1. Cont, control.