Osteoblasts support MK lineage cell expansion in vitro and in vivo. (A) Quantification of FACS analysis of the fraction of CD41⁺CD42d⁺ mature MK in BM of 3-week-old OSX-TSC1 and OSX-Raptor mice and control littermates (n = 6, Student unpaired t test). (B) Quantification of FACS analysis of CD41⁺Sca-1⁺ MK proliferation rates in BM of OSX-TSC1 and control littermates (n = 6, Student unpaired t test). (C) Quantification of FACS analysis of CD41⁺Sca-1⁺ MK proliferation rates in BM of OSX-Raptor and control mice (n = 6, Student unpaired t test). (D) Quantification of FACS analysis of proliferation rate of CD41⁺Sca-1⁺ and CD41⁺CD42d⁺ cells (n = 3, 1-way analysis of variance [ANOVA]). BM cells were cultured in conditioned medium (CM) from osteoblast, monocyte, BMSC, or endothelial cell culture supernatant for 4 days. The lymphoid and granulocytes were then removed from the CM-stimulated BM cells by direct lineage cell depletion kit (130-110-470; Miltenyi, Bergisch Gladbach, Germany) and the proliferation rate of CD41⁺Sca-1⁺ and CD41⁺CD42d⁺ cells was analyzed by FACS. Data are representative of 3 independent experiments and are represented as mean ± SD. **P < .01; ***P < .001.