Control of BM homeostasis by extramedullary neutrophils. (A) Number of CFUs in culture (CFU-C) at ZT5 in the blood of WT and Fut7^−/− mice; n = 6 mice from two experiments. (B) Levels of CXCL12 protein in the BM of WT and Fut7^−/− mice; n = 6–10 mice from two experiments. (C) Reduced number of CXCL12-producing cells in the BM of Fut7^−/−; Cxcl12^GFP mice, as measured by flow cytometry; n = 5–8 mice from two experiments. Density plot at left shows the gating strategy to identify endothelial cells (EC), osteoblasts (OB), and reticular cells (CAR). (D) Experimental design (left) and number of WT^RED or Fut7^RED-derived circulating progenitors (CFU-C) after 1 mo in parabiosis with WT or the indicated mutant mice. For comparison, the levels of circulating HPCs in WT control parabionts is shown; n = 3–20 mice. Cxcr4^ΔN and Mcl1^ΔN are neutrophil-specific mutants (characterized in Fig. S3). (E) Relative number of CXCL12-producing cells in the BM of Fut7^−/−; Cxcl12^GFP mice after 1 mo in parabiosis with Fut7^−/− or WT mice; n = 8–9 mice from three independent experiments. (F) Micrographs showing GFP⁺ niche cells (green) in the BM of Fut7^−/−; Cxcl12^GFP mice after parabiosis with Fut7^−/− or WT mice. Dashed lines outline the bone surface. Data from three mice per group. Bars, 20 µm. Data shown as mean ± SEM. *, P < 0.05; **, P < 0.01; ***, P < 0.001; n.s., not significant, as determined by unpaired Student’s t test (A, B, C, and E), or ANOVA with Tukey’s multigroup correction (D).