Megakaryocyte migration to the endosteal niche is induced by TBI and inhibited by c-MPL deficiency and CD41 blockade. (A) Immunostaining of metaphyseal bone (BO) and BM sections (20×) for CD41-expressing (red) megakaryocytes (black arrowheads) from WT (left column) and mpl^−/− mice (right column) at baseline (top) and at 48 hours post-TBI in the presence (bottom) or absence (middle) of CD41 blockade. (B) Percentage of endosteal megakaryocytes (mean ± SEM) at baseline (no TBI) or 48 hours after TBI in WT and mpl^−/− mice receiving TBI only, anti-c-MPL–blocking antibody (MPL), CXCR4 blockade with AMD3100 (CXCR4), or anti-CD41–blocking antibody (CD41) (n ≥4 mice per group). *P < .05 vs unirradiated group within same strain, +P < .01 vs WT mice receiving TBI only in other TBI-treated groups, ‡P < .05 vs mpl^−/− mice receiving TBI only in groups with specific receptor blockade (comparisons performed by 1-way ANOVA and Dunnett posttest analysis). (C) Immunostaining (20×) for BrdU incorporation (black) in CD41-expressing (red) megakaryocytes (arrowheads) in WT vs mpl^−/− BM 48 hours after TBI. (D) At 48 hours post-TBI in WT BM (left), few BM cells remain, and none with the morphologic appearance of megakaryocytes display TUNEL⁺ (brown stain, black arrow) apoptosis, compared with a ligase-treated positive control serial section (right), with megakaryocytes (arrowheads) present in each section (20×). (E) Total megakaryocyte number per HPF (20×) in the same groups as in (B). +P < .01 vs baseline WT or WT TBI-only group (comparisons performed by 1-way ANOVA and Dunnett posttest analysis).