TPO administration before BMT enhances megakaryocyte-mediated expansion of niche osteoblasts and HSC engraftment. (A) Schedule of TPO administration. (B) Immunostained (CD41) sections (20×) comparing 48-hour post-TBI BM endosteal megakaryocytes (red arrowheads) after high-dose TPO (HDTPO, 50 µg/kg), low-dose TPO (LDTPO, 10 µg/kg), or sham treatment. (C) BM sections (20× top, 63× bottom) at 48 hours post-TBI showing increased endosteal osteoblast expansion (black arrowheads) after HDTPO treatment vs LDTPO- or sham-treated mice. (D) Quantitative assessment of 48-hour post-TBI osteoblast layers (mean ± SD) in sham-, LDTPO-, and HDTPO-treated WT mice (n ≥5 mice per group). *P < .005 vs sham-treated group. (E) Absolute BM GFP⁺ engraftment at 24 hours post-BMT (5 × 10⁶ cells) in post-TBI 1° recipients receiving sham, LDTPO, or HDTPO treatment (n = 5 per group). *P < .05 vs sham-treated group. (F) Secondary (2°) recipient GFP⁺ peripheral RBC, myeloid (GR-1⁺), B-cell (B220⁺), and T-cell (CD3⁺) engraftment at 6 weeks after 2° BMT with 2 × 10⁵ WT competitor BM cells and either sham-, LDTPO-, or HDTPO-treated primary (1°) recipient BM at 24 hours after 1° recipient BMT. *P < .02 vs 2° recipients of sham-treated 1° recipient BM. (G) Percentage of 2° recipients of sham, LDTPO, or HDTPO 1° recipient BM with GFP⁺ BM engraftment (>0.25% each) in myeloid, B-cell, T-cell, and Lin^- BM populations at 28 weeks after 2° BMT. *P < .05 vs 2° recipients of sham-treated 1° recipient BM. (H) Representative dot plots of GFP vs Lin in BM at 28 weeks after 2° BMT in recipients of sham- vs HDTPO-treated 1° recipient BM.