Retention efficiency of transplanted progenitor-enriched cells in nonirradiated and irradiated trabecular or compact bone. Irradiation was associated with a markedly reduced efficiency of BM seeding. This is depicted as number of recovered donor cells/bone section, normalized to 1 × 10⁶ donor cells. Seeding efficiency in trabecular bone was approximately 6-fold higher than in compact bone. α4 integrin-deficient progenitor-enriched cells showed an overall reduced seeding, confirming all prior data on impaired homing of α4 integrin-deficient cells. All columns and cross bars shown represent mean values ± SEM; *P < .05.