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Background: The behavior of host and graft cells during the healing process after autologous tendon graft has not been elucidated. Hypothesis: Host cells will integrate into the bone-tendon interface and contribute to cellular repopulation of the graft. Study Design: Controlled laboratory study. Methods: Twelve-week-old, genetically identical, female green fluorescent protein transgenic rats (n = 20) and wild-type rats (n = 20) were used. The rats were divided into 2 experimental groups. In group A, the Achilles tendons of wild-type rats were harvested and transplanted into the transcondylar femoral bone tunnels of green fluorescent protein rats. In group B, the Achilles tendons of green fluorescent protein rats were transplanted into a transcondylar femoral bone tunnel of wild-type rats. Immediately after transplantation (time zero) and at 1, 2, and 4 weeks after the transplantation, distal femoral epiphyses were harvested and cut into 14-μm serial sagittal frozen sections. The sections were examined with a confocal laser-scanning micro-scope to quantify green fluorescent protein-positive cell survival. Results: At time zero, only host cells in group A and only graft cells in group B demonstrated green fluorescent protein signals. At 1 week in group A, many green fluorescent protein-positive cells were found in the graft. In group B, a few green fluorescent protein-positive cells were found in the graft. At 2 and 4 weeks in group A, many green fluorescent protein-positive cells were detected in the graft, but green fluorescent protein-positive cells had disappeared completely in group B. Conclusion: Host cells, rather than graft cells, contribute to repair of the bone-tendon interface and the remodeling of grafts after simulated autologous tendon graft. [ABSTRACT FROM AUTHOR]