MyoD-lineage cells contribute to open but not closed fracture repair. Histologically, only the occasional hAP+ labeled cell was detected in the early repair matrix in the closed fracture model (arrowhead in A'). No contribution from MyoD cells were seen in the latter stages of closed fracture repair (B-C). In the open fracture model, hAP+ cells resembling mesenchymal cells (arrowheads) and chondroblasts (arrows) were observed in the 1 week fracture template (D, enlarged in D'). Numerous hAP+ cells were visible in the fracture callus at 2 weeks (E) and although reporter expression decreased by 3 weeks, hAP+ labeled cells were still observed (F). No staining was observed in endogenous bone, or in the fracture calluses of Z/AP control mice (G-I). Scale bar = 100 μm.