Donor cells found at fracture and bone formation sites. (A) Visualization of donor cells by fluorescence in situ hybridization (FISH). Magnification of a single cell (small panel; 100×/0.75 NA oil objective) showing multiple chromosomal signals. Cells clustered at callus fractures and at sites of active bone formation (hematoxylin and eosin [H&E] staining; middle). (B) Human-specific OP staining in the callus area at site of new bone formation. (C) DAB staining revealing OP deposited in the primary spongiosa below the growth plate (GP; H&E staining, middle) and (D) within the matrix of the callus. (E,F) Visualization of donor cells (circled) by immunostaining using human-specific vimentin antibody and Cy3-conjugated secondary in the bone marrow cavity (BM) of the diaphysis (H&E staining, middle). (G,H) Donor cells (vimentin staining) clustered at site of bone formation in the periosteum (H&E staining, middle; G) and in the endosteum (DAB staining; H). (I,J) OP-positive staining within trabecular bone using immunofluorescence (I) and DAB staining (J).