(A–F) At 14 days, cell patches (dotted lines in A and B) were found by in vivo (A) or ex vivo (B) imaging, GFP fluorescence (C), and anti-GFP staining in cross sections (D). Note positioning of the patches near α-SMA⁺ (red in E) vessels and the surrounding CD45⁺ (red in F) clusters. (G–I) At 14 days, confocal imaging revealed some GFP⁺ cells coexpressing CD31 or α-SMA (arrowheads in G and H, respectively). A limited number of regenerating SkMBs were GFP⁺ (thus donor derived; revealed by anti-GFP staining; I). (J–L) At 5 weeks, mMAPC-U persisted (shown by in vivo imaging, J; and anti-GFP staining, K and L). Note that donor-derived regenerating fibers (GFP⁺ cells in K) were still apparent and that cell number was less than it was at 14 days (compare D and L). (M–P) At 30 days, hMAPC-U stably engrafted (revealed by large human vimentin⁺ [green] cell patches intercalated between muscle fibers [M and N] and as scattered cells around vessels [P]). Cells were detected in the endothelial layer of vessels by a human-specific anti-CD31 antibody (green in O), indicating EC differentiation, and in some cells the human vimentin signal (green) colocalized (yellow; white arrowheads) with α-SMA (red), indicating their SMC identity in P. Images in A, C, D, E, G–J, L, M, O, and P are from adductor; images in B, F, I, and K from gastrocnemius and N from quadriceps muscle. DAPI was used as nuclear counterstain in C and E and M–O and Topro (Molecular Probes) in H and P. Scale bars: 50 μm (G–I, K), 100 μm (C, D, F, L, M, O, and P), 200 μm (E and N), and 500 μm (A, B, and J).