Long-term engraftment of mesoangioblasts in PBS and of polyethylene glycol-fibrinogen (PF)-embedded mesoangioblasts injected intramuscularly into injured tibialis anterior (TA) muscle. Sections of injured TA from Rag2 γ-chain null mice after 1, 3, and 5 weeks, respectively of treatment with nuclear (n)lacZ mesoangioblasts in PBS (A-C) or in PF (E-G) stained with X-Gal (blue) and laminin (red). Histological analyses revealed a higher number of lacZ-positive cells in TA treated with the PF mesoangioblasts, compared with TA treated with the PBS mesoangioblasts. (H) High-magnification views of X-Gal and laminin staining showing the localization of lacZ-positive nuclei at the periphery of the host’s mature regenerating muscle fibers (arrow) in TA injected with PF mesoangioblasts. (D) The muscle treated with PBS mesoangioblast presented lac-Z positive cells mainly located in the extracellular matrix (arrow) of the TA muscle fibers. The histograms show the number of nlacZ-positive nuclei detected in five randomly selected fields of different, non-adjacent sections (n = 3 mice per group) of X-Gal/laminin-stained TA. (I,J) Mean ± SD of nlacZ-positive nuclei (I) in the whole TA (cell engraftment evaluation) and (J) inside myofibers (cell integration evaluation). Black bars indicate mesoangioblasts injected in PBS, and white bars indicate mesoangioblasts injected in PF, analyzed at 1, 3, and 5 weeks after treatment. Differences were significant (P<0.05) by ANOVA. Scale bar: (A–C,E–G) 500 μm, (D,H) 20 μm.