Survival and proliferation of implanted mesoangioblasts in injured tibialis anterior (TA) muscle of Rag2 γ-chain null mice. Shown are representative sections 48 hours after intra-muscular injection with nuclear (n)lacZ- mesoangioblasts in (A-F) PBS or (G-L) polyethylene glycol-fibrinogen (PF). Graft survival is documented by X-Gal (blue) and laminin (red) staining. The results show higher lacZ-positive cell engraftment in TA treated with the PF mesoangioblasts (G,J) than with the PBS mesoangioblasts (A,D). The high-magnification regions (black squares) reveal the localization of lacZ-positive nuclei; these are at the centre of the host’s regenerating muscle fibers (black arrowheads) in the TA muscle treated with the PF mesoangioblasts (J), whereas they are mainly located in the extracellular matrix in the TA muscle treated with PBS mesoangioblasts (D). Proliferation and apoptosis was assessed by staining with 5-bromo-2-deoxyuridine (BrdU;green) (B,H) and terminal dUTP nick-end labeling (TUNEL; red) (C,I); both sets include a nuclear counterstain with 4,6-diamidino-2-phenylindole (DAPI). The decrease in apoptosis in TA sections treated with PF mesoangioblasts (I) is striking compared with sections treated with PBS mesoangioblasts (C). High-magnification regions (white arrows) of the BrdU- and TUNEL-labelld sections imaged by fluorescence under phase-contrast microscopy show proliferating and apoptotic mesoangioblasts in PBS (E,F) and PF (K,L), juxtaposed with the regenerating host muscle fibers. Scale bar: (A,B,C,G,H,I) 500 μm, (D,E,F,J,K,L) 40 μm, (insets) 50 μm.