Young Aged Isochronic Heterochronic Isochronic Heterochronic Figure S2 Young Aged Isochronic Heterochronic Isochronic Heterochronic Laminin/BrdU Figure S2 Heterochronic parabiosis restores fiber regeneration in old mice. As in Figure 1, five days after injury, muscles from isochronic and heterochronic parabionts were analyzed for indices of regeneration. BrdU was injected intraperitoneally 2 days after the injury to label proliferating cells. Shown are muscle sections immunostained for laminin (red) and BrdU (green). BrdU+ nuclei were present in regenerating fibers of muscle from young mice. In isochronically paired aged mice, most BrdU+ nuclei were located in the interstitial spaces. However, in aged mice in heterochronic pairings with young mice, the enhanced regeneration (see Fig. 1) was accompanied by an increase in the incorporation of BrdU+ nuclei into regenerating fibers.