Scientists have succeeded in regenerating a living organ for the first time. Researchers rebuilt the thymus -- an organ in the body located next to the heart that produces important immune cells. The advance could pave the way for new therapies for people with damaged immune systems and genetic conditions that affect thymus development. The team reactivated a natural mechanism that shuts down with age to rejuvenate the thymus in very old mice. After treatment, the regenerated organ had a similar structure to that found in a young mouse.