Researchers are making progress towards building useful muscle tissue from scratch, suitable for transplant: "exercise is a key step in building a muscle-like implant in the lab with the potential to repair muscle damage from injury or disease. In mice, these implants successfully prompt the regeneration and repair of damaged or lost muscle tissue, resulting in significant functional improvement. ... For the study, small samples of muscle tissue from rats and mice were processed to extract cells, which were then multiplied in the lab. The cells, at a rate of 1 million per square centimeter, were placed onto strips of a natural biological material. The material, derived from pig bladder with all cells removed, is known to be compatible with the body. Next, the strips were placed in a computer-controlled device that slowly expands and contracts - essentially 'educating' the implants on how to perform in the body. ... The next step was implanting the strips in mice with about half of a large muscle in the back (latissimus dorsi) removed to create functional impairment. While the strips are 'muscle-like' at the time of implantation, they are not yet functional. Implantation in the body [prompts] further development. ... The scientists compared four groups of mice. One group received no surgical repair. The other groups received implants prepared in one of three ways: one was not exercised before implantation, one was exercised for five to seven days, and one had extra cells added midway through the exercise process. The results showed that exercising the implants made a significant difference in both muscle development and function. ... The implant that wasn't exercised, or pre-conditioned, was able to accelerate the repair process, but recovery then stopped. On the other hand, when you exercise the implant, there is a more prolonged and extensive functional recovery. Through exercising the implant, you can increase both the rate and the magnitude of the recovery."