A good demonstration of the state of practical tissue engineering for muscles and connective tissue can be found at ABC Online: "The researchers used a synthetic scaffold seeded with ligament cells to regenerate the new tissue in the damaged anterior cruciate ligament (ACL) of rabbits. The bunnies were able to begin bearing weight on their knees 24 hours after surgery, and by the end of the 12-week experiment, the animals had fresh collagen and blood vessels growing in the damaged area. ... The ACL is the stabilising ligament that connects the thighbone to the leg bone. It unravels like a plait when ruptured, making healing difficult. In humans, the standard treatment for this is reconstructive surgery. Surgeons remove healthy tissue from tendons around the knee and graft it onto the damaged ligament to regenerate it. But it can take five to six months for a full recovery, and surgeons would prefer not to harvest healthy tissue if possible. Researchers have tried to craft ligament-like scaffolds to help the healing process before, but success has been limited. This is the first time that researchers have combined synthetic materials with ACL cells and been able to substantially engineer new ligament tissue."