The problem is not specialization, but formation of correct shape. Our cells have instruction how to build the system from the ground up, but they does not have instructions how to repair major injuries, so once leg lost it cannot be rebuilt.

Indeed, even just a bone 'knowing' how to heal back into the right shape, this kind of healing power kind of has a nice body-instruction-manual.. and good thing too, or every fracture would immediately heal-wrong before you had a chance to put a cast on it.