From the Tartan: "As for regrowing whole limbs, Badylak is unsure of when technology will be advanced enough for such a feat. 'That's an impossible question really to answer. I do think that we will be able to stimulate the regrowth of at least digits. The only way we will be able to do that is by understanding the signaling mechanisms that occur in the fetus, when these sorts of structures are normally developed. In a fetus if you amputate these structures at an early enough stage, the fetus will regrow a limb. Yet we lose that ability as we develop into a newborn.' The hardest obstacle of regenerative medicine lies within the cellular communication system. Badylak explained that to regrow a tissue, the cell requires the right kind of signal to start, so that it can in turn stimulate the right cells. The biggest challenge is finding the master switch, a molecular signal which will tell the injured tissue to respond with the means to regrow instead of just heal, and afterward, finding out how activate it. These switches are genes that need to be activated. When activated, the genes can then express proteins that can trigger a cell regeneration cascade."

Link: http://thetartan.org/2008/10/13/scitech/stemcells