But humans can’t make an entire new arm. “There must be something in humans that prevents the regenerative process from going that far,” Gardiner says.
Some scientists think it may be an evolutionary tradeoff.
“If an amphibian chews off one of its arms, it could hide away for weeks without eating and regenerate,” Enrique Amaya, a developmental biologist at the University of Manchester in Great Britain, recently told BBC News. “That’s out of the question for an animal whose high metabolic rate requires it to feed constantly. It has to heal quickly and dirtily.”
NUI-Galway’s Frank, whose research examines heat-shock proteins and Wnt signaling, which are found in both stem cells and tumors, thinks that humans and other mammals may lose their embryo-like regenerative ability because it would make them more vulnerable to developing cancers.
“Because these (embryonic-like stem) cells are so versatile, it is difficult to keep them under control,” Frank explains. “They are more likely to ‘misbehave' or form tumors than differentiated cells. We hypothesize that only animals that have very simple body plans, like Hydractinia, can manage this problem because they have less complex organs and 'misbehaving' cells are less of a problem. But complex animals, like humans, need better control of their cells to maintain their highly complex organs. They have to get rid of them during early development before they become too complex.”
Gardiner, however, suspects that humans may still have the latent capability to grow limbs and organs -- and he thinks that scientists eventually may come up with a way to turn certain switches on or off to reactivate their regenerative capabilities, without unleashing the cancer process. He says that recent advances, such as researchers’ discovery in 2007 of how to convert differentiated cells back into induced pluripotent stem cells, have eliminated what many once thought were insurmountable barriers to regeneration.
As Gardiner explains, growing new human limbs or organs may be a matter of providing a different set of genetic instructions to our cells -- essentially, a blueprint that would show guide them in creating various types of differentiated cells and organizing them into a structure.
“When you look at blastemas (at the cellular level), they look just like tumors -- except that they stop and differentiate, and form an arm,” Gardiner says. The difference, he explains, “ is the information that’s controlling the growth and patterns. “
Some skeptics have argued that growing a new human arm would be such a time-consuming process that it would be impractical. Gardiner, however, disagrees.
“Salamander arms are just as complex as human arms,” he notes. “What’s important is that you need structure to regenerate. The fibroblasts (a type of cell that forms the framework for tissue) make the blueprint in the salamander ... I think that over the long term we’ll be able to regenerate something like a salamander can, but to do that, we’ll need to figure out the information grid.”
Skeptics also have argued that growing an adult human arm or another organ might take so many years that it would be impractical. Gardiner, again, disagrees. “How long does it take to make a baby’s arm to grow? Probably, a couple of months. What would happen would be that you’d grow back a baby-sized arm—there seems to be a limit on how big you can regenerate.”
Afterward, however, the arm might be programmed at the cellular level to grow to adult size more rapidly than the rest of your body parts did. “A salamander only regenerates a small arm, too, but it grows back faster than the rest of the animal grows, so that it catches up.”
It’s difficult to say how soon scientists might be able to unlock and program the human regenerative process, Gardiner says, because no one yet knows how many stages are involved. “There might be only two or three steps, in which case it might take us 10 years,” he speculates. “But if there turn out to be a lot more steps, it might be 50 years.” But he expects that someday it will happen.