Keating, at the Novartis Institutes for Biomedical Research in Cambridge, Massachusetts, believes stem cells can ordinarily undertake only very limited repairs of organs like the liver and heart, and the scarring often seen in these tissues is a fallback mechanism put in place when the stem cells' capacities are exceeded.
If the genes that boot up the zebra fish blastema also exist in people but are not switched on, perhaps some drug might goad them into action. Once a blastema had been induced at a wound site in the body, regeneration researchers suggest, it might regrow the missing limb or organ with no further intervention required. "Maybe there are residual abilities that could be enhanced" in mammals, said Shannon Odelberg, a University of Utah researcher who studies regeneration in the newt with the goal of inducing blastemas to form in mammals.
Emerging field
Regeneration is studied in only a few laboratories. It was not even on the agenda of the research planning meeting held last October by the California Institute of Regenerative Medicine, which was dominated by stem cell biologists. One reason for this orphan status is that the animals used by most biologists, like the roundworm, the fruit fly and the mouse, are ones that do not regenerate.
The genetics of regenerating animals, like the salamander, are largely unknown, so the process of regeneration has received little attention from research biologists. But there is a group of vertebrates that can regenerate very successfully, said Brockes. "It would be rather surprising if there weren't some interesting and important lessons one could learn from them."
Keating believes the expense of stem cell therapy, should it work, is a major consideration. Even if the approach works, he said, developing cells for every patient who needs them would be very expensive. Switching on the regenerative process with drugs, should that prove possible, would be cheap by comparison.
Robert Weinberg, a biologist at the Whitehead Institute in Cambridge, said therapeutic regeneration was "decades away" because the cells of animals that regenerate are so different from those of people. But he said there is great hope of taking embryonic stem cells and making them yield primitive adult stem cells that still possess regenerative capability. He placed less confi-dence in using fully mature adult stem cells, which may have lost the ability to build new tissue.
In the light of new knowledge, some stem cell biologists are making more guarded predictions about the imminence of stem cell therapy. Ron McKay, an expert on neural stem cells at the National Institutes of Health, noted that stem cells inserted into the developing brain of a fetal animal "become incorporated in an extraordinary way, as if local cues were controlling their behavior."
But in the adult brain, he said, nothing happens, suggesting that the concept of using stem cells to treat Alzheimer's disease is illusory.
Regeneration and stem cell therapy are promising aspects of regenerative medicine but both are still at the research stage. "I'm very bullish on regenerative medicine," said Keating, alluding to both types. "I think it's going to happen and it will be a revolution, but it will take time. It would be a mistake to oversell it and promise too much too early."
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