STANFORD, Calif., Sept. 5 (UPI) -- Scientists said Thursday new research suggests a type of adult stem cells found in bone marrow is not as plastic or as capable of becoming as many different cell types as previously believed, casting doubt on their potential to treat disease.
The research could have implications for the debate over whether to pursue adult stem cell research over the much more controversial embryonic stem cell research -- which requires destruction of embryos to obtain stem cells that have the capability of becoming any cell in the body and potentially treating various diseases.
Previous research suggested adult blood-forming stem cells found in bone marrow could give rise to brain, heart and muscle tissue but the new findings show that is "just not true," Irving Weissman, a cancer biologist at the Stanford University Medical Center and lead author of the study, told United Press International.
"Blood-forming stem cells from adults make blood," Weissman said. "They don't make brain, they don't make heart muscle or any of these things."
Some research has suggested the plasticity of adult stem cells is limited but more recent studies have shown a rare type of adult stem cell could be coaxed into becoming many different types of cells and thus have nearly the same potential for replacing damaged tissue and treating diseases as embryonic stem cells.
Opponents of embryonic stem cell research have cited such studies to argue only research with adult stem cells should be pursued since they appear to have the potential to differentiate without the controversy of using embryos.
Weissman argued his study refutes that notion and instead shows the plasticity of adult stem cells is so limited their potential for treating disease is questionable. He was quick to point out this does not mean adult stem cell research should not be pursued but it does suggest it certainly does not hold the same potential as embryonic stem cell research.
Adult stem cell research "is really important," he said, "but it's got a long way to go."
Ronald Green, professor of religion and director of the ethics institute at Dartmouth College in Hanover, N.H., who has closely followed the stem cell debate, agreed.
"This is certainly confirmation that at this time we may not be able" to coax adult stem cells into becoming all the different cell types in the body, Green told UPI. "But the real take home message is that we have to keep both pathways of research open."
Adult stem cells could one day prove just as capable as embryonic stem cells of becoming all the different cell types, but if they do not and only adult stem cell research is allowed, then "we may find ourselves at a dead end for many diseases three or four years out," Green said.
That length of delay could be devastating to millions of people with incurable diseases that potentially could be treated with embryonic stem cells, he noted.
Edward Scott, director of the Program in Stem Cell Biology at the University of Florida in Gainesville, told UPI the new findings "don't rule out developing a therapy based on adult stem cells, but it's not going to be as easy as just doing a transplant and forgetting about it."
Scott noted growth factors and other stimulators -- which were not used in the new study -- may be necessary to spur adult stem cells into becoming different cell types.
"Adult stem cells like to do their normal thing unless they get some sort of extra outside signal," he said.
Weissman noted, however, even using stimulating factors, no one has yet been able to get adult blood-forming stem cells to become brain, heart or muscle tissue.
In the study, Weissman's group obtained adult stem cells from the bone marrow of mice. These stem cells give rise to cell types that make up the blood and immune system.
The cells were engineered to express a fluorescent green protein so they could be identified. A single cell was injected into mice that had their bone marrow -- which generates cells of the blood and immune system -- knocked out by irradiation. The single cell replaced the blood and immune system cells over a period of several weeks but only eight other green cells were identified -- one in the brain and seven in the liver.
It is not clear whether the adult stem cells actually gave rise to the brain and liver cells or simply fused with cells already in those areas. But even assuming all eight cells were generated by the adult stem cell, it clearly is not enough production to warrant using adult stem cells as a therapy to treat disease, Weissman said.
In another experiment, the researchers found adult stem cells did not give rise to new intestinal tissue when it had been destroyed by radiation.
The study appears in Thursday's edition of Science Express, an advance online edition of the journal Science.
(Reported by UPI medical correspondent Steve Mitchell in Washington.)
