Researchers have found evidence that stem cells circulating in the bloodstream can grow new tissue in the liver, gut and skin.
The study is the latest to suggest the versatility of adult stem cells, the body's manufacturing equipment for new tissue. In adults _ unlike in embryos _ they have long been thought to be programmed to make just one kind of cell, such as liver or brain, depending on where they are located.
Stem cells found in the bone marrow were once believed to make only new blood cells. But recent studies have upset this dogma, finding that they apparently develop into a variety of cells throughout the body.
Stem cells in the blood are virtually identical to the bone marrow variety. The latest work, published in Thursday's New England Journal of Medicine, suggests they, too, can morph into many different kinds of tissue.
The finding boosts scientists' hopes of one day using stem cells to repair injuries and treat diseases.
Researchers at the University of Texas M.D. Anderson Cancer Center in Houston examined tissue samples collected from 12 cancer patients after stem cell transplants and found evidence they manufacture new tissue in unlikely places within two weeks.
Six women got transplants from a brother, so the researchers looked for male cells in tissue taken from the women's liver, gastrointestinal tract and skin. They found cells with a male Y chromosome accounted for up to 7 percent of the female samples.
``The school of thought for many years was that stem cells only make cells of their own tissue. This has changed and things are pretty much upside down now,'' said Dr. Martin Korbling, one of the researchers.
He said it is possible that blood stem cells could one day be used to help repair or replace tissue in various organs. It is easier to collect stem cells from the blood than from the bone marrow, he noted.
Dr. Janis L. Abkowitz of the University of Washington cautioned that the research is not definitive because of technical limitations, and that it will be far more difficult to show that the reprogrammed cells actually function.
``It would be very surprising as a stem cell biologist ... if they saw a difference between one source or another,'' she said. ``But from a practicality standpoint, it's much nicer and easier if one is ever thinking of a therapeutic benefit to collect'' from a donor's blood than to remove bone marrow.