RENEWED controversy has broken out over the abilities of adult stem cells, whose apparent potential has generated much excitement.
It has long been known that there are different kinds of adult stem cells in the body that give rise to particular tissues: new blood cells, say. But recent studies have shown that a few adult stem cells can transform themselves into a much wider range of specialised cells. The way suddenly appeared open for doctors to turn a patient鈥檚 adult stem cells into matched tissues for treating diseases, avoiding the ethical and practical problems of using embryonic stem cells.
Now two independent groups are challenging these claims. Blood stem cells from bone marrow can only form liver tissue by fusing with existing liver cells, they say. If this is true for all adult stem cells, it means that turning them directly into specialised tissues in the lab, prior to re-implanting them, will be far more difficult than thought. That could rule out some therapies.
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鈥淩ather than bone marrow cells morphing themselves into liver cells, they fuse first with liver cells, then change into liver cells,鈥� says Markus Grompe, whose team at Oregon Health and Science University published its results this week (Nature, DOI: 10.1038/ nature01531). David Russell鈥檚 group at the University of Washington in Seattle also reported similar results (DOI: 10.1038/nature01539).
Both teams studied mice destined to die because they lacked a liver enzyme for breaking down the amino acid tyrosine. But the mice survived when injected with normal bone marrow stem cells, which rapidly colonised their livers. Under the microscope, though, the team saw that the transplanted cells were fusing with existing liver cells instead of forming pristine new liver cells.
Grompe thinks fusion reprograms stem cells to act as liver cells 鈥� and that the same mechanism is responsible for reports of blood stem cells forming other specialised cells. 鈥淚 think this will turn out to be the dominant mechanism.鈥�
But many others disagree. They say such fusion might be peculiar to the liver. By fusing with each other, healthy liver cells can ramp up production of enzymes when facing overloads of a particular toxin or compensate for damaged genes in individual cells, suggests Eva Mezey of the National Institute of Neurological Disorders and Stroke in Maryland.
鈥淭here鈥檚 absolutely no evidence of fusion in vivo in anything other than the liver,鈥� says Mezey. So while she does not dispute Grompe鈥檚 findings, she thinks fusion might not occur in other organs.
For instance, Mezey has found that between 2 and 12 per cent of the cheek cells from women who had received bone marrow transplants from men years earlier were of male origin, and so must have come from the transplanted cells. Only 2 out of the 9700 cells examined showed signs of having fused, she reports in this week鈥檚 Lancet.