Many ethical and political issues could be averted if adult stem cells prove to be as successful at differentiating into all tissue types in the human body as their embryonic counterparts are. Numerous studies have so far reported success at coaxing adult stem cells into various cell types under different conditions. But a report published online today by the journal Science suggests otherwise. The findings indicate that stem cells cultivated from adult blood may resist change more strongly than previously thought.

Amy J. Wagers and her colleagues at the Stanford University School of Medicine isolated blood-forming stem cells from adult mice in order to test their plasticity. To track the adult stem cells' transformation, the team genetically engineered them to produce a green fluorescing protein that is easily identifiable under a microscope (see image). When mice whose bone marrow had been destroyed by radiation received a single stem cell, green-glowing cells replaced the animals' missing blood and immune cells within a few weeks. A comprehensive search of more than 15 million muscle, brain, liver, kidney, gut and lung cells, however, found only one brain and seven liver cells that appeared green. As a result, the scientists suggest that the capabilities of adult stem cells so far have been overstated. Says Wagers: "It's not to say that nobody should think about adult stem cell plasticity--of course people should look into it--but it's not as robust as it is claimed to be."