A recent paper from researchers at the medical center is raising fresh questions about the developmental potential of adult stem cells. This research, published in the Sept. 5 issue of Science Express, an advance online publication of the journal Science, highlights differences of opinion in the scientific community over whether stem cells taken from adults can fulfill the same potential as stem cells taken from embryos.

The debate over the developmental potential of adult stem cells could weigh heavily on the fate of research in embryonic stem cells. Some policy-makers want to ban embryonic stem cell research in favor of using less-controversial adult stem cells — a decision that’s possible only if adult stem cells can form all tissue types.

In the Science Express paper, Irving Weissman, MD, the Karel and Avice Beekhuis Professor of Cancer Biology, and postdoctoral fellow Amy Wagers, PhD, tried — and failed — to coax adult blood-forming stem cells in mice to form tissues other than blood and immune cells.

"This is the first time somebody injected a single adult stem cell and showed that it made only blood," Weissman said. His findings coincide with those of other studies in which researchers reported being unable to induce adult stem cells into forming different types of adult tissues.

However, other scientists report that they have succeeded in turning adult stem cells into a variety of tissues in mice. Among them is Stanford researcher Helen Blau, PhD, the Donald E. and Delia B. Baxter Professor of Pharmacology. In her lab, Blau has seen adult stem cells form brain, muscle and other cell types.

Despite their differences, Weissman and Blau both urge caution in banning an entire field of research before scientists reach an agreement about the meaning of their results.

"Stem cells may hold tremendous potential for treating disease," Blau said. "But the work is in the early stages and we need further research to settle these discordant results." She added that even if adult blood-forming stems cells do form other adult tissues — as her work suggests — that’s no reason to eliminate research with embryonic stem cells. "All the research should go on in parallel — we don’t know for which disease which approach will be the best," she said.

In their recent paper, Weissman and Wagers studied whether stem cells taken from adult mice could integrate into adult tissues. They first isolated adult blood-forming stem cells from the bone marrow. These cells were engineered to make a green fluorescent protein that’s visible under a microscope. The researchers then injected a single stem cell into mice whose bone marrow had been knocked out by irradiation. (Bone marrow produces all blood and immune cells.)

After several weeks, the green fluorescent stem cell had single-handedly repopulated the blood and immune cells of the mice. When the researchers searched through more than 15 million muscle, brain, liver, kidney, gut and lung cells, they found only one brain and seven liver cells that were green under the microscope, indicating that these cells were either formed from the adult stem cell or that one of the stem cell’s progeny had fused with the original cell.

"The important thing is that this paper teaches people in the lay community that you need to wait before you jump to judgment," said Weissman. "Especially when you are jumping to a political judgment that has big policy repercussions." Another advocate for embryonic stem cell research, Paul Berg, PhD, the Robert W.and Vivian K. Cahill Professor of Cancer Research, Emeritus, and a Nobel Laureate, said that based on Weissman’s work it’s questionable whether blood-forming adult stem cells can differentiate into other tissues.

"That doesn’t mean that there aren’t cells in the bone marrow that can form other types of cells," Berg said. He sees value in pursuing both adult and embryonic stem cell research.