Originally created 12/30/00

Scientists find two types of stem cells



Canadian scientists have made an intriguing discovery about the mechanics of human blood that is bound to have an impact on everything from cancer treatments to genetic therapies.

Based on a complex experiment 15 years in the making, researchers at the Hospital for Sick Children in Toronto have learned that stem cells - mother cells within bone marrow that give rise to the entire human blood system - are not created equally.

After transplanting human stem cells into laboratory mice, researchers found that one type of stem cell blossoms quickly, but produces new blood only for about a month before disappearing. A second type of stem cell appears to lie dormant for several months before springing into action for a much longer period.

Without knowing that both short- and long-term types of stem cells existed, doctors have never made a distinction between the ones they target when transplanting bone marrow in cancer or anemia patients, or in which cells they try to insert corrective genes, or which stem cells they manipulate in experiments to grow new human tissue and organs.

The Sick Kids finding may, as a result, help explain why many of these treatments have been less than successful. It also offers some hope there will be fewer failures in the future.

"Because we always thought a stem cell is a stem cell is a stem cell, we did not have the knowledge that some of these things would be better suited for long-term re-populating cells than the short-term re-populating cells and vice versa," Toronto geneticist John Dick, who led the Sick Kids research team, said.

"Because the stem cells have different functions, the clinical use of these cells will also be different."

For cancer patients who withstand high chemotherapy doses and then undergo a bone marrow transplant to save their battered blood systems, short-term, fast-acting stem cells could help quickly generate the white blood cells that bolster their vulnerable immune systems.

Meanwhile, researchers interested in chemically coaxing stem cells to grow into various organs and tissues for transplant, would probably be more interested in the potential of long-term stem cells; as would scientists who increasingly view stem cells as an effective agent to administer gene therapies.

This month, for example, Los Angeles researchers began inserting artificial genes that block the AIDS virus into stem cells that will be transplanted into children with AIDS. The idea is that as the stem cells proliferate in the blood system, they will transfer the HIV-resistance to all their offspring cells.

Molecular biologists Ihor Lemischka of Princeton University and Craig Jordan of the University of Kentucky, who reviewed the Canadian research to be published in the January issue of Nature Immunology, wrote that the work was ""the most compelling characterization of human stem cells to date."

This is no small feat, since human stem cells are a rare find to begin with: Just one in every million bone-marrow cells is a stem cell. What's more, under a microscope, stem cells look like any other cell. The only way to identify them is to find a way to monitor them in action.

As the reviewers put it: "Finding and characterizing the relatively rare stem cell is literally the scientific equivalent of looking for a needle in a haystack."