Some experts predict that doctors will someday use stem cells to treat many different diseases. Yet so far there has been less progress in stem cell therapies than many had hoped.

For years, scientists have studied stem-cell treatment of cancer. Doctors now use stem cells in therapies for several forms of the disease. But at the same time, researchers increasingly are examining a possible connection between stem cells and cancer.

A large number of researchers now point to stem cells as a possible cause of solid tumors. Studies have reported identifying cancer stem cells.

Embryonic stem cells

Stem cells can develop into any kind of cell, like skin, blood or brain cells. Embryos have more stem cells than adults. Embryonic stem cells can develop into all the many different tissues that form the body.

But there are stem cells that remain throughout a person's life to replace cells that become damaged. When a stem cell divides, one of the two cells remains a stem cell. The other becomes a specialized cell.

Many questions remain to be answered. But the general thinking seems to be that cancer stem cells represent a small population of the cells in a tumor. Some researchers think these cancer stem cells have the ability to divide and change into other types of cells. As a result, they think the stem cells help the cancer to metastasize, or spread to other organs.

The researchers also think cancer stem cells are able to repair and feed tumors so they continue to grow. They suspect that the cells are even able to repair damage from radiation treatment and form new tumors. This could explain why some cancers resist drug therapies.

Much of the research into cancer stem cells is being done in California and Canada. Last week, Canadian Health Minister Tony Clement and California Governor Arnold Schwarzenegger announced joint efforts. Their governments agreed to provide one hundred million dollars each over the next three years for research into cancer stem cells.

A California company says it is close to tests in humans of a drug that would directly target cancer stem cells. Two scientists who led the discovery of cancer stem cells in solid tumors, Michael Clarke and Max Wicha, started OncoMed Pharmaceuticals four years ago. OncoMed has signed an agreement with a major drug company, GlaxoSmithKline, to work on the experimental treatment.

And that's the VOA Special English Health Report, written by Caty Weaver. I'm Steve Ember.