Human embryonic stem cells have helped repair stroke-induced brain damage in rats, said Stanford Medical School researchers in a study published this week.

These stem cells can form any cell type in the body, but coaxing them to form neural cells in particular has been tough. The cells also have a tendency to form tumors when transplanted.

To overcome these obstacles, researchers grew the embryonic stem cells in a particular combination of growth factors that induced them to form neural cells only. After six months of growth in the lab, none of the stem cells developed into tumors.

The researchers next took a group of rats and induced a stroke that left the animals with one weak forelimb. Researchers transplanted the stem cells into the brains of 10 of these rats. After two months, the cells were incorporated into the damaged brain tissue. Testing showed those rats who received the stem cell transplants were able to use their damaged forelimbs more normally than untreated rats with the same type of brain damage. None of the rats developed tumors.

750,000 people in the United States suffer from strokes each year, and the study’s senior author Gary Steinberg, a professor of neurology, said he hopes the technique can be implemented in clinical trials for human stroke patients with five years.

The study was published in the Feb. 20 issue of “Public Library of Science-ONE.”