In a new study, rats were spared the limb-weakening effects of a stroke if they were treated with brain tissue cultivated from human embryonic stem cells. But unlike similar experiments, the transplanted cells gave no sign of causing tumors, according to a report this week in the online journal PLoS One.

Researchers say that if they can build a string of such successes in a range of animal models, they can make a stronger case for testing the cells in people. "This is really exciting, just to overcome this obstacle of tumorigenicity," says Stanford University stem cell biologist Marcel Daadi, a co-author of the study.

Investigators have had success of late creating stem cells, or cells very similar to them, from new sources such as adult human tissue. But the ongoing scientific challenge is to harness those cells' ability to morph into the different adult cell types and thereby develop new treatments for debilitating diseases such as stroke, which strikes about 700,000 Americans every year, according to the U.S. Centers for Disease Control and Prevention.

Daadi and his colleagues transplanted specially grown human neural stem cells—precursors of neurons and other neural cell types—into the brains of rats made to suffer a stroke in the right hemisphere of the brain, which had sapped the strength from their front left paws. Rats that received the transplant recovered strength in the impaired limbs, as judged by a test in which the rodents explore a tube. Animals given a sham injection regained little or none of the lost strength, the group reports.

The researchers found no sign of tumor growth in the brains of the healed rats or after stem cell injections into the bodies of healthy rats. Daadi attributes the success to their way of harvesting neural stem cells from human embryonic stem cells, which he says weeded out unwanted cell types that might grow into tumors.

An application for early human testing of a stroke treatment using cells derived from human fetal brain tissue, developed by the Guildford, England–based stem cell company ReNeuron Group, PLC, is currently on hold with the Food and Drug Administration, pending additional data.