An injection of embryonic stem cells helped brain-injured mice perform markedly better on physical tasks but did not improve their cognitive ability, researchers at the University of Pennsylvania School of Medicine have learned.

The study may ultimately have implications for the millions of human beings who have traumatic brain injuries, said Tracy McIntosh, senior author of the study published in this month's issue of the journal Neurosurgery. No known treatment can restore function in damaged areas of the brain, said McIntosh, who directs Penn's Head Injury Center.

Any treatment for people based on this research is years away, he said, but these results are promising because injured mouse and human brains behave similarly.

The Penn researchers compared results in 65 mice. Thirteen were given anesthesia and a surgical incision - mock surgery - but were not injured or treated. In the remaining 52 mice, a part of the cortex that is important for motor skills was damaged. Half of those mice were given an injection of kidney cells, which would not be expected to function in the brain. The other half were injected with about 300,000 embryonic stem cells.

Although the hippocampus, a key center in the brain for memory and learning, was not directly injured, cells there often die when other parts are damaged, McIntosh said. The mice in the study showed cognitive as well as physical impairment.

Embryonic stem cells can differentiate into working cells in any part of the body. Once they have been harvested, a continuing line can be grown easily in a culture, so it's not necessary to repeatedly gather them from embryos, McIntosh said.

In this study, the researchers found that an estimated 30 percent of the injected embryonic stem cells were still alive at three months. Some were functioning as neurons, which transmit nerve signals, and others had turned into glial cells, which supply nerve cells with food and energy.

They had migrated to damaged areas of the brain. "They seem to want to go where they're needed," McIntosh said.

The animals were then given two tests that measured balance and sure-footedness. One required the mice to walk across a rotating pole. Injured mice who had been treated with stem cells made about half as many "foot faults" as those who had not received the treatment.

However, the treated mice were no better at negotiating a maze than those in the control group.

McIntosh said the cells were injected close to where the physical damage to the brain occurred and tended not to migrate to the hippocampus. A future study will measure what happens when the cells are delivered closer to that part of the brain.

In this study, the stem cells were injected three days after the injury occurred.