No relevant blood abnormalities were observed. Some patients experienced transient nausea and vomiting, and 78 percent had temporary headaches related to the transplant procedure.

"Patients improved by several standard measures, and their improvement was not only statistically significant, but clinically meaningful," Steinberg said. "Their ability to move around has recovered visibly. That's unprecedented. At six months out from a stroke, you don't expect to see any further recovery."

Interestingly, the study showed that the implanted stem cells begin to disappear about one month after the procedure and are gone by two months.

That meant something secreted by these cells during their early postoperative presence near the stroke site stimulates lasting regeneration or reactivation of nearby nervous tissue.

The study also revealed that the stroke patients' postoperative improvement was independent of their age or their condition's severity at the onset of the trial.

"Older people tend not to respond to treatment as well," Steinberg said. "But here we see 70-year-olds recovering substantially."

The researchers also called for new thinking regarding the permanence of brain damage.

"The notion was that once the brain is injured, it doesn't recover -- you're stuck with it," Steinberg said. "But if we can figure out how to jump-start these damaged brain circuits, we can change the whole effect."

Some 800,000 people suffer a stroke each year in the United States alone, with about 85 percent of these strokes being ischemic in nature.

Although approved therapies for ischemic stroke exist, to be effective they must be applied within a few hours of the event -- a time frame that often is exceeded by the amount of time it takes for a stroke patient to arrive at a treatment center.