From Scientific American:
A new study finds that neural stem cells may be able to save dying brain cells without transforming into new brain tissue, at least in rodents. Researchers from the University of California, Irvine, report that stem cells rejuvenated the learning and memory abilities of mice engineered to lose neurons in a way that simulated the aftermath of Alzheimer’s disease, stroke and other brain injuries.
Researchers expect stem cells to transform into replacement tissue capable of replacing damaged cells. But in this case, the undifferentiated stem cells, harvested from 14-day-old mouse brains, did not simply replace neurons that had died off. Rather, the group speculates that the transplanted cells secreted protective neurotrophins, proteins that promote cell survival by keeping neurons from inducing apoptosis (programmed cell death). Instead, the once ill-fated neurons strengthened their interconnections and kept functioning.