Enlarge Image Direct delivery. Neurosurgeons inject human neural stem cells into bundles of nerve axons, hoping the cells will become myelinating oligodendrocytes. Credit: Kenneth Probst Four young boys with a rare, fatal brain condition have made it through a dangerous ordeal. Scientists have safely transplanted human neural stem cells into their brains. Twelve months after the surgeries, the boys have more myelin—a fatty insulating protein that coats nerve fibers and speeds up electric signals between neurons—and show improved brain function, a new study in Science Translational Medicine reports. The preliminary trial paves the way for future research into potential...