Inside the human brain, branching neurons grow beside, around and on top of one another like trees in a dense forest. Scientists used to think that any neurons that wilted and died from injury or disease were gone forever because the brain had no way to replace those cells. By the 1990s, however, most neuroscientists had accepted that the adult brain cultivates small gardens of stem cells that can turn into mature neurons.
Researchers are still trying to determine exactly how often these stem cells become new neurons and how well these differentiated cells survive and join established brain circuits. Some evidence suggests that the brain's neural stem cells help the organ heal itself in modest ways—helping to replace small populations of neurons that suffocated during a stroke, for example. But this minimal self-repair does not restore the millions of neurons lost to stroke, traumatic brain injury and neurodegenerative diseases such as Alzheimer's and Parkinson's.