Researchers have discovered a type of brain cell that continuously regenerates in humans.
A store of regenerating cells has been found in the human brain
A pool of "resting cells" migrate to create new nerve cells in the part of the brain which deals with smell.
The system has been shown in mice and rats but it was believed it did not exist in the human brain.
Experts said the findings, published in Science, opened up the potential for research into repairing brains in conditions such as Alzheimer's disease.
The researchers from the University of Auckland, New Zealand and the Sahlgrenska Academy in Sweden showed stem cells rest in certain areas of the brain, just beneath large fluid-filled chambers called ventricles.
But then they needed to work out how they got to the right part of the brain.
In many species, it was known that a tube filled with brain fluid enabled these cells to travel to the olfactory bulb - the region of the brain that registers smells - turning into nerve cells as they went.
But until now, this system had not been shown in humans.
Using several techniques, including a powerful electron microscope, the team identified the tube, and showed it contained stem cells as well as cells which were gradually turning into nerve cells as they travelled along.
The researchers said the addition of new nerve cells in the olfactory bulb in humans helped the system respond to different stimuli throughout a person's life.
Experts said the findings could be important for future research into brain cell repair in patients with neurodegenerative diseases such as Alzheimer's disease and, importantly, that studies in mice would be applicable to humans.
Dr Mark Baxter, Wellcome Trust senior research fellow at Oxford University, said: "This study is exciting because it reveals a group of brain cells in the adult human brain that are continuously regenerating.
"This opens another direction by which we may discover ways to repair human brains that are damaged from injury or diseases, and underscores the importance of animal research in guiding biomedical research in humans."
Professor Sebastian Brandner, head of the division of neuropathology at the Institute of Neurology, University College London, said it has been known for decades that such cells were present in mice and rats.
"Understanding stem cell biology is essential to study brain repair in neurodegenerative diseases such as Alzheimer's and it is even possible that stem cells are the source of some brain tumours."
Professor Clive Ballard, director of research at the Alzheimer's Society, said: "For the first time, this study demonstrates that stem cells are routinely involved in replenishing nerve cells in at least one part of the adult human brain.
"This process raises exciting new questions for the treatment of Alzheimer's disease, such as whether stem cells could be stimulated into action when the brain has been injured.
"These findings are the first step to unlocking potentially exciting new treatments."