Previous research has clearly demonstrated adult brain cell regeneration – also known as neurogenesis – in many other species. But until 1998, scientists lacked good evidence that this process occurred in adult humans.
In that year a study of the adult human brain created a stir when it showed the birth of new neurons in the hippocampus, a brain region involved in memory (Nature Medicine, vol p 1313).
To find out if neurogenesis occurs in other adult brain regions, Peter Eriksson at the Sahlgrenska Academy in Gothenburg, Sweden and colleagues looked at the brains of cancer patients who had received injections of a compound known as bromodeoxyuridine, or BrdU. The chemical label is incorporated into the DNA of new cells formed near the time of injection. Doctors had given the patients BrdU to locate areas of uncontrolled cell growth in the body, where cancer lurked.
When Eriksson and colleagues later examined the brains of the patients killed by their cancer, they found a population of BrdU-positive cells in the olfactory bulb, the region at the front of the brain that processes smells. Since the patients ranged in age from about 38 to 70 years, the results revealed that neurogenesis continues in that brain area in adults.
Brain stem cells
Experts say the continued supply of new neurons in this brain region might help people register and remember new smells even when they are older. “It’s most likely that we are relying more on our olfactory system that we believe,” says Eriksson.
The BrdU labelling also helped researchers identify a reservoir of brain stem cells – which develop into neurons – in the fluid-filled chambers called ventricles that sit deep in the brain. By looking at individual brain slices the team also identified a tube in each hemisphere – about 1.5 millimetres in diameter and 35 mm in length – that connected the ventricles with the olfactory region.
“We were amazed when we found this anatomical structure that had gone undiscovered until now,” says Eriksson. Brain cells born in the fluid-filled ventricle migrate to the olfactory bulb via this small “superhighway” that connects them, he says.
The dark side
Other experts note that similar tube-like structures connect these brain regions in rodents. “What has been known for decades to happen in mice and rats is now shown to exist in humans,” explains neuropathologist Sebastian Brandner of University College London, UK. “Stem cells are resting in certain areas of the brain, and the novelty is that they can make their way to the olfactory bulb.”
Scientists stress that a continued supply of new cells through adulthood probably helps to replenish and restore the brain. But they add that these brain stem cells might also have a dark side.
“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,” says Brandner.
Journal reference: Science (DOI: 10.1126/science.1136281)