Feb. 16, 2007 -- In a dramatic new discovery, scientists have found that the adult human brain can create new cells, which could open the door to new therapies to halt and even reverse paralysis and damage from degenerative nerve disease. Researchers caution more study is needed, but say this finding may provide hope to patients suffering from such disorders as multiple sclerosis, muscular dystrophy and Parkinson's.
Until recently, scientists believed the adult brain was incapable of generating new nerve cells, commonly called neurons. But then tantalizing clues emerged from animal research suggesting that rats and monkeys did have the ability form new neurons.
Other mammals were found to have new neurons in the olfactory bulb, the part of their brains used to process smells. If these other creatures used new neurons to smell, researchers reasoned that humans might also have them.
The search for new neurons in the human brain has been difficult. After all, one cannot slice open the skull of a living person to try to pinpoint where the neurons might be born.
But this week, scientists from New Zealand and Sweden report that they not only located this elusive area in the human brain, but they also caught new neurons in the process of forming.
How Did They Do It?
They began their search by dissecting 30 brains of people -- ages 20 to 80 -- who had died and aiming high-powered microscopes at roughly the same area where they had located the new neurons in rat brains. To their surprise, they found streams of cells in the process of dividing, indicating more cells were forming.
They then confirmed the location of the cell streams leading from the center of the brain to the olfactory bulbs (on either side of the forebrain) using magnetic resonance imaging (MRI) in six living patients.
Researchers have a theory about why the new neurons follow a specific path in the brain.
"We suspect [the cells] may be following this tube because the tube is filled with liquid that contains some kind of growth factor or some type of attractant," says Peter Eriksson, a neuroscientist at Sahlgrenska Academy in Göteborg, Sweden, and coauthor of the study appearing in this week's issue of the journal Science.
Researchers found an average of about 110,000 cells at different stages of development along the route. By the time the cells arrived at the olfactory bulbs, they were well on their way to becoming neurons.
"There was some controversy about adult human neurogenesis in the olfactory bulb," says Fred H. Gage, a biologist at the Salk Institute for Biological Studies and a former collaborator with Eriksson on a 1998 paper on neurogenesis (new nerve cell growth) in cancer patients. "But, this recent manuscript is very persuasive."
Pasko Rakic, a neurobiologist at the Yale University School of Medicine, says these findings echo previous work in monkeys.
"This doesn't come as a surprise to me,'' he adds, "because we have seen a present [study], but very small one, compared to rodents, in non-human primates."
Discovery Could Broaden Disease Knowledge
Now that the team has located the path that the new cells follow, Eriksson says researchers can begin to assess whether there is a relationship between the new neurons and degenerative nerve disease.
In their Science paper, the authors note that a reduction in new neurons can dull rodents' sense of smell, a symptom in humans that can be a harbinger of Parkinson's disease.
Eriksson hypothesizes that new neurons are needed for smell because continuous renewal of cells may be needed to process the myriad scent molecules encountered during life.
What is not known is whether new neurons could be dispatched to other parts of the brain if needed. For example, neurons could conceivably be recruited to injury sites if the brain suffers damage.
Eriksson says there is already animal research that shows brain injuries trigger chemical signals to start the growth of new neurons. Next he plans to look for evidence of this in humans.
"The first question," Eriksson says, "would be to see if there are differences between normal, healthy subjects and subjects [who] have had a stroke or some kind of insult [to] the brain to figure out whether we actually do get activation of stem cells also in the human brain."