A crucial research is challenging the widely held view that adult human brains make new neurons. The study of brain samples from59 people of various ages found no immature neurons in anyone older than 13,scientists report online Wednesday in the journal Nature.
New research led by the University of California,San Francisco suggests that the human hippocampus, or the brain region that is important for memory and learning, stops making new cells in childhood.
New findings, which have been published in the journal Nature, are likely to fan the flames of an already heated debate about the human brain's capacity to heal itself through "neurogenesis," or the birth of new brain cells.
"We find," explains Arturo Alvarez-Buylla, who is a professor of neurological surgery working at the University of California, San Francisco (UCSF) and the head of the laboratory behind the study, "that if neurogenesis occurs in the adult hippocampus in humans, it is an extremely rare phenomenon, raising questions about its contribution to brain repair or normal brain function."
The research challenges a growing body of evidence that suggests that it may be possible to treat brain-wasting disorders such as Alzheimer's disease by promoting neurogenesis to replenish the brain cells, or neurons,which are destroyed by disease.
However,the undaunted authors point instead to the new questions that their findings rise,such as how does the human brain adapt and learn if it cannot make new neurons?
Perhaps the answers to this question will open an even better path to new treatments for brain diseases, they suggest.
Does neurogenesis occurs in humans brains?
Over the past 30 years, Prof. Alvarez-Buylla and his team have been finding increasing evidence that the brains of songbirds and rodents can make new neurons throughout their lives. However, more recently, they have begun to question whether this is true of the human brain.
Now the new finding challenges decades of research suggesting that new neurons continue to appear in the hippocampus, an area of the brain involved in memory and emotion. It also calls into question claims that both exercise and some anti-depressant drugs could boost the production of new neurons in the hippocampus.
The finding is bound to be controversial. "I'm sure that there are going to be people who challenge [our results]" says Alvarez-Buylla, a professor of neurological surgery at UCSF.
For example, the UCSF group has cast doubt on whether neurogenesis in the olfactory bulb — which is an ancient part of the brain that is important for sensing odors — continues into adulthood in humans as it does in rodents.
For their new study, Prof. Alvarez-Buylla and colleagues analyzed 59 human hippocampus samples collected from China, Spain, and the United States. Some of the tissue samples — which ranged from before birth into adulthood — we retaken from postmortems, while others were retrieved during surgery on epilepsy patients.
The team carefully analyzed the samples to look for changes in new neurons and stem cells over the lifespan. They focused on an area known as the dentate gyrus, a "simple cortical region" that forms an integral part of the hippocampus and is important for memory formation.
Study found no evidence of neurogenesis in adult brain cells
The researchers found prolific evidence of neurogenesis occurring before birth and right after it. They calculated that the average number of new neurons per square millimeter of dentate gyrus tissue in newborns was 1,618.
However,neurogenesis fell sharply after birth: the number of new neurons per square millimeter had reduced fivefold by age 1.
The evidence also revealed that the decline continued through childhood: there was a 23-fold decrease between the ages of 1 and 7 years, and another fivefold reduction by the age of 13.
At this point, by early adolescence, the concentration of new neurons per square millimeter of brain tissue had fallen to just 2.4.
The researchers found no evidence of neurogenesis in adult dentate gyrus tissue —in neither the 17 postmortem samples nor in those retrieved from 12 adult epilepsy patients.
"In young children," says Mercedes Paredes, an assistant professor of neurology at UCSF and who co-led the tissue analysis, "we were able to see that substantial numbers of new neurons continue to be made and integrated into the dentate gyrus, but neurogenesis fades away completely by early adolescence."
Can neurogenesis play a major role in plasticity?
While they analyzing the neural stem cells which are the precursor cells that spawn new neurons — the researchers discovered that they, too, were abundant in the brain before birth, but by early childhood, they had almost disappeared.
The research team also found no evidence of early accumulation of neural stem cells in the sub-granular zone of the human dentate gyrus.
This is in contrast to mice, where this early concentration does happen. The team suggests that this means that it might be a necessary step for neurogenesis to continue into adulthood.
Finally,the investigators accept that, while they searched extensively, they cannot say for certain that the adult human hippocampus never makes new neurons.
"But,"says Dr. Shawn Sorrells, who is a senior researcher in Prof. Alvarez-Buylla's group, "I think that we need to step back and ask what that means."
"If neurogenesis is so rare that we can't detect it, can it really be playing a major role in plasticity or learning and memory in the hippocampus?" Dr.Shawn Sorrells
Furthermore, if the results hold up, scientists will have to figure out how the adult human brain can continue to change throughout life without adding new cells, the way a rodent brain does.
One clue is that it can take many years for a new human brain cell to mature, Snyder says, an assistant professor at the University of British Columbia who wrote a commentary accompanying the study.So even a grown-sup's brain may contain many cells whose functions aren't yet fully defined.
"Cells that were born in childhood could play a big role in learning, in memory and in emotional disorders," Snyder says. "So their impact on adult brain life could still be big."
It also may be possible to use drugs or other therapies to get an old brain to start making new cells, Snyder says. That might help repair the damage caused by a brain injury or Alzheimer's disease, he says.
"I think the possibility is real that someday we'll be able to rejuvenate the aging brain," Snyder says.