New findings fuel debate over whether humans make new nerve cells as adults
An examination of the brains of extraordinarily intelligent older people reveals clues to their unusual abilities. Deep inside these exceptional brains were signs of what some scientists believe were newborn nerve cellsborn to an old age.
The results, published on February 25 in Natureadd data points to the scientific debate whether adults can make new neurons, a process called neurogenesis, and if they can, what those neurons do.
Whether this debate is now settled depends on who you ask – because not everyone agrees that the reported signs come from dividing neurons.
Neuroscientist Orly Lazarov of the University of Illinois at Chicago and his colleagues set out to study how different brains age and what kinds of changes might keep some people alert for decades. Their study focused on brain samples taken after death, giving researchers access to brain tissue that would otherwise be inaccessible. The tissues came from five groups of six to ten people each: young, healthy adults; healthy older adults; elderly people showing the first signs of dementia; elderly people with Alzheimer’s disease; and “SuperAgers,” adults at least 80 years old with the memory capacity of someone 30 years younger.
Studying a set of brains with such diversity in age and cognitive status is “an incredible and unprecedentedly exciting thing,” says neuroscientist Shawn Sorrells of the University of Pittsburgh, who was not involved in the study.
For this study, the researchers focused on the seahorse-shaped hippocampus; Located on either side of the brain, the hippocampi are essential for memory formation and other tasks such as navigation. Specifically, they looked at particular genetic signatures – collections of active or inactive genes – inside the nuclei of brain cells taken from this region. These signatures belong to cells involved in neurogenesis, including newly created neurons and their parents, the scientists explained.
Signatures appeared in all groups to varying degrees. But there were some key differences between them.
The analysis suggests that SuperAgers had about 2.5 times more immature cells than people with Alzheimer’s disease. Other comparisons yielded less clear-cut results, although there was evidence of greater numbers of new neurons in SuperAgers than in young adults, older adults, and older adults with early signs of dementia. Researchers suspect that this abundance of neurogenesis in youth may be behind SuperAgers’ mental strength.
Because of the small number of brains studied, it’s hard to say how robust this trend might be, Lazarov says. “We have to be a little careful with that.” The key idea, she says, is that genetic signatures are distinct in SuperAgers.
Not everyone agrees that neurogenesis occurs, much less that it provides benefits. “The hypothesis that these cells are actually dividing is a big step forward, not supported by their data,” says Sorrells. He suspects that the genetic analysis method used in the new study could have mistakenly classified the cells as new neurons.
Still, Lazarov says, “the best I can say is that given the tools we have now, this is the best evidence we have.”
The results don’t mean SuperAgers don’t age. “We could clearly see that their profile was very different from that of young adults,” says Lazarov. But “they had a unique signature, a unique genetic profile that allowed them to cope with the aging process.” Neurogenesis, she adds, could be one element of this adaptation process.
It’s important to explore brain changes related to aging, Sorrells says. “It’s super interesting, super exciting – a fantastic question. But it all hinges on the idea that they correctly identify the cells.”
The debate, which hinges on what counts as evidence when it comes to unambiguously detecting newborn neurons, speaks to the complexity of the human brain, Sorrells says. “The brain holds many mysteries that have yet to be revealed.”
