Do we continue to add new neurons to our brain circuitry throughout our lives? Or does our neuron count remain fixed once we reach adulthood?
The scientific debate rages on.
In a report published Thursday in Cell Stem Cell, scientists from Columbia University present new evidence that our brains continue to make hundreds of new neurons a day, even after we reach our 70s, in a process known as neurogenesis.
To come to this conclusion, lead author Dr. Maura Boldrini, a research scientist at Columbia University's department of psychiatry, and her colleagues looked at the brains of 28 deceased people aged 14 to 79. Their goal was to see whether aging affects neuron production.
Previous research had shown that neurogenesis slows down in aging mice and nonhuman primates. Boldrini's group wanted to see whether a similar pattern occurred in humans.
In each brain sample the researchers looked for evidence of neurons in various stages of development, including stem cells, intermediate progenitor cells that would eventually become neurons, immature neurons that had not fully developed, and new neurons.
The team looked only at the hippocampus, in part because it is one of the few areas of the brain that previous research has shown can produce new neurons into adulthood. This region is involved in emotional control and resiliency, as well as memory, Boldrini said.
In all their samples the researchers found similar numbers of neural progenitor cells and immature neurons, regardless of age. This led them to conclude that the human brain continues to make neurons even into old age.
However, the researchers did uncover some differences in the brains of young people and older people.
Specifically, they found that development of new blood vessels in the brain decreases progressively as people get older. They also discovered that a protein associated with helping new neurons to make connections in the brain decreased with age.
"We don't find fewer of the new neurons or fewer of the progenitors of new neurons, but we find that new neurons might make fewer connections," Boldrini said.
This might explain why some older people suffer from memory loss or exhibit less emotional resiliency, she said.
These new findings were published one month after a team of researchers from UC San Francisco reported in Nature that it was unable to find any evidence of neurogenesis after adolescence in humans at all.
In an email statement, that group, which works out of developmental neuroscientist Arturo Alvarez-Buylla's lab, said that while they found the new study's evidence of declining blood vessel growth in the adult hippocampus interesting, they are not convinced that Boldrini and her colleagues found conclusive evidence of adult neurogenesis.
"Based on the representative images they present, the cells they call new neurons in the adult hippocampus are very different in shape and appearance from what would be considered a young neuron in other species, or what we have observed in humans in young children," they wrote.
They added that in their study, they looked not just at protein markers associated with different types of cells, as Boldrini and her team did, but also performed careful analysis of cell shape and structure using light and electron microscopes.
"That revealed that similarly labeled cells in our own adult brain samples proved to be neither young neurons nor neural progenitors, but rather non-neuronal glial cells expressing similar molecular markers," they wrote.
Boldrini points out that the two groups were working with very different samples.
She and her team examined more than two dozen flash-frozen human brains, which were donated by families of the deceased at the time of death. The brains were immediately frozen and stored at minus-112 degrees Fahrenheit, which keeps the tissue from degrading.
The other research team received brain samples from hospitals in China, Spain and the U.S., and the brain tissue they examined had not been preserved in the same way. Boldrini said the chemicals that were used to fix the brains could have interfered with their ability to detect new neurons.
She also noted that while both groups were looking for signs of neurogenesis in the hippocampus region of the brain, her group had access to the entire hippocampus while the UCSF team was looking at thin slices of the tissue representing a small fraction of the brain.
"In science, the absence of evidence is not evidence of absence," she said. "If you can't find something it doesn't mean that it is not there 100%."
The debate continues.
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