In a report published in Cell Stem Cell, scientists from Columbia University present new evidence that human brains continue to make hundreds of new neurons a day, even after we reach our 70s, in a process known as neurogenesis.
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 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 reach this conclusion, lead author Dr. Maura Boldrini, a researcher in Columbia University’s department of psychiatry, and her colleagues looked at the brains of 28 dead people aged 14 to 79. Their goal was to see whether aging affects neuron production.
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Previous research had shown that neurogenesis slows in aging mice and nonhuman primates. Boldrini’s group wanted to see whether a similar pattern occurred in humans.
In each brain sample, 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 the samples, 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, 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 age. They also discovered that a protein associated with helping new neurons 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 experience memory loss or exhibit less emotional resiliency, she said.
These new findings were published one month after a team of researchers from the University of California, San Francisco (UCSF) 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.
Boldrini said 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 United States, 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, 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.
The debate continues.