Human brains keep growing neurons even in old age, study finds for first time
This long-held view painted the adult brain as a fixed organ, incapable of generating fresh cells in the very region responsible for memory and learning.
But a landmark new study turns that dogma on its head, offering the clearest evidence yet that adult humans continue to form new neurons well into the old age.
A team of researchers from Karolinska Institutet in Sweden has identified and tracked the formation of new neurons in the hippocampus, a region critical for memory, learning, and emotional regulation.
In 2013, a team of researchers led by Professor Jonas Frisén made headlines with a study showing that new neurons can form in the hippocampus of adult humans.
They used carbon-14 dating in DNA extracted from brain tissue to pinpoint when individual cells were created, providing rare evidence of adult neurogenesis.
But while the study proved that new neurons could exist, it did not answer where these neurons come from.
Until now, scientists lacked direct proof that the neural progenitor cells were present and actively dividing in the adult human brain.
'We have now been able to identify these cells of origin, which confirms that there is an ongoing formation of neurons in the hippocampus of the adult brain,' said Frisén.
In the new study, the team analyzed post-mortem brain tissue from individuals ranging in age from infancy to 78 years to discover that neural progenitor cells—precursors to fully developed neurons—not only exist in the adult brain but are also actively dividing.
The study used advanced techniques like single-nucleus RNA sequencing to map gene activity in individual brain cell nuclei.
Combined with flow cytometry and machine learning, the approach allowed researchers to identify various stages of neuronal development, ranging from stem-like cells to immature, still-dividing neurons.
To visualize where these new neurons were forming, the scientists employed RNAscope and Xenium, two powerful spatial transcriptomics tools. These confirmed that the cell formation was happening in the dentate gyrus, a part of the hippocampus linked to learning, cognitive flexibility, and the encoding of new memories.
The findings reveal that adult human neural progenitor cells closely resemble those seen in mice, pigs, and monkeys, though some gene activity patterns differ between species.
What's especially intriguing is the variability between individuals. While some adults had abundant neural progenitor cells, others had very few, raising new questions about what factors might influence adult neurogenesis.
'This gives us an important piece of the puzzle in understanding how the human brain works and changes during life,' Frisén explained.
'Our research may also have implications for the development of regenerative treatments that stimulate neurogenesis in neurodegenerative and psychiatric disorders.'
The findings could also pave the way for new regenerative therapies for neurodegenerative and psychiatric conditions, potentially helping to restore or enhance brain function by stimulating neuron formation in targeted ways.
The groundbreaking study has been published in the journal Science.
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