The dogma was simple: you are born with all your neurons, then you gradually lose them as you age. End of story. But science, as often happens, rewrites the ending. Swedish researchers used artificial intelligence to identify progenitor cells in the human hippocampus, the ones that give rise to new neurons. The result? Even at 78, the brain continues to churn out fresh nerve cells. A discovery that not only confirms adult neurogenesis, but opens exciting prospects for understanding memory, learning and brain aging.
The Brain's Time Machine
For years, we have known that mice and monkeys continue to produce new neurons even as adults. But for humans, the question remained open, shrouded in a debate that divided the scientific community. Adult neurogenesis in the human hippocampus seemed like a mirage: some studies confirmed it, others categorically denied it.
The team led by Jonas Frisen at Karolinska Institutet decided to cut to the chase using a completely new approach. They created AI models trained on brain samples from six children, teaching the machines to recognize progenitor cells based on the activity of about 10.000 genes. A true “molecular identikit” of the cells that give rise to neurons.
The strategy proved brilliant. The models correctly identified 83% of the progenitor cells in the mice and accurately predicted the absence of these cells in the adult cortex, where neurogenesis does not occur. A reliable recognition system, finally. Yes, but what about humans? Now I get there.
The missing link in neurogenesis
The real breakthrough came when the researchers applied this system to the brains of 14 people who died between the ages of 20 and 78. They first selected only cells that were dividing at the time of death, a trick that allowed them to exclude mature neurons and focus on rare progenitor cells.
Outcome: nine out of 14 donors showed clear signs of active neurogenesis. The progenitor cells were there, located precisely in the dentate gyrus of the hippocampus, exactly where we expected to find them. As I I was underlining some time ago, the brain's ability to regenerate has no pre-established age limits.
This study, published in the journal Science, finally provides the missing link we have been looking for: direct evidence that neural progenitor cells exist and divide in the adult human brain.
Adult Neurogenesis: Individual Differences
One fascinating aspect of the research is the enormous individual differences. Some brains are teeming with progenitor cells, others have very few. Five of the 14 donors showed no detectable signs of neurogenesis. Why the variability?
Researchers hypothesize that genetic and environmental factors may heavily influence our ability to produce new neurons. Stress, exercise, cognitive stimulation: all could play a role in keeping these special cells active. As suggested by Professor Hongjun Song of the University of Pennsylvania, this diversity could explain why some people age cognitively better than others.
Newborn neurons are not an absolute novelty: already in 2013, Frisén's team had demonstrated that in the human hippocampus about 700 new neurons are formed every day. But the crucial piece was missing: seeing the progenitor cells in action.
Implications for memory and disease
The finding has profound implications for our understanding of memory and learning. The hippocampus, where this neurogenesis occurs, is crucial for forming new memories and navigating space. New neurons may be key to our ability to adapt to new experiences.
Sandrine Thuret of King's College London points out that in mice, just a few newborn neurons are enough to influence memory and learning. The same could be true for us. Although the rate of adult neurogenesis is low (less than 0,03% of hippocampal neurons), the functional impact could be enormous.
The link to neurodegenerative diseases is equally intriguing. If we can figure out how to stimulate neurogenesis, we could develop new therapies for Alzheimer's, depression, and mood disorders. Neuroscientist Eugenia Jumps of the Netherlands Institute for Neuroscience sees this discovery as an important piece of the puzzle for understanding how the human brain changes over the course of life.
The Future of Adult Neurogenesis
This study closes one chapter and opens many others. The next frontier, according to Gerd Kempermann of the University of Dresden, is to study whether differences in the rate of neurogenesis contribute to cognitive decline in Alzheimer's. A finding that could lead to targeted therapies.
Research shows that the prediction of the neuronal future of our brain is not written in stone. We continue to produce nerve cells throughout our lives, maintaining a capacity for renewal that we thought was lost. The human brain, once again, proves more plastic and surprising than we imagined.
As Frisén concludes: “We hope that this discovery will no longer create controversy, but unite.” Adult neurogenesis is no longer a hypothesis to be proven, but a reality to be exploited. The aging brain is not condemned to decline: it still has arrows in its quiver, fresh neurons ready to spring into action.
