Adult Neurogenesis Discovered in Human Brain

A landmark discovery has been made by molecular biologist Ionut Dumitru and his team at the Karolinska Institute in Sweden, providing near-certain evidence that adult neurogenesis occurs in the human brain.

For seven years, Dumitru's team searched for concrete proof of this process, which involves the formation of new neurons in the adult brain. Their findings are part of a slew of new discoveries that have finally put to rest the long-standing debate among neuroscientists about whether this is possible.

The researchers' exhaustive search has yielded positive results, with evidence pointing to the occurrence of adult neurogenesis in the human hippocampus, a region crucial for learning and memory. This area of the brain was previously thought to be largely static after childhood.

The discovery of adult neurogenesis challenges long-held assumptions about how our brains function and change over time. It also raises exciting possibilities for harnessing this ability to stimulate neurogenesis and treat neurological conditions that are currently incurable.

Scientists have been searching for evidence of adult neurogenesis for decades, with each new study seeming to either confirm or contradict the findings of its predecessors. The latest research suggests that this process is indeed a reality in the human brain.

The potential implications of adult neurogenesis are profound, particularly given the devastating impact of conditions such as Alzheimer's disease, stroke, and amyotrophic lateral sclerosis (ALS), which involve massive neuronal loss. If our brains can replace lost neurons, new avenues for treatment may become available.

Neuroscientist Hongjun Song at the University of Pennsylvania believes that this discovery has significant implications for the field, stating, "To many of us, we think the question is no longer whether it exists or not."

Humans were long thought to be born with all the neurons they would ever have. The brain would undergo significant changes over a person's lifetime, forming new connections between existing neurons and pruning away those that were no longer needed.

However, this view was challenged by Joseph Altman, a biologist at the Massachusetts Institute of Technology, in the early 1960s. Altman conducted an experiment on adult rats, injecting them with a radioactive tracer to track cell division.

The results of Altman's study suggested that the rats' brains were generating new neurons, which contradicted the prevailing view in the scientific community. Despite this finding, many experts were skeptical and dismissed it as flawed.

Fred "Rusty" Gage, a neuroscientist at the Salk Institute for Biological Studies in California, notes that the idea of adult neurogenesis was deeply ingrained in the scientific literature. Altman's work was largely ignored until he replicated his findings in additional rats and even cats.

Further research on various species, including canaries and monkeys, revealed that adult neurogenesis played a crucial role in learning and memory. For example, birds were able to learn new songs due to the generation of new neurons in their brains.

However, the question remained as to whether human brains also underwent neurogenesis. To answer this, researchers needed to find an alternative method for studying the brain without resorting to invasive procedures.

In the 1990s, some cancer patients received injections of a chemical called BrdU, which marked dividing cells so that doctors could monitor disease progression. When these patients died, their brains were autopsied, and researchers found evidence of neurogenesis in hippocampal cells.

The quest to prove adult neurogenesis has been ongoing for years, but results have been inconsistent. Some studies suggested that next to no new neurons are formed in the adult brain, while others claimed that the hippocampus adds 700 new cells every day.

To resolve this discrepancy, Song and his colleagues took a fresh approach. Instead of relying on specific markers to identify immature neurons, they analyzed the genetic profile of individual brain cells to pinpoint those with characteristics of newly generated cells.

This innovative method revealed evidence of neurogenesis in individuals who had passed away at various stages of life, including some as old as 92. However, simply spotting young-looking neurons was not enough to confirm that new cells were being formed in adulthood.

There's a catch: immature neurons can remain in a state of arrested development for years, even decades. This means they may not necessarily be newly generated in adulthood but rather stalled at an early stage of growth. To prove adult neurogenesis, researchers needed to capture each stage of the process and identify the cells responsible for producing new neurons.

The elusive step was pinpointing the proliferating progenitors, the cells that give rise to new neurons. To tackle this challenge, Dumitru's team analyzed over 400,000 brain cells from individuals who had died between infancy and adulthood using a machine learning model.

Their analysis revealed genetic patterns in human brain cells that resembled those of animal progenitors and developing infants. The researchers were initially hesitant to accept their findings but eventually felt confident after re-testing their results multiple times.

Despite the breakthrough, the team's discovery was not without limitations, they found only around 350 progenitor cells across all the brains sampled, suggesting that these cells may not be as widespread as previously thought.

The discovery of active neurogenesis in the hippocampus has sparked debate among scientists. While some researchers have found evidence to support this notion, others remain skeptical about its validity.

Developmental neurobiologist Arturo Alvarez-Buylla from the University of California, San Francisco, is one such sceptic. He questions the reliability of machine learning models used to categorise cell types and believes that only by tracking a stem cell as it transforms into a neuron can we be certain about adult neurogenesis in humans.

However, Alvarez-Buylla's views are increasingly at odds with the growing consensus among researchers. Many studies have now provided evidence for active neurogenesis in adults, including those related to learning and memory.

At least 38 clinical trials are currently underway to investigate this phenomenon further. The fact that these trials exist suggests a high level of interest and investment in understanding adult neurogenesis.

Neuroscientist Sandrine Thuret from King's College London is among those who firmly believe that adult neurogenesis occurs. She points out that the process appears almost exclusive to the hippocampus, the brain region responsible for learning and memory.

The hippocampus is highly plastic, able to reorganise itself and form new connections in response to new information. Research has shown that this ability may be linked to cognitive function, with studies indicating that neurogenesis plays a role in helping us learn and remember new things.

Research suggests that enhanced neurogenesis may allow the brain to bypass or overcome factors contributing to cognitive decline.

Studies have found that neurogenesis is impaired in other conditions such as Parkinson's disease and depression.

Neuroscientist René Hen has spent 20 years investigating the connection between neurogenesis and depression, sparked by a study showing rats treated with antidepressants had increased rates of neurogenesis.

Hen ran a follow-up experiment manipulating rodents' brains to stop producing new neurons, which also stopped them responding to antidepressants.

This led Hen to investigate further the link between neurogenesis and depression, as well as other psychiatric conditions such as schizophrenia and bipolar disorder.

In these conditions, hippocampal neurogenesis seems crucial for pattern separation, distinguishing between different objects, situations and emotions, but is often impaired, leading to overgeneralisation.

Hen suggests that smaller hippocampal volumes associated with these conditions may be due in part to impaired neurogenesis, which could be addressed by stimulating the process.

Researchers are optimistic that harnessing neurogenesis could provide a shield against cognitive decline, allowing people to maintain their mental acuity as they age.

This approach may even have benefits for conditions not directly related to neurogenesis, such as traumatic brain injury or stroke, which can cause lasting damage to the brain's neural networks.

A 2025 study demonstrated that transplanting neural progenitor cells into the brains of mice after a stroke led to the production of new neurons and improved motor function in the animals.

However, it will take several years to determine whether any neurogenesis interventions work and even longer to bring them to market if they are effective.

Scientists are working to speed up this process by developing miniature models of the human brain, called organoids, which could make it easier to identify therapies that boost neurogenesis.

These organoids capture what happens in the living human brain more accurately than animal models, allowing researchers to better understand how to stimulate new neuron growth.

Other scientists are focused on developing non-invasive brain-imaging techniques to measure neurogenesis in living people, with similar goals of identifying effective therapies.