New neurons (red) being generated from a neuronal stem cell (green) in a mouse brain. Credit: University of Zurich

Neuron creation in the region of the brain linked to learning and memory decreases sharply during childhood and is almost undetectable in adults, new research shows. The study, published in Nature this week, casts doubt on the previously held belief that neurogenesis in the hippocampus persists throughout our lives.

The hippocampus is an area of the brain that is tasked with generating and preserving memories, and it had previously been thought that it did so by creating hundreds of new nerve cells each day. While previous research has demonstrated that hippocampal neurogenesis decreases with age, there has been evidence that a small number of neurons are still generated in humans during old age. It is because of this that several research teams are attempting to harness this renewal ability for the treatment of selected brain disorders, including Alzheimer’s disease and Parkinson’s.

The new study indicates that our previously held beliefs about neurogenesis are inaccurate. By analysing hippocampus samples from 29 patients (17 post-mortem and 12 from surgical resection of epilepsy patients) between 18 and 77 years old, the team were unable to find any evidence of young neurons. Similarly, when they tested samples from patients between 7 and 13 years old, they only found a small number of isolated young cells. From these observations, the team concluded that hippocampal neurogenesis only persists for a short time after birth and is almost entirely absent by the time we reach adulthood.

“If neurogenesis continues in the adult human hippocampus, this is a rare phenomenon, raising questions of how human dentate gyrus plasticity differs from other species in which adult hippocampal neurogenesis is abundant,” the authors wrote. “Interestingly, a lack of neurogenesis in the hippocampus has been suggested for aquatic mammals (dolphins, porpoises and whales), species known for their large brains, longevity and complex behaviour. Understanding the limitations of adult neurogenesis in humans and other species is fundamental to interpreting findings from animal models.”

Some researchers have voiced their scepticism about these results, however. In particular, there is a concern that just because the team could not find any young neurons, it does not mean that some were not present. This could be a result of inaccurate or inadequate tagging of the target cells, or of the process by which the tissues were preserved after collection.

Despite the hesitation, these results have been sufficient to pique interest within the field, and will likely result in other research groups launching their own investigations into hippocampal neurogenesis.

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