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Over the past 30 years, scientists have established that animals, such as songbirds and rodents, produce new neurons in their brains throughout their lives, which allow them to recover and regenerate after an injury.

As a result, researchers have always wondered if this neurogenesis also occurs in people.

However, new research conducted at UC San Francisco has revealed that neuron production in the human hippocampus declines throughout childhood, ending sometime in adolescence.

“We find that if neurogenesis occurs in the adult hippocampus in humans, it is an extremely rare phenomenon, raising questions about its contribution to brain repair or normal brain function,” said study author, Arturo Alvarez-Buylla, Ph.D., the Heather and Melanie Muss Professor of Neurological Surgery at UC San Francisco.

Starting in the 1960s, researchers at MIT first proposed that rodents were able to create new neurons in their brain even in adulthood. However, this finding wasn't confirmed until the 1980s when scientists at Rockefeller University conclusively demonstrated neurogenesis in songbirds.

In particular, studies have shown that a part of the hippocampus called the dentate gyrus produced new neurons throughout life to aid in the formation of new memories. This finding has suggested that neurogenesis-aided therapies could be used to treat brain diseases, like Alzheimer's disease.

Building on this research in animals, the Alvarez-Buylla laboratory analyzed changes to the number of newborn neurons in 59 samples of human hippocampus. While there was plenty of evidence of neurogensis in both prenatal and newborn brains, the number of newborn cells sharply declined beginning at just a year of age.

By age 13, there were fewer than 2.4 new cells per square millimeter of tissue. At adulthood, the researchers were unable to find any newborn neurons.

The research team was surprised that these findings were so different than what had been known about animal models--so they also examined the brains of nonhuman primates.

"Using labeling techniques that are not typically possible in humans for ethical reasons, we tracked the generation of new neurons in living animals. We discovered that the neural stem cells that generate new neurons coalesce into a ribbon-like layer in the monkey hippocampus before birth. This layer was present and contained dividing cells even in juvenile monkeys. When we looked back at our data from the newborn human hippocampus we saw that the stem cells did not organize themselves in this fashion – a clear developmental difference between human brains and those of other primates," the authors wrote in The Conversation.

The discovery, published March 7 in Nature, opens the door to learn more about how the brain learns without forming new neurons. The researchers hope a deeper understanding will lead to new treatments and therapies for brain diseases.

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