The Brain's Hippocampus Stops Adding New Neurons in Early Teens: Study

Only seven years after birth, the rate at which new neurons are formed in the adult human hippocampus falls by 23-times the rate at birth.

For many decades now, it has been thought that new brain cells – neurons – are formed in the adult human brain and that our lifestyle could affect the process. But a new study has stirred this pot. Researchers have found that most neurons we have as adults in one part of the brain, the hippocampus, are what we were born with, that rarely do new neurons form after 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,” Arturo Alvarez-Buylla, a professor of neurological surgery at the University of California in San Francisco, whose lab led the study, said in a statement.

The hippocampus is a seahorse-shaped part of the brain responsible for forming memories, for spatial cognition and navigation. Dementia, including Alzheimer’s disease, that leads to memory loss and the inability to find one’s way – even to different parts of one’s own house – is the result of a floundering hippocampus.

For a long time, scientists believed that most of the adult human brain was unchanging and that all brain cells are what we were born with. At the end of the last century, a study revealed that new neurons did form in the hippocampus of an adult brain. Since then, several studies have established that new nerve cells do form in the hippocampus.

And this is what Alvarez-Buylla, Mercedes Paredes, a neuroscientist at the University of California, San Francisco, and their colleagues thought they would find when they set out to investigate neurogenesis.

“We actually began the study assuming that we would find some evidence of adult neurogenesis in the human hippocampus,” Paredes told The Wire. “However, this prevailing view was not supported by our observations.”

They found that neurons rarely regenerated in the adult hippocampus, and that the last time that really happened was in our teens.

This conclusion contrasts what scientists have found in animals such as songbirds and rodents. Studies by Alvarez-Buylla have shown that new neurons continue to be made in adult canaries and ring doves.

Investigations with rodents have revealed that their brains form new neurons throughout their lives, and that neurogenesis can be boosted by exercise and reducing stress. Thus emerged the idea that therapies that stimulated neurogenesis could make us smarter and help protect us against brain diseases prompting research into treating depression.

In addition, other researchers have demonstrated that new neurons continue to form in adult monkeys. Adult macaques, which are similar to humans in their life-history patterns and development, also showed signs of neurogenesis, although the rate at which new brain cells formed was lower than in rats and mice.

But Alvarez-Buylla and co. found something else in humans. The team studied samples of brain tissue obtained from 37 corpses and surgically removed from 22 living people, the latter being treated for epilepsy, aged up to 77 years. The researchers sliced the tissues thin and looked for young and dividing cells. They used fluorescent antibodies to label and identify the different types of cells and their maturity. They also studied the shape and size of the cells using electron microscopy.

In foetuses and newborns, about 1,618 new neurons were being formed per millimetre square of brain tissue. This rate decreased sharply with age. About a year after birth, the number of new neurons decreased by 5x; at seven years, by 23x; at 13 years, by a further 5x, although by this time they had matured well.

In the 39 adult human samples obtained from people aged 18 or more, the researchers could find no evidence of young neurons. They also looked for stem cells that gave rise to neurons. Although such stem cells were found in large numbers in the womb, they became very rare in toddlers and young children.

“It seems like a very well done study,” said Sachin Deshmukh, a professor of neuroscience at the Indian Institute of Science, Bengaluru, before warning of statistical limitations: “I would take this particular human study … with a pinch of salt as we are still talking about pretty small numbers here.”

Before we can think about implications for how we understand how our brains work, the results will have to be reproduced by other researchers. Then, they will have to be correlated with behavioral studies showing a decline in the ability to form particular types of new memories.

For example, a person with Alzheimer’s disease has difficulty forming new memories. So pharmaceutical companies developed drugs that attempted to increase neural growth. However, if the results of the new study are true, then those drugs could become useless.

Deshmukh did caution that this is all still speculation.

Although animal models have shown adult neurogenesis, the new study suggests the human brain may be different. For example, if we know that the brain changes in adults, then it may not be in the form of new neurons but, say, in synaptic transmission or in the shapes/sizes of neurons.

But the lack of neurogenesis in adult humans is similar to what some studies have found in dolphins and whales, both smart creatures that live long lives. So we need more studies to help us interpret what we’re finding. Additionally, the new study doesn’t preclude adult neurogenesis in other parts of the brain.

This is one conclusion from a small number of samples that has piqued scientists’ curiosity, says Deshmukh. But it is too early to tell how it will change our conception of the human brain.

Lakshmi Supriya is a freelance science writer based in Bengaluru.