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Scientists have always known that a majority of the brain’s neurons, specialized cells responsible for transmitting information throughout the body, are formed at the fetal stage. However, after studies on mammals, like rats, showed that neurogenesis continues in the dentate gyrus, a part of the hippocampus area of the brain vital to memory formation, through adulthood, it was assumed the same was true for humans as well. However, scientists from the University of California, San Francisco are challenging this long-held belief with a new study which asserts the human brain stops adding new neurons by age 13.
For their research, the team, led by UCSF neurological surgery professor Arturo Alvarez-Buylla, studied brain tissue from 59 people who had died or had samples of the tissue removed during brain surgery. The study, conducted over five years, included a wide range of specimens – from newborns to adults as old as 77.
The scientists began by identifying young neurons, characterized by their long and slender shapes, and then used fluorescent antibodies to detect proteins specific to cells at different stages of maturity. They found that while there were a lot of young neurons in the brain tissue at birth, the production of new hippocampal neurons slowed down substantially by age 7 and was nonexistent by age 13. Dr. Mercedes Paredes, an assistant professor of neurology at UCSF and one of the study’s leaders, says, "We went into this work thinking we were going to find evidence of neurogenesis because other groups did. So we were actually surprised when we didn't see any evidence of it in our adult samples." Given that similar observations have been made in aquatic mammals like dolphins and whales, the researchers think the lack of regeneration could be a hallmark of complex brains.
However, not everyone agrees with the results of the study, published in the journal Nature on Wednesday, March 7, 2018. Among them is Rusty Gave, interim director of the Salk Institute in La Jolla, California, whose 1998 study concluded new neurons are continually being formed in the hippocampus of adult humans. The researcher believes the UCSF study is flawed because testing for neurogenesis is not reliable when using a dead person’s brain tissue. He asserts, “I feel confident in my lab's results. And results from many labs repeatedly confirm that neurogenesis can occur in the adult brain."
Jason Synder, an assistant professor at the University of British Columbia, says Alvarez-Buylla’s research is "as convincing as you can be." However, the expert knows it will probably not be enough to satisfy all researchers. Whether we are ever able to solve this age-old mystery remains to be seen. That’s because human neurogenesis is impossible to observe in real time since high-quality brain tissue is hard to obtain. Even when researchers can get them, each sample only provides a snapshot of what’s going on at a single point in time in a specific brain area. But the new UCSF data will undoubtedly instigate a race among experts to try their best to find a definitive answer.
Resources: sciencemag.com,npr.org, iflscience.com