New research from scientists at UC San Francisco is challenging half a century of conventional wisdom by suggesting the human brain may cease producing new neurons beyond childhood. While the divisive study may prove a blow to some research aimed at birthing new neurons to battle neurodegenerative disorders, it offers a new perspective on how the human brain can adapt in later life without such a capability.
The team generated its data by studying brain specimens of 59 subjects, from babies to the elderly. The strategy was to look for the presence of young neurons or dividing cells by using certain antibodies that bind to those cells of interest. The focus was on the hippocampus region of the brain, known to be crucial for memory, and a comprehensively studied area previously suggested to be a key location for neurogenesis.
The results were fairly comprehensive. Young or immature neurons were identified in plentiful volumes in prenatal and newborn samples but the rate consistently declined over childhood. The oldest sample that immature neurons were found in was 13 years of age, and adult samples displayed no evidence of new neurons.
"In young children, we were able to see that substantial numbers of new neurons continue to be made and integrated into the dentate gyrus, but neurogenesis fades away completely by early adolescence," says Mercedes Paredes, a senior researcher on the project. "The fact that we could compare newborn brains, where new neurons were clearly present, to the adult, where we saw no evidence for young neurons, gave us added confidence that what we were seeing was correct."
The results were unexpected as adult neurogenesis has been discovered in other animals, including rodents and mice. It is fair to ask how this revelation can sit alongside such seemingly contradictory evidence in animals? Jason Snyder, from the University of British Columbia, examines this discordancy in a commentary written for Nature. Snyder suggests that this evidence may point to an extended period of maturation in human neurons that could take decades and explain a degree of neural plasticity in adult humans.
"Rodents are born with relatively immature nervous systems, so adult rodent neurogenesis could be a decent model of neurogenesis in children or adolescents," Snyder also suggests.
Of course, not everyone is convinced this new study is definitive. Jonas Frisen, from the Karolinska Institute in Stockholm, is senior author of a significant 2013 study that claimed 700 new neurons are added to the adult hippocampus every day in an ongoing process of neurogenesis. Frisen suggests the antibody model used to trace new neurons in this study was not as effective as his method.
"Since it is a rare phenomenon they are looking for, they may just not have looked carefully enough," says Frisen in an interview with Scientific American.
Shaun Sorrells, a senior researcher on the new study acknowledges that it is virtually impossible to definitively say the adult hippocampus produces no new neurons, but this evidence certainly suggests that if adult neurogenesis exists, it may be a very rare and/or minimal process that doesn't adequately explain plasticity in an adult human brain.
"If neurogenesis is so rare that we can't detect it, can it really be playing a major role in plasticity or learning and memory in the hippocampus?" asks Sorrells.
While the research may prove dispiriting to those hoping that ways of boosting adult neurogenesis could be the way to treat devastating diseases such as Alzheimer's, it does raise exciting new questions about adult brain plasticity. If we can better understand how the brain can continually learn and adapt without producing new neurons then we can better treat a whole host of degenerative brain diseases.
The research was published in the journal Nature.
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