New hippocampus neuron advancement additionally dwindled after some time in macaque monkeys, the researchers found.
'The early decline in hippocampal neurogenesis raises questions about how the function of the dentate gyrus differs between humans and other species in which adult hippocampal neurogenesis is preserved'.
The disclosure repudiates past discoveries recommending that hippocampal neurons renew themselves all through adulthood, as they do in numerous different warm blooded creatures. That might help fix the damage caused by a brain injury or Alzheimer's disease, he says.
"Our findings in humans pertain to normal brains, and we cannot rule out the possibility that quiescent stem cells could be reactivated by injury or under some circumstances in this brain region", they add. As a graduate student in the Nottebohm lab at the time, Alvarez-Buylla contributed to understanding the mechanism of adult neurogenesis in songbirds. And hippocampus regeneration was thought to be lacking in big-brained dolphins, porpoises and whales. Animal experiments have also suggested that neurogenesis-boosting therapies could treat brain disorders of aging such as Alzheimer's disease, and leading researchers have proposed that antidepressant medications like fluoxetine (Prozac) may work by increasing DG neurogenesis.
In a News and Views article accompanying the new Nature paper, Jason Snyder, a behavioral neuroscientist at the University of British Columbia, says the team has used a careful approach to studying neurogenesis in adult humans and explains the results are "in stark contrast to the prevailing view that human hippocampal neurogenesis extends throughout adult life". However, these findings, some of which were based on small numbers of brain samples, have remained controversial. "It's by far the best database that has ever been put together on cell turnover in the adult human hippocampus", says Steven Goldman, a neurologist at the University of Rochester Medical Center and the University of Copenhagen. Whether these markers can reliably tag young neurons depends a lot on the quality of the tissue, which is influenced by how soon after death the samples are treated to keep them from decaying, Kempermann says.
"These findings are certain to stir up controversy", he said, and underlined they would have to be confirmed by other researchers.
Sorrells and Paredes also note that evidence of young neurons or dividing progenitors did show up in the walls of the brain ventricles.
The quantity of creating neurons in the dentate gyrus decreased with age and the most established example in which any were found was from a 13-year-old.
A few years ago, the group looked at a well-preserved adult brain sample and spotted a few young neurons in several regions, but none in the hippocampus.
"In all of the adult samples we looked at, we couldn't find any evidence of a young neuron", says Shawn Sorrells, the study's lead author and a senior researcher in the lab of Arturo Alvarez-Buylla at the University of California, San Francisco. "It remains essential to study the process of adult neurogenesis in animal models because of the importance of this work to brain fix".
One reason is that it is very hard to study the birth of new neurons, a process called neurogenesis, in people.
And they found lots of these cells in babies and young children. They noted that these cells fail to cluster early on into a concentrated "niche" in a region of the human DG known as the subgranular zone (SGZ). He adds that other teams using the same techniques have seen adult neurogenesis. If the proteins showed up under the microscope, new neurons were likely emerging. "I wouldn't close the books on [that]", says neuroscientist Heather Cameron of the US National Institute of Mental Health in Bethesda, Maryland. Past studies have suggested that neurogenesis may bolster learning and memory and possibly also stress response and emotion.
The first evidence of neurogenesis in adult humans came in 1998 from the brains of deceased cancer patients who had received injections of a chemical called bromodeoxyuridine while they were still alive.
Although Alvarez-Buylla acknowledges the limitations of the study, he stands by its results.
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