Are there no new neurons in the adult brain?

Are there no new neurons in the adult brain?

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No more new neurons in adults

Over the past decades, various studies have concluded that we still form nerve cells in adulthood, which raised hopes for new therapeutic approaches against Alzheimer's and other neurodegenerative diseases. However, in a current study, researchers come to the conclusion that the production of neurons - the so-called neurogenesis - decreases sharply after early development and comes to a standstill until adulthood.

Evidence has grown over the past 20 years that adults can produce hundreds of new nerve cells a day, fueling hope that this effect could also be used therapeutically. A promotion of neurogenesis could make depression, dementia and other brain diseases preventable or treatable, speculated doctors. However, a study now published in the specialist journal "Nature" has shattered these hopes.

New generation of neurons controversial in adults

In the so-called subgranular zone of the dentate gyrus, new neurons continue to be formed in the adult hippocampus, according to the previous assumption. A process that is associated with learning and memory, stress and movement. An influence on neurological diseases was also suspected. Some studies have concluded that hundreds of new neurons are formed in the adult dentate gyrus every day, while other studies have found much fewer new neurons, the researchers report on the starting point in their current investigation.

The formation of new nerve cells quickly decreases with age

The research team led by Arturo Alvarez-Buylla from the University of California San Francisco (USA) has now used tissue samples that were taken from the brains of test subjects post-mortem or as part of an epilepsy operation to investigate to what extent the production of neurons also persists in adulthood. "We found that the number of proliferating progenitor cells and young neurons in the dentate gyrus decreased sharply in the first year of life and that only a few isolated young neurons aged 7 and 13 years could be observed," write the scientists. In samples from adults, the researchers were no longer able to detect young nerve cells in the tissue samples.

Disappointing study results

Studies of the monkey's hippocampus (Macaca mulatta) had also shown that the proliferation of neurons in the subgranular zone occurs in early postnatal life, but this greatly decreases during juvenile development, the researchers report. "We conclude that the neurogenesis in the dentate gyrus does not, or only very rarely, progress in adult humans," the scientists continued. The current results of the study will disappoint many, stresses neuroscientist Paul Frankland from the Hospital for Sick Children in Toronto (Canada) in a supplementary article by the journal "Nature".

Other studies have come to the opposite conclusion

Only reliable evidence of the neurogenesis in adult humans was presented in 1998 when examining the brains of deceased cancer patients. The patients had received chemotherapy based on bromodeoxyuridine during their lifetime. This chemical marks newly divided cells and in the brain tissue of the test subjects there was an accumulation of young neurons in the hippocampus. In 2013, Jonas Frisén's laboratory at the Karolinska Institute in Stockholm confirmed the suspicion of carbon dating of individual neurons in the brain tissue of 55 deceased people. Using the age of the cells, the researchers calculated that humans regenerate 700 of their neurons in the dentate gyrus every day.

No supply of new neurons found in adults?

In the current study, the researchers conclude from the examination of the tissue samples that people have a large number of neural stem cells and progenitor cells early in life - an average of 1618 young neurons per square millimeter of brain tissue at birth. "But these cells did not develop into a proliferating layer of neural stem cells, and the production of new nerve cells decreased 23-fold between the first and seventh year of life," reports the research team. In adulthood, the supply of young neurons had finally disappeared completely.

Although other scientists have expressed doubts about the current study, Arturo Alvarez-Buylla and colleagues are convinced of their results. To what extent adult people still have neurogenesis remains controversial and the debate will probably continue for some time, according to the neuroscientist Paul Frankland. (fp)

Author and source information

Video: Adult neurogenesis and the plasticity of hippocampal networks (August 2022).