Are not nerve cells regenerated in the adult brain?

No new neurons in adults

Over the past several decades, several studies have concluded that we still form nerve cells in adulthood, raising hopes for new therapeutic approaches to Alzheimer's disease and other neurodegenerative diseases. However, in a recent study, researchers come to the conclusion that the production of neurons - the so-called neurogenesis - decreases sharply after early development and comes to an end to adulthood.

Over the last 20 years, there has been growing evidence that adults can produce hundreds of new nerve cells per day, raising hopes that this effect could be therapeutically useful. By promoting neurogenesis depression, dementia and other brain diseases could be preventable or treatable, speculated physicians. However, a study published in the journal "Nature" has shattered these hopes.

Are new neurons still forming in the brain of adults? (Image: Sergey Nivens / fotolia.com)

New formation of neurons in adults controversial

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

The formation of new nerve cells decreases rapidly with increasing age

The team of researchers led by Arturo Alvarez-Buylla of the University of California San Francisco (USA) has now examined the extent to which the production of neurons is also examined by means of tissue samples taken from the brains of test persons either post-mortem or in an epilepsy operation in adulthood persists. "We found that the number of proliferating progenitor cells and young neurons in the dentate gyrus decreases dramatically in the first year of life and only a few isolated young neurons can be observed at age 7 and 13," the researchers write. In samples from adults, the researchers could no longer detect any young nerve cells in the tissue samples.

Disappointing study results

Investigations on the hippocampus of the monkey (Macaca mulatta) have also shown that the proliferation of neurons in the subgranular zone occurs in early postnatal life, but this decreases greatly during juvenile development, the researchers report. "We conclude that neurogenesis in the dentate gyrus is not or only very rarely progresses in adult humans," the researchers added. The current study results will disappoint many, emphasizes the neuroscientist Paul Frankland of the Hospital for Sick Children in Toronto (Canada) in a supplementary contribution of the journal "Nature".

Other studies came to the opposite conclusion

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

In their current study, the researchers were unable to detect any young neurons in tissue samples from the brains of adult humans. (Image: Tatiana Shepeleva / fotolia.com)

No supply of new neurons detectable in adults?

In the current study, the researchers come from the study of tissue samples to the conclusion that humans early in life have a large number of neural stem cells and progenitor cells - 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 dropped 23-fold between the first and the seventh year," the research team reports. In adulthood, the supply of young neurons finally disappeared completely.

Although doubts about the current study are voiced by other scientists, Arturo Alvarez-Buylla and colleagues are convinced of their findings. The extent to which adult people still have neurogenesis remains controversial and the debate is likely to continue for some time, according to neuroscientist Paul Frankland. (Fp)