Atomic bomb tests like this one in Nevada in 1957 provide surprising insights into the human brain. Image: Thinkstock
Reading aloud And they multiply! In the adult brain again and again, new nerve cells are formed, researchers could now prove beyond doubt - using an unusual method: they exploited the drastically increased amount of a certain carbon form in the air after the atomic bomb tests between 1955 and 1963 to determine the year of birth of individual brain cells, It turned out that in a certain part of the hippocampus, a crucial brain area, among other things, new nerve cells are formed every day - throughout life. The bad news is that the brain does not grow bigger, because old nerve cells are broken down in parallel. "For a long time it was assumed that we were born with a certain number of brain cells, and that it is not possible to form new neurons after birth, " explains study leader Jonas Frisén from the Karolinska Institute in Stockholm. In many animals, especially in the rodents, which are very popular as models, this is different: there is also in the adult brain regularly produced cell replenishment, so that certain parts of the brain even grow constantly.

Central questions: where and how many?

However, there has been some evidence for some time that the situation in humans may not look as hopeless as expected and that there are definitely new nerve cells in the course of life. How many are and whether they are even sufficient to play a role in the functioning of the brain remained completely unclear. Therefore, Frisén and his colleagues are now looking for a way to quantify the new members in the brain group. Their idea: If one could determine, so to speak, the year of birth of the individual cells, one could directly see whether there are new brain cells in adulthood and also calculate how many that are.

They were helped by a method that Frisén had developed a few years ago. It uses a side effect of the many nuclear weapons tests that were carried out during the Cold War, especially in the years 1955-1963. At that time, the proportion of carbon isotope C-14 in the atmosphere drastically increased. After agreeing to a stop of aboveground tests in 1963, he then began to fall again - and with a now almost exactly known speed. As the C-14 contained in the carbon dioxide of the air is absorbed by plants, it also enters the food chain. display

Through the food, therefore, the human body gets exactly the amount of heavy carbon at any time, which is also found in the atmosphere. Tissue that forms at a certain time fixes the C-14 concentration like a snapshot of the current situation. If one measures this concentration, one can say in reverse quite exactly when the corresponding tissue has formed - a principle that Frisén and his colleagues have already successfully used, for example, for the age determination of unknown corpses.

After the corpses now the brain cells

Now the team transferred this measurement method to brain cells that they gained from tissue donations of the deceased. The donors had been between the ages of 16 and 92 when they died, covering both birth years before 1955 and after 1963. The researchers focused on the DNA of the brain cells in the hippocampus: If it is formed, carbon is always required, so that from the ratio of the existing isotopes and the amount of C-14 directly on the year of education can be closed.

The very first data showed the researchers interesting connections. For all samples, the C-14 scores were different than for the year of birth of the donor - so it must definitely have taken place after birth still a neuron formation, the team explains. In addition, this new formation does not seem to stop altogether in old age: the DNA of the oldest donor, who was already 42 years old in 1955, contained more C-14 than in the period before the tests in the air was. It must therefore have formed after 1955 and thus in the fifth year of life of the donor. However, the new formation seems to take place only in certain areas of the hippocampus, as the researchers report.

The conclusion from this first assessment is therefore that there is a not inconsiderable new formation of brain cells in the adult hippocampus, which is only slightly slower in old age, but is limited to a specific part of the brain area, explains the team. With the aid of various mathematical models, the scientists were then even able to calculate the extent of the new formation: About 1.400 new nerve cells form each day, whereby the exact number varies from person to person. Overall, just under a third of all hippocampal neurons participate in this renewal - this fits in well with the size of an area called Gyrus dentatus, which was one of the main suspects, says the team.

Recycling instead of growth

However, according to the researchers, it is not directly related to nerve cell growth, as is the case with mice, but rather a kind of recycling system. Because parallel to the new development of the neurons old cells are degraded, so that the size of the hippocampus even slightly decreases in the course of life. But why then all the effort? The researchers also have an answer to that: Shortly after birth, neurons have very special properties that they later lose, such as a very high level of flexibility. The new formation therefore has a sense: By ensuring that there is always a certain amount of young neurons in the hippocampus, it can maintain functions that would not be possible with exclusively old nerve cells.

This includes, for example, a good pattern recognition, which is important for the formation of new, differentiated memories. Older neurons, on the other hand, specialize in recognizing similarities in patterns, and thus in summarizing similar memories and generalizing. The scientists want to investigate this connection in more detail. It is well known that people with depression or other mental health problems often tend to generalize and that may be due to a reduced formation of neurons in the hippocampus .

Kirsty Spalding (Karolinska Institute, Stockholm) et al .: Cell, doi: 10.1016 / j.cell.2013.05.002 Ilka Lehnen-Beyel

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