For the first time, researchers from Sweden have measured the rate at which new neurons are added in the adult human brain, an achievement popularly held impossible. They used fallout from nuclear tests in the mid-20 century to find that about 1,400 neurons are added daily to the hippocampuses, parts of the brain responsible for cognition and learning.

Their findings were published in Cell on June 6. They noted that the rate of neuron production, called neurogenesis, decreases only modestly with age. Across a lifetime, about one-third of the hippocampal cells are renewed.

While neurogenesis has been known to occur in mammals for a decade, this is the first time their rate has been measured in humans.

The team, from Karolinska Institute, Stockholm, used a technique similar to carbon dating to measure when different neurons were born in the brain, with deceased humans as subjects.

Carbon dating makes use of an isotope, carbon-14, which is generated in the upper atmosphere when cosmic rays strike nitrogen atoms. It then finds its way into different substances.

When scientists measure the amount of the isotope in them, they can calculate how much of it there was originally based on how quickly it is radioactively decaying. This number can reveal the age of the substance, too.

The process

“When a cell divides, it will build carbon-14 into its DNA with a concentration corresponding to that in the atmosphere at that time, effectively making a timestamp in the cell’s DNA,” Dr. Jonas Frisén, professor of stem cell research, Karolinska Institutet, told this correspondent via email.

Frisén’s team, however, measured carbon-14 produced by above-ground nuclear tests before the Limited Test Ban Treaty was signed in 1963. The fallout from these bombs has contributed to the carbon-14 pool worldwide, but this input has been steadily declining at a known rate since 1963.

When the team employed this phenomenon to measure carbon-14 content in neurons, the electrochemical messenger cells in the brain, they had a surprising result.

They found that about 700 neurons had been continuously added to each of the two hippocampuses, parts of the brain crucial for memory and learning, on a daily basis.

Studies conducted since 2009 have shown that the birth of new neurons helps rodents sharpen their memory. The researchers have found that the neurogenesis rate is comparable between mice and middle-aged humans, suggesting adult hippocampal cells could contribute to brain function, especially to tasks like separating patterns.

“It has long been suspected that depression is related to reduced hippocampal neurogenesis,” Frisén said.

His and his team’s findings could potentially be developed to target this process.