It was once thought that the brain did not generate new neurons in adult life, but the evidence for ongoing neurogenesis was found a few decades ago. Levels of neurogenesis in humans have been hard to pin down, but knowing the degree to which it happens naturally has some relevance to attempts to induce a higher rate of neuron creation with the aim of reversing age-related loss of cognitive function. Here researchers find a way to quantify the level of cell turnover in at least one part of the brain:
The birth of new neurons in the adult brain sharpens memory in rodents, but whether the same holds true for humans has long been debated. A [study] reveals that a significant number of new neurons in the hippocampus - a brain region crucial for memory and learning - are generated in adult humans. The researchers used a unique strategy based on the amount of carbon-14 found in humans as a result of above-ground nuclear testing more than half a century ago. The findings suggest that new neurons are born daily in the human hippocampus, offering the tantalizing possibility that they may support cognitive functions in adulthood.
Due to technical limitations, until now it was not possible to quantify the amount of neurogenesis in humans. To overcome this hurdle, [researchers] developed an innovative method for dating the birth of neurons. This strategy takes advantage of the elevated atmospheric levels of carbon-14, a nonradioactive form of carbon, caused by above-ground nuclear bomb testing more than 50 years ago. Since the 1963 nuclear test ban treaty, atmospheric levels of "heavy" carbon-14 have declined at a known rate. When we eat plants or animal products, we absorb both normal and heavy carbon at the atmospheric ratios present at that time, and the exact atmospheric concentration at any point in time is stamped into DNA every time a new neuron is born. Thus, neurons can be "carbon dated" in a similar way to that used by archaeologists.
By measuring the carbon-14 concentration in DNA from hippocampal neurons of deceased humans, the researchers found that more than one-third of these cells are regularly renewed throughout life. About 1,400 new neurons are added each day during adulthood, and this rate declines only modestly with age.