17 June 2013. Stem cells are present at certain locations of the brain where they may develop into mature brain cells when needed. This has been demonstrated in animal models, in which injected radioactive tracers are incorporated in newly formed DNA of brain cells. This happens especially in the hippocampus, the part of the brain that is home to learning and memory functions (see Chapter 1 in Only one Chance). But how important is that, and what does it mean?
In an article just published in Cell, scientists demonstrated that some nerve cells in the hippocampus were only recently formed. So why we get only one chance to develop the brain as a whole, some renewal does take place. An innovative research strategy made it possible for the researchers to identify the time of formation of the cells in adult brain tissue obtained from autopsies. Some of the carbon in the cells is in the form of the (weakly) radioactive isotope C-14, some of which originated from nuclear bomb tests conducted until 1963, when a test ban treaty came into force. Since then, the amount of C-14 in the environment – and in people – has declined. By measuring the relative concentration of C-14 in brain cells, it is therefore possible to calculate an approximate “time stamp” or “birth date”. Overall, the researchers showed that several hundred new neurons are apparently added in the hippocampus every day, corresponding to an annual turnover of 1-2 % of the neurons within the area involved. So, during the life-span, as much as one-third of the hippocampus neurons may be replaced.
Is this good news that reveals hidden plasticity and repair functions? Probably not. The cell renewal seems to be concentrated in a specific part of the brain, where new learning and memory may require exchange of cells, whose functions and connections are no longer needed. It does not mean that we can count on stem cells to compensate for any damage such as chemical toxicity. The fact that we rely on stem cells for renewal or learning and memory may even represent a sensitive target for toxic chemicals. Thus, stem cells seem to be particularly vulnerable to brain drainers such as methylmercury. In regard to lead, some studies suggest that memory functions decline faster in highly-exposed subjects. If such declines are due to insufficient numbers of active stem cells in the hippocampus, we need to determine to which degree stem cell losses are related to early brain drain and/or toxic exposures later in life.