Carbon dating nuclear testing
Ever since it was shown that neurogenesis takes place in hippocampus of the adult brain (see ARF related news story), an outstanding question has been whether or not cortical neurons can be replenished in adults.To tackle this question, the authors isolated neuronal and non-neuronal nuclei from cerebral cortex tissue samples.While they found that total DNA C14 levels in these samples are younger than the donor, indicating cell turnover, they found that the cortical neurons are always as old as the individual donor.This was true for people born prior to, during, or after the spike in atmospheric C14.The authors first established that there is a relationship between the C14 content of DNA and the atmospheric C14 in the local area when that DNA was made.Unlike many other macromolecules in a cell, DNA is chemically stable once laid down, so its C14 levels are not expected to change even if the DNA ages.As Paola Arlotta and Jeffrey Macklis from the Harvard Medical School write in an accompanying Cell perspective, historically, methods to label newborn cells, such as the use of tritium, bromodeoxyuridine (Brd U), or other halogenated urides, are toxic and cannot be used in humans, so the strategy developed by Spalding and colleagues “enables a more direct understanding of cell turnover, aging, and lifespan throughout the human body and those of other long-lived animals.” There is one small caveat, however.Because atmospheric C14 levels are falling relatively quickly, the method will decrease in sensitivity with time, so the period during which it will be useful is limited and people born around the time of the nuclear tests will remain the most suited for study.
This matches the pattern that would be expected when one considers the time taken for development of the human brain and the relatively rapid turnover of epithelial cells in the intestine.
The authors estimated that using their method, they can put an age on human cells to within /- 2 years.
Turning to individual cell types rather than tissues, Spalding and colleagues set about to determine if cortical neurons are as old, or perhaps younger, than the individual.
These two areas have been named “neurogenic” because they are now widely believed to support adult neurogenesis.
However, similar evidence has been reported for “non-neurogenic” brain regions, including the neocortex, striatum, amygdala, hypothalamus, and substantia nigra (6,7,9,12,30,32).