But the almost tenfold increase in atmospheric C14 that peaked around the mid-1960s has been followed by a rapid decline since the nuclear test ban treaties and the cessation of high-yield, above-ground nuclear tests.
In fact, C14 is assimilated so rapidly that from about 1963, its half-life in the atmosphere has only been about 11 years.
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).
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.
Nevertheless, as Spalding and colleagues point out, there are plenty of tissue banks with archived material which will always be useful.—Tom Fagan About 40 years ago, Joseph Altman and his colleagues used the 3H-thymidine autoradiographic method to birthdate cells in the brains of adult rats and cats, and reported evidence for neurogenesis in the olfactory bulb, hippocampus, and neocortex (1-5).
Next, examining samples from single individuals born after the test ban treaty went into effect, the authors found that the C14 content in DNA isolated from the cerebellum, cortex and intestine were, as would be expected, the same age.
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.
However, others have not corroborated these findings (20,22,23), or have found evidence for adult neurogenesis only under conditions of damage (21,25,31).
Methodological flaws have been proposed as explanations for putative false positive and false negative data on this subject (13,26).
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.