15 Jul 2005

Analysis of growth rings from pine trees in Sweden shows that the proliferation of atomic tests in the 1950s and 1960s led to an explosion in levels of atmospheric carbon 14. Now, Jonas Frisen and colleagues at the Karolinska Institute in Stockholm have taken advantage of this spike in C14 to devise a method to date the birth of human cells. Because this test can be used retrospectively, unlike many of the current methods used to detect cell proliferation, and because it does not require the ingestion of a radioactive or chemical tracer, the method can be readily applied to both in vivo and postmortem samples of human tissues. In today’s Cell, Frisen and colleagues report how they used the dating method to dismiss the possibility that neurogenesis takes place in the adult human cortex.

Because of its extremely long half-life (over 5,000 years), carbon 14 content has typically been used to date only very old artifacts or fossils. The method has traditionally failed to resolve dates of samples that differ in age by less than a few hundred years—accurate enough perhaps to date the youngest and oldest parts of the most ancient redwood trees, but not to tell how many newborn cells might be present in the human brain. 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. Current atmospheric C14 is about twice the level it was before the 1950s.

First author Kristy Spalding and colleagues capitalized on this relatively rapid decline in C14 to develop their dating method. 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. Indeed, when Spalding and colleagues examined samples that dated prior to or after the proliferation of atmospheric nuclear tests, they found that the C14 content in the DNA correlated with the predicted atmospheric C14 at the time the DNA would have been synthesized. 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, not the same age. Cerebellar DNA was a few years older than the DNA in the cortex, which in turn was about 12 years older than the DNA in the small intestine. 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. 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 suggest that previous reports of adult cortical neurogenesis might be technical artifacts. But they also state that their method is only sensitive enough to detect a minimum of 1 percent of newborn cells in a given cell population, so that leaves open the possibility that there is some, albeit a very small amount, of newborn cells in the cortex.

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 (BrdU), 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. Nevertheless, as Spalding and colleagues point out, there are plenty of tissue banks with archived material which will always be useful.—Tom Fagan

References News Citations New Neurons in Old Brains Make New Contacts 22 Nov 2002