Many of the body's tissues are constantly renewed. In fact, the average age of cells in a human adult body might be only 7–10 years, according to stem cell biologists from the Karolinska Institute in Stockholm, Sweden.

Jonas Frisén and colleagues have developed a new technique for measuring the age of cells, and published their first findings in Cell.

The Frisén team hypothesized that the dramatic increase in atmospheric radioactive carbon 14 (14C) as a result of nuclear testing during the Cold War in the early 1960s and the exponential decline after 1963 might provide a modern-day 'carbon dating' equivalent.

As DNA does not exchange carbon after it is generated, the amount of 14C in a cell's DNA reflects the amount of atmospheric 14C at the time of its birth.

Looking at tissue samples from deceased adult subjects that had been exposed to a changing 14C environment, Frisén and colleagues concluded that cells from the cerebral cortex were as old as the subjects themselves. “The reason these cells live so long is probably that they need to be wired in a very stable way,” said Frisén (news@nature.com, 14 July 2005).

However, all other tissues they studied were considerably younger — the average age of cells in the intestine is ∼10 years and in skeletal muscle ∼15 years.

So, if the body is capable of regenerating most of its tissues, why do we age? “The notion that stem cells themselves age and become less capable of generating progeny is gaining increasing support,” according to Frisén (The New York Times, 2 August 2005). And he is hopeful that the new carbon dating technique might shed light on this theory.