© 2005 Elsevier Inc.
The cells in our body are continually replaced, especially those exposed to the harsher environments of the skin or intestine. Even cells in tissues that generally experience less turnover often regenerate after an injury. While scientists believe other cells in the human body, such as those in the brain, have longer life spans, they still don't know much about how often cell replacement occurs, if at all.
Molecular proliferation markers can capture the number of cells in a cycle at any given time but still fail to elucidate the number of mature cells that are actually produced or end up contributing to the tissue, notes Kirsty Spalding of the Medical Nobel Institute in Sweden. And, such methods use modified nucleotides and retroviruses that are toxic and cannot be used for human studies.
Inspired by archaeologists' use of 14C dating, Spalding and colleagues from the Karolinska Institute in Stockholm, Sweden, and the Lawrence Livermore National Laboratory in Berkeley, Calif., devised a way to calculate cell age without having to deliver any chemical to a subject before analysis.1
Because the level of genomic 14C is proportional to atmospheric levels – which skyrocketed following increased nuclear weapons testing in the 1950s and 1960s, and then dropped exponentially after the test ban treaty in 1963 – they were able to establish the birth date of cells by using this relationship to determine when DNA synthesis occurred. The group has also used dating of 14C in tooth enamel to determine the age of an individual to within 1.6 years, a technique with potential forensics applications.2
Geoscientists and forensic specialists use carbon dating to date specimens, says Paula J. Reimer, director of the Centre for Climate, the Environment and Chronology at Queen's University, Belfast. "But [what] Spalding et al. have [done is] pinpoint cell turnover times with 14C measurements of DNA extracted from various tissues," Reimer says.
Spalding's group discovered that neurogenesis continues in isolated regions of the brain throughout life in the mammals they studied. They also found that cortical neurons are as old as the individual.
"Our analysis revealed that neurons from the adult occipital cortex have 14C levels in their genomic DNA corresponding to the time when the individual was born," Spalding says. " [This lends] little support to any long-term stable integration of new neurons in this region, and these results have important implications as far as understanding cortical functioning and cognition."
But the technique has a downside, says Gerd Kempermann, head of the neural stem cell group at the Max-Delbrück Center for Molecular Medicine in Berlin: Its resolution is low and may miss regions of low cell genesis. "This is not a fundamental breakthrough, but a very intelligent strategy. Perhaps one can refine the method to learn even more."