Jonas Frisén wanted to answer a question: How often, if at all, do brain cells in the hippocampus turn over in adult humans? But it was clear to him that the usual plan of attack for addressing such a question in lab animals—namely, feeding them a radioactive tracer and then examining their tissues postmortem—couldn’t be adapted to human subjects. “You will not be able to find a healthy volunteer to first drink a toxin and then donate the part of their brain to you, of course,” he explains.
Mulling over the problem, the stem cell researcher at the Karolinska Institute in Sweden considered how archeologists who also can’t feed their subjects radioactive tracers address dating of samples. Archaeologists often tests the ratio of carbon isotopes to determine approximate dates when an organism was alive, taking advantage of the fact that 14C decays at a measurable and steady rate.
That rate of decay is far too slow to be useful in differentiating the ages of an individual’s brain cells. But as Frisén read up on 14C, he learned that above-ground nuclear bomb tests conducted in the late 1950s and early 1960s during the Cold War had created a global spike in atmospheric 14C levels, followed by a gradual decline over the subsequent decades as the isotope has been taken up by plants and other photosynthesizers. Those documented, changing atmospheric levels, he realized, provided “time dynamics of these varying carbon-14 concentrations that actually were very suitable for the type of questions we were interested in.” So Frisén and his colleagues attempted to detect differences in the 14C levels in cells that reflected atmospheric concentrations at the time of the cells’ births.
See “The Oldest of Them All”
But available detection techniques weren’t sensitive enough to pick up the small differences that would distinguish neurons of varying ages in the postmortem brain samples Frisén’s group had access to. So he worked with physicists who were able to increase the sensitivity of a method known as accelerator mass spectrometry. In 2005, the group published its first studies using the technique. By looking at the carbon isotope ratios in the genomic DNA of brain cells in the cerebral cortex of the human brain samples, they found that while the neurons’ carbon isotope ratios reflected atmospheric levels of 14C at the time of their bearers’ birth, some of the glial cells had lower 14C that indicated they had turned over at some point in the people’s lifetimes. The researchers later established that tooth enamel, which is generated at known times during childhood and does not turn over during an individual’s lifetime, has a 14C signature that can reveal its’ bearers’ approximate birth year. The latter approach has since been used to estimate the birth year of victims in forensics cases where their identities couldn’t be determined by other means.
The findings of those two studies hinted at the range of biological information that can be mined by taking advantage of what’s known as the radiocarbon bomb pulse. To date, researchers have used it to determine the ages of everything from classes of proteins to sharks to vintage wines—in some cases, overturning long-held assumptions.
Since his initial 2005 publications, Frisén has had no shortage of human cell types to date. He and collaborators have used 14C to examine the dynamics of human heart muscle cells, adipocytes, microglia, and more. It wasn’t until 2013 that his group published its findings on the cell type that had sparked Frisén’s search for a new technique in the first place, neurons in the hippocampus. While most of those cells do stay with us throughout our lifetimes, the researchers found, about one-third of hippocampal neurons belong to a subpopulation that does divide periodically, with about 700 new neurons born in the region each day.
See “Brain Gain”
These brain cell investigations, in particular, have turned up a few surprises, says Frisén. In contrast to what’s been found in some other mammals, his group found that postnatal humans have little to no generation of new cells in the smell-processing area known as the olfactory bulb. By contrast, they later reported, humans do birth new neurons in an area called the striatum, where new cells are rarely found in other animal brains.
More recently, the group found that the rate of turnover of myelin-generating oligodendrocytes in the brain was very different in people with multiple sclerosis compared with the turnover of these cells in animal models of the disease. The researchers have also found that in the immune system, the generation of new naive T cells slows with age, and the group identified molecular pathways associated with that slowdown.
Types of materials in the body that don’t turn over at all—such as the enamel Frisén’s group analyzed early on—can shed light not just on particular biological processes, but on an organism’s age. Shortly after Frisén’s initial 14C papers were published, University of Copenhagen forensic pathologist Niels Lynnerop and colleagues set out to see whether proteins in the eye known as lens crystallines reflect a person’s age. Overall, they found, the age of these proteins does align with the year a person was born.
Lynnerop says he and colleagues conducted the study with the idea that lens crystalline dating could help identify victims in situations such as mass casualty events. But its utility has turned out to be limited, he explains, partly because of the use of DNA testing to help make such identifications. Lynnerop knows of only one case where the lens-dating technique has been used forensically, he says: to date the births and deaths of three newborns found in a freezer in Germany more than a decade ago. Because of their ages, the babies didn’t yet have tooth enamel that could be dated.
Although lens-crystalline dating has been little-used in forensics, the technique has been put to a very different use: revealing the age of sharks. Sharks, unlike humans, constantly lose teeth and grow new ones, so researchers can’t use enamel to determine the animals’ ages. So in 2009, when a University of Copenhagen–based team set out to figure out how long the slow-growing Greenland shark lives, they analyzed 14C levels in lens crystallines from 28 animals caught as bycatch. Only the three smallest sharks from the sample had been born during or after the bomb pulse, the researchers reported. After using the bomb-pulse dating combined with the smaller sharks’ sizes to estimate the species’ growth rate, the team calculated that the oldest shark in their sample had been somewhere between 272 and 512 years old. The results positioned Greenland sharks as the longest-lived vertebrate known to science.
This technique has a pretty good record of turning population biology on its head for species that we thought we understood.—Allen H. Andrews, University of Hawai'i at Manoa
Bomb pulse 14C has revealed surprisingly long lifespans for some other aquatic species as well. By dating the core of an ear bone known as the otolith, for example, Allen H. Andrews of the University of Hawai’i at Manoa and colleagues reported in 2016 that the bluespine unicornfish (Naso unicornis) can live for more than 50 years. “The utility, really, of this approach is putting ourselves in a better position to manage the fishery, because it turns out that if it’s a really long-lived species, they may not be as productive as we originally thought,” Andrews tells The Scientist. “This technique has a pretty good record of turning population biology on its head for species that we thought we understood.”
That’s the case, he says, with the bigmouth buffalo (Ictiobus cyprinellus), a freshwater fish that he and his colleagues have shown to have a lifespan of more than 100 years. Anglers hunt the species using bows and arrows in some midwestern states, he says—which, given its long lifespan, is cause for concern.
In addition to addressing questions of significance to science and conservation, it’s perhaps inevitable that a method to determine the vintage of living organisms would eventually be applied to wine. In a 2011 book chapter, researchers at the University of Adelaide detailed how they’d verified the ages of decades-old bottles using bomb pulse 14C. And a team at the Institute of Earth Environment in Xi’an, China, reported in 2016 that a similar technique can be used to authenticate the ages of vintage liquors.
But as the nuclear blasts’ 14C signature gradually fades from the atmosphere, its scientific utility, too, is declining. “Bomb pulse dating works exactly best when we had that very, very steep increase during the bomb testings,” which allowed for relatively precise dating of biological material that was living in that time period, says Lynnerop. “Now the curve is going flatter and flatter which means that the precision is not so good.”