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When the scientists examined the postmortem brain tissue, they found that new neurons had indeed sprung forth during adulthood. The cells were located in a part of the hippocampus—a pair of seahorse-shaped structures located deep within the brain and involved in memory and learning. The compound was later found to be toxic, however, and the experiment was never repeated. Since , a number of studies have demonstrated that new neurons are generated in the same small region of the hippocampus in mice and appear to play an important role in memory and learning, says Kirsty Spalding, a molecular biologist at the Karolinska Institute in Stockholm and lead author of the new study.
Because the work was never confirmed by independent research, however, scientists have fiercely argued over whether the neuron birth seen in mice also occurs in people. The method, which has taken Spalding more than a decade to develop, hinges on a massive pulse of radioactive carbon isotopes released by nuclear explosions in the s and '60s, which doubled the amount of carbon in the atmosphere. This pulse stopped with the Limited Test Ban Treaty of , which banned aboveground tests of nuclear weapons, and the unstable carbon isotopes have steadily decayed.
Because cells incorporate carbon from the atmosphere into their DNA as they divide, the proportion of carbon to the more stable carbon isotope carbon acts as a time stamp for when a cell was born.
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Spalding has been using this ratio to determine the age of teeth in forensic investigations and the turnover rate of fat cells. But she had to improve the sensitivity of the technique so that it could detect the isotopic ratio in DNA from the roughly 6-gram sliver of neural tissue in the hippocampus thought to produce new neurons, the dentate gyrus. At best, the isotope is present in only one out of every 15 neurons, she says, making it difficult to detect in small amounts of tissue.
For the first 5 years, Spalding worked on finding an effective way of separating the roughly 20 million neurons in the dentate gyrus from other types of hippocampal cells and then extracting their DNA. Discovering that she could use a fluorescence-activated cell sorting machine to distinguish non-neuronal cells from neurons by making them glow in different colors was "a high point," she says. The next 5 years were largely spent on finding ways to purify the DNA samples and extract and analyze the carbon atoms using high-powered particle accelerators.
After finally getting the technique down pat, Spalding decided that it was time to try it on some real human brain tissue. She and her colleagues extracted hippocampi from 55 deceased people who had given informed consent to have their brains studied.
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They then ground up the tissue samples, sorted the cells, and extracted the DNA. Next, she sent the purified genetic material to the Lawrence Livermore National Laboratory in California, where it was reduced to pure carbon pellets and split into different carbon isotopes by weight in a particle accelerator, allowing the researchers to calculate the ratio between carbon and carbon More than a third of hippocampal neurons were regularly replaced, with roughly new neurons added each day during adulthood , they report online today in Cell. Kempermann says that his own and other's studies in mice indicate that fresh adult neurons have a specific function in the hippocampus—for example, in helping the brain distinguish between things that belong to the same category, or comparing new information to what it has already learned from experience.