The vast majority of neurons are generated during fetal development, but scientists have shown that in some regions of the brain, new neurons can continue to be made in adult animals.
"It is really a feat of biology," said Alvarez-Buylla. "The cell has to be born, then migrate and integrate into the tissue, make new extensions to connect with other cells, and then it has to contribute to the brain function."
Although this process has been well studied in mice, rats and canaries among other animals, only a handful of studies have sought to discover if neurogenesis also occurs in people after childhood.
"It's tricky," Paredes said. "It's hard to study human brain tissue, not only to get the samples, but to know how to analyze them and have confidence in the result."
The samples used in this work were collected from hospitals in China, Spain, Los Angeles and San Francisco. Most of the brain tissue came from people who had just died, but 22 samples came from brain operations that were performed on living people as a treatment for epilepsy.
"In those cases we were able to get the tissue very quickly, preserve them in the best way possible and then analyze them with less concern for degradation," Paredes said.
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Instead of looking for new neurons themselves, the authors analyzed their tissue samples for combinations of proteins that are associated both with young neurons and with the stem cells that would make new neurons.
To make sure there was no mistake with their detection method, the authors looked to see if they could find evidence of new neuron growth in the brain tissue of fetuses, where they were certain that new neurons were developing. And indeed, when they looked at the fetal hippocampus, they were able to see that it was filled with young neurons.
"So in that case, we thought, we definitely have the tools to see them," Paredes said.
Next they wondered if perhaps their methods were only capable of detecting neurogenesis in young brains, and not in the brains of adults. To see if that was the case they analyzed two post-mortem autopsy samples and looked for evidence of young neurons in another region of the brain that is known to produce new neurons into childhood. There, the authors did find rare examples of young neurons, but very few.