Researchers have shown for the first time that new neurons can form in one part of adult human brains. The finding, reported in the November issue of the journal Nature Medicine, holds out a distant hope that researchers may one day learn how to stimulate regeneration in areas of the brain that atrophy in Alzheimer's and other neurodegenerative diseases.
For years, scientists have thought that all human brain neurons are formed during fetal development, and no new ones are generated after birth. That theory was based on experiments done more than 10 years ago by Pasko Rakic and his colleagues at Yale University. They found no evidence that neurons are dividing in the brains of rhesus monkeys. In the following years, researchers did turn up evidence of new neurons in other adult mammals, including rodents, tree shrews, and marmoset monkeys. But it seemed that the ability to grow new neurons had been lost along the evolutionary path to humans.
To answer the question once and for all, neuroscientists Fred Gage of the Salk Institute in La Jolla, California, and Peter Eriksson and his colleagues at the Sahlgrenska University Hospital in Göteborg, Sweden, took advantage of a cancer study in Sweden. Patients had been given a marker chemical called bromodeoxyuridine (BrdU), which is incorporated into the DNA of dividing cells, to follow the growth of their tumors. After five of those patients died, the scientists examined samples of their brain tissue. They found BrdU-labeled neurons in the dentate gyrus, a small part of the brain area called the hippocampus, which is involved in forming memories. This was a telltale sign that those cells had been recently formed by the division of precursor cells.
Although this overturns the notion that new neurons do not arise in the human brain, the practical significance is unclear. "We don't have any evidence that these cells are hooked up in any way," Gage says, noting that neurons are only useful if they are connected to networks of other neurons. And no one knows why the dentate gyrus in particular would need to generate neurons throughout life, or what keeps this from happening elsewhere in the brain.
But the finding may suggest that researchers might someday learn how to encourage regeneration in other parts of the brain. Rakic sees an opportunity to learn how to produce new cells in the areas that are hit by neurodegenerative disorders. That's a tall order, but it is a concept that people "weren't even thinking about" before this finding, says Gage.