One of the great discoveries of the late 20th century was that the brain was also capable of generating new brain cells: neurogenesis.
Previously, it was thought that we were born with every last brain cell we would ever have, and that no more could be created. This turns out not to be the case. Neurogenesis is the creation of new brains cells from pre-cursor cells that occur in the hippocampus, which is so important for memory.
Although there had been some evidence of this from work done on bird brains and other animals, it was eventually identified in human brains from the tenacious work of a Swedish stem cell neuroscientist Peter Eriksson, from the Sahlgrenska University Hospital, Gothenburg. The biochemical marker that had been used in animal experiments couldn’t, ethically, be used for human experimentation for this purpose at this time. However, elsewhere, approved medical studies were being carried out in terminal cancer patients – where new cells are created by the cancer – this same biochemical marker was being used to try and evaluate the effectiveness of treatment, because it would flag up neurogenesis. Eriksson’s scientific assumption was that if the brain was capable of neurogenesis, these new cells would show evidence of the same biochemical marker that was being used to identify new cancer cells. So he asked if he could have the brains of these patients, after their death, to examine and see if there was – as he thought there may be – any evidence of neurogenesis.
This work was also being explored by the eminent neuroscientist Professor Fred Gage, at the Salk Institute for Biological Studies, California. After extensive study, and ruling out the possibility that new cells in the brain could be secondary cancer cells, the scientists finally got their ‘Eureka!’ moment in 1998. The brain was capable of neurogenesis.
‘All the brains had evidence of new cells exactly in the area where we had found neurogenesis in other species,’ said Gage. ‘And we could prove through chemical analysis that they were mature neurons. The neurons were born in the patients when they were in their fifties and seventies, and these neurons stayed alive until the people died. That was the first evidence for neurogenesis in the adult human brain. So now we know that in some areas of the brain, new neurons are being made all the time. It was a surprise because we thought the brain was stagnant. But in this region of the hippocampus, there are these little baby cells that are dividing, and over time, they mature and migrate into the circuitry and become a full-blown adult neuron with new connections. And this is occurring throughout life. The finding brought us an important step closer to the possibility that we have more control over our own brain capacity than we ever thought possible.’
What was also becoming clear from continuing research with mice was that voluntary exercise was as crucial to neurogenesis as environmental enrichment. So not only did the old adage, ‘use it or lose it’, apply to neurogenesis, physical activity is crucial too.
‘We think that voluntary exercise increases the number of neural cells that divide and give rise to new neurons in the hippocampus,’ says Gage. ‘But we think it is environmental enrichment that supports the survival of these cells. Usually, 50% of the new cells reaching the dentate gyrus of the hippocampus die. But if the animal lives in an enriched environment, many fewer of the cells die. Environmental enrichment doesn’t seem to affect cell proliferation and the generation of new neurons, but it can affect the rate and the number of cells that survive and integrate into the circuitry.’