For years, scientists have believed that hippocampal neurons stop being generated as we reach old age. But, a new study dramatically overturns this long-held belief.
As the population of the United States lives longer and the number of older adults slowly rises, understanding how the brain reacts as it ages is becoming ever more important.
Of particular interest is the production of new neurons, or neurogenesis, in the hippocampus. This is a brain region vital for turning short-term into long-term memories, among other tasks, such as navigation.
If the hippocampus degenerates, so too does memory performance.
Over the years, the idea that neurogenesis in the hippocampus stops as we enter our twilight years has been hotly debated. In rodents and primates, for instance, the ability to grow new neurons in this region has been shown to slow with age.
As this occurs, one part of the hippocampus called the dentate gyrus, particularly important for the formation of new memories, shrinks in volume. For some time, scientists have believed that this occurs in humans, too.
Recently, researchers from Columbia University and New York State Psychiatric Institute, both in New York City, NY, ran an experiment in the hope of reaching a definitive conclusion.
Previous studies have explored hippocampal volume in aging humans, but the results have been hampered by the technical difficulties of accurately measuring parts of the brain using scanning technology.
To circumvent these issues, the researchers inspected the whole, autopsied hippocampi of 28 men and women, all aged 14–79, who had died suddenly. None of the individuals had long-term health problems or cognitive deficits, and none had had a significantly stressful life event in their last 3 months of life.
The team also ensured that none of the individuals had been depressed or taking antidepressant medications. This is important because previous research by the same team demonstrated that antidepressants negatively affect neurogenesis.
This recent study was the first to assess the numbers of “newly formed neurons” and blood vessels in the whole human hippocampus following death. The results are published this week in the journal Cell Stem Cell.
Surprisingly, the scientists found that older men and women can generate the same level of new brain cells as younger people. Lead study author Dr. Maura Boldrini, an associate professor of neurobiology at Columbia University, explains the results.
“We found,” she says, “that older people have similar ability to make thousands of hippocampal new neurons from progenitor cells as younger people do. We also found equivalent volumes of the hippocampus (a brain structure used for emotion and cognition) across ages.”
Even the oldest brains, in their 8th decade of life, were still producing new brain cells. The authors write, “We found similar numbers of intermediate neural progenitors and thousands of immature neurons.”
However, there were certain deficits detected. As Dr. Boldrini continues, “[O]lder individuals had less vascularization, and maybe less ability of new neurons to make connections.”
So, the older adults had fewer blood vessels and a reduced supply of progenitor cells, which are similar to stem cells.
The authors believe that this reduced pool of progenitor cells may play a part in explaining the drop in cognitive-emotional resilience in older adults. Additionally, the reduction in blood vessels and a decrease in cell-to-cell interaction in the hippocampus could also result in a cognitive slowing.
The take-home message is that brain cells in the hippocampus continue to be produced into later life, but also that those cells are less connected and have a reduced supply of oxygen and nutrients.
Following on from this discovery, Dr. Boldrini wants to continue investigating how hippocampal neurogenesis is affected by other factors, such as transcription factors, hormones, and other biochemical pathways.