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| Neuronal Stem Cells |
What neuronal processes have led to the changes in the hippocampi of London taxi drivers? Perhaps this is achieved by neurons migrating from one region to the posterior hippocampus? Another intriguing possibility is that the changes are the result of new neurons going to the region.
New neurons? Don't we have our complete store of neurons by early childhood? That previous dominant paradigm had been found incorrect. In the past two decades, researchers have shown that neurons are continually produced in a variety of animals, including humans. It isn't that neurons divide. They don't. Instead, the brain maintains a reservoir of stem cells that are capable of generating new neurons (neurogenesis). One area of the brain where stem cells have been found is the hippocampus.
The discovery of stem cells and neurogenesis began with basic research with songbirds. During each breeding season male songbirds need to recall their mating song. Starting in the 1980s researchers noted that the number of neurons in certain areas of the brain (especially the hippocampus) would increase in male birds around the start of the breeding season. The number of neurons in these areas would decrease after the mating season. This striking evidence led other researchers to look for neurogenesis in the brains of mammals. Studies on rats found substantial neurogenesis. In one part of the hippocampus alone, nearly 10,000 new neurons are generated each day in adult rats. Starting in the 1990s Elizabeth Gould of Princeton University found that the adult brains of several species of monkeys also undergo considerable neurogenesis.
Following these animal studies, researchers examined whether humans have the capacity for neurogenesis. They studied postmortem brain tissue from humans, using various stains to determine whether new neurons were being generated from dividing progenitor cells. They were able to find such new neurons in the hippocampus, showing that neurogenesis proceeds throughout life in at least some regions of the human brain.
Engaging in mental and physical activity is one important way elderly people can maintain their mental acuity. This aspect of conventional wisdom has been vindicated by medical research. Mental and physical activity reduces the risk of neurodegenerative disorders and improves the prognosis of stroke patients. Yet, we know little about the molecular mechanisms behind this effect. Studies in mice of neurogenesis in the hippocampus, however, point to one possible reason for why activity keeps the mind sharp. Mice who were exposed to an enriched environment for the second half of their lives showed a dramatic increase in neurogenesis in the hippocampus as compared with control subjects. The hippocampi from the mice that received the enriched treatment also appeared like those of younger animals. These results strongly suggest that activity maintains the proper function of the brain by increasing neurogenesis in the hippocampus.
Elizabeth Gould and other researchers studying neurogenesis think that the new neurons generated in the hippocampus are involved in modulation of the stress response as well as learning. There are some complications, however. Learning enhances neurogenesis but only under certain conditions. Moreover, experimental blockage of neurogenesis interferes with some types of learning but not others.
Our understanding of neurogenesis remains far from complete. Yet, tremendous progress has been made during the last two decades andurther progress is expected. In addition to what these studies tell us about how the brain works, they may also pave the way toward treatment of degenerative diseases like Alzheimer's and Parkinsons as well as brain trauma.
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| ||End Notes |
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- Campbell, N. A., and J. B. Reece. 2002. Biology. 6th ed., 1022. San Francisco: Addison-Wesley Longman, Inc.
- Jiang, Y., A. Lee, Y. Chen, V. Ruta, M. Cadene, B. T. Chait, and R. MacKinnon. 2003. X-ray structure of a voltage-dependent K+ channel. Nature. 423:33-41.
- Sullivan, J. M. 2002. Cannabinoid receptors. Curr. Biol. 12:R681
- Kempermann, G. 2002. Why new neurons? Possible functions for adult hippocampal neurogenesis. J. Neuroscience 22:635-38.
- Maguire, E., R. Frackowiak, and C. Firth. 1997. Recalling routes around London: Activation of the right hippocampus in taxi drivers. J. Neuroscience 17:7103-10.