Provided by the National Institute of Mental Health
The Adult Brain Can Generate New Nerve Cells
Once, neuroscientists believed that our complement of nerve cells was created
prenatally and during the first years of life, and that no new neurons could be
generated. Now we know that this belief was wrong. It had been thought that
unlike other bodily organ systems, such as skin which continuously generates
cells to replace those that die or are injured, neurons that were lost due to
trauma, stroke or disease were irreplaceable. Recent research has shown that the
brain can add nerve cells during adult life. This process is called neurogenesis.
These findings and their implications for therapeutic interventions are
currently under investigation.
The first solid evidence that adult brains may be able to add nerve cells
emerged several years ago from basic animal research involving songbirds.
Researchers showed that increases and decreases in the number of neurons in
certain brain areas occurred in conjunction with the mating season.1 Previous
research had indicated that a low level of neurogenesis occurs in certain
regions of the rodent brain, including the hippocampus (a brain region required
for the formation of certain types of memory) during the adolescent period, long
after the generation of neurons in most brain areas had ceased.2 But the
songbird research yielded such dramatic evidence of neurogenesis that interest
in higher animal models was rekindled. Animal investigators went on to show that
not only does the rodent brain continue to generate neurons during late
adolescence, but that this process continues even into adulthood.3,4
With interest spurred by new technical developments in imaging, numerous
laboratories are developing a clearer and encouraging picture of neurogenesis.
In 1998 and 1999, NIMH-supported investigators showed that the hippocampus in
adult monkeys also generates neurons.5,6 Within a few months of these reports,
other researchers demonstrated the phenomenon of neurogenesis in the adult human
brain!7
Ongoing work in laboratories nationwide is finding that the rate at which the
new nerve cells are generated can be influenced by environmental factors. For
example, stress inhibits the formation of new neurons.8 These findings are
changing the way neuroscientists think about the nervous system, and about
possible future interventions to address nerve cell loss due to trauma, stroke
or, eventually, diseases like schizophrenia or autism. Information gained to
date about neurogenesis also fits well with data from brain imaging studies that
reveal a relative decrease in hippocampal volume in patients suffering from
recurrent depressive illness with its accompanying increase in circulating
levels of stress hormones.9 It also offers hope that if the rate of generation
of new neurons is open to outside influences, perhaps therapeutic interventions
may be developed that are capable of actively and precisely repairing the damage
wreaked on brains by severe, protracted mental illnesses.
For More Information
Please visit the following link for more information about organizations that
focus on the human brain.
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All material in this fact sheet is in the public domain and may be copied or
reproduced without permission from the Institute. Citation of the source is
appreciated.
NIH Publication No. 01-4602
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References
1Nottebohm F. A brain for all seasons: cyclical anatomical changes in song
control nuclei of the canary brain. Science, 1981; 214(4527): 1368-70.
2Bayer SA. Development of the hippocampal region in the rat. I. Neurogenesis
examined with 3H-thymidine autoradiography. Journal of Comparative Neurology,
1980; 190(1): 87-114.
3Bayer SA. Changes in the total number of dentate granule cells in juvenile and
adult rats: a correlated volumetric and 3H-thymidine autoradiographic study.
Experimental Brain Research, 1982; 46(3): 315-23.
4Stanfield BB, Trice JE. Evidence that granule cells generated in the dentate
gyrus of adult rats extend axonal projections. Experimental Brain Research,1988;
72(2): 399-406.
5Gould E, Tanapat P, McEwen BS, et al. Proliferation of granule cell precursors
in the dentate gyrus of adult monkeys is diminished by stress. Proceedings of
the National Academy of Sciences USA, 1998; 95(6): 3168-71.
6Gould E, Reeves AJ, Graziano MS, et al. Neurogenesis in the neocortex of adult
primates. Science, 1999; 286(5439): 548-52.
7Eriksson PS, Perfilieva E, Bjork-Eriksson T, et al. Neurogenesis in the adult
human hippocampus. Nature Medicine, 1998; 4(11): 1313-7.
8Gould E, Tanapat P. Stress and hippocampal neurogenesis. Biological Psychiatry,
1999; 46(11): 1472-9.
9Sheline YI, Sanghavi M, Mintun MA, et al. Depression duration but not age
predicts hippocampal volume loss in medically healthy women with recurrent major
depression. Journal of Neuroscience, 1999; 19(12): 5034-43.
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