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Neurogenesis in the Adult Human Brain

Until recently, most biopsychologists and other brain scientists believed that the development of new neurons in the brain of an adult human was impossible. However, a recent report suggests that this long-held notion is invalid and that neurogenesis (the birth of new neurons) occurs in the brain over much of the adult human lifespan.

Researchers led by Peter Eriksson of the Sahlgrenska University Hospital in G`teborg, Sweden and Fred H. Gage of the Salk Institute in San Diego, California have found that neurogenesis occurs in the brains of adult humans as old as 72 years of age! This neurogenesis is believed to be due to neural stem cells that exist in the adult human brain. Neural stem cells are cells that are able to differentiate into many different types of neurons. Neural stem cells are common in the brain during embryonic development (see BIOPSYCHOLOGY, pp. 403), but they were believed to disappear from the mature brain...a belief that supported the idea that neurogenesis in the adult brain was difficult if not impossible. However, Eriksson, Gage and their colleagues used a chemical marker to specifically identify newly-formed neurons in the adult human brain. These new neurons could only be due to the proliferation and division of neural stem cells into new neurons, because mature neurons are incapable of cell division.

Interestingly, this neurogenesis was observed in the dentate gyrus of the hippocampal formation, a brain region that plays a key role in certain types of memory (see BIOPSYCHOLOGY, Fig 14.7 and pp. 394-397). Neurogenesis in this part of the brain has also been demonstrated in a variety of nonhuman primates and in rodents, thus opening the door for animal studies to better understand the phenomenon. To date, such studies have indicated that the stem cells that give rise to new neurons in the dentate gyrus lie in the hilar region that separates the dentate gyrus from area CA4/CA3 of the hippocampus. Many newly formed cells seem to die almost as soon as they develop; however, other cells are able to migrate into the dentate gyrus and appear to integrate themselves into the existing neural circuitry.

At this point, an obvious question for any biopsychologist is "Are these newly developed neurons functionally significant? Do they contribute to the behavior of the organism?" The answer to this question is still open to some debate. However, indirect evidence from several sources suggests that neurogenesis is accompanied by behavioral changes, at least in laboratory rodents. For example, Gage and his colleagues have demonstrated that environmental enrichment enhances both neurogenesis and the ability of laboratory rodents to perform a task of spatial memory (e.g., the Morris water maze, BIOPSYCHOLOGY pp. 394). Similarly, Elizabeth Gould and her colleagues from Princeton University have shown that learning enhances neurogenesis in the hippocampal formation. Gould had earlier shown that stress appears to inhibit neurogenesis in the hippocampus (BIOPSYCHOLOGY, p. 482), a finding that might account for some of the behavioral dysfunctions associated with chronic stress. And lest one assumes that neurogenesis is always beneficial to an animal, Parent and his colleagues have demonstrated that seizures promote neurogenesis in the dentate gyrus region of the hippocampus and that these new neurons contribute to a pathological rewiring of the neural circuitry in this brain region.

Thus far, the clinical relevance of the discovery of neurogenesis in the adult human brain remains a matter for speculation and further research. Many years of experimentation and study will be required before biopsychologists and other brain scientists will be able to answer questions such as "How is neurogenesis in the hippocampus controlled? How widespread is neurogenesis in the adult central nervous system? What IS the behavioral significance of neurogenesis in the adult central nervous system?". Future research will also undoubtedly focus on the use of manipulated neural stem cells in the treatment of neurodegenerative diseases such as Parkinson's Disease or Huntington's Disease (BIOPSYCHOLOGY, pp. 148-151). Neural plasticity at the level of connections between existing neurons in the adult brain has been recognized for years; the recent demonstration that neural plasticity extends to the level of neurogenesis raises a host of exciting new questions for further biopsychological inquiry!

Links:

Scientific American.
Science and the Citizen, November 1998: Dogma Overturned.

Letters, September 1999.
Growing Nerve Cells.

New York Times on the Web. Science/Health, June 15th, 1999.
Brain Stem Cell is Discovered, Twice.

Related Articles:

Peter S. Eriksson et al. (1998). Neurogenesis in the Adult Human Hippocampus. Nature Medicine (4): 1313-1317.

Elizabeth Gould et al. (1997). Neurogenesis in the Dentate Gyrus of the Adult Tree Shrew Is Regulated by Psychosocial Stress and NMDA Receptor Activation. Journal of Neuroscience (17): 2492-2498.

Gerd Kempermann & Fred H. Gage (1999). New Nerve Cells for the Adult Human Brain. Scientific American (280): 48-53.

Jack M. Parent et al. (1997). Dentate Granule Cell Neurogenesis Is Increased by Seizures and Contributes to Aberrant Network Reorganization in the Adult Rat Hippocampus. Journal of Neuroscience (17): 3727-3738.


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