Neurogenesis happens in humans, too

Neurogenesis indeed occurs in the human adult brain, according to a new paper in this week's Science. The findings provide new evidence for the long-controversial theory, and suggest that neurogenesis occurs in the olfactory bulb and follows the same pattern as in other mammalian brains.

The new paper "disproves the dogma that the human brain doesn't have similar pathways of neurogenesis to that found in other mammalian brains," author Richard L.M. Faull of the University of Auckland said in an Email.

"The data are very nice and very clear, just what we'd expect based on rodent work," agreed Heather Cameron, of the National Institute of Mental Health in Bethesda, MD, who is not connected to the study.

Many vertebrates create new central nervous system neurons at all stages of life. This talent is linked to the ability to regenerate after CNS injury, so is most common in fish and amphibians. For most of the 20th century, conventional wisdom held that neurogenesis in mammals happened exclusively during embryonic and early postnatal life. Only within the last decade have researchers come to accept that adult neurogenesis does occur in selected mammalian brain regions, notably the hippocampus and olfactory system. However, neurogenesis in the human adult olfactory bulb has remained controversial.

Faull and his colleagues found that baby neurons called neuroblasts, born in the subventricular zone (SVZ), reach the human adult olfactory bulb, and do it via the rostral migratory stream (RMS). Rodents use an RMS too, although it differs anatomically from the human RMS. Using MRI, cell-specific markers, and electron microscopy, the researchers found progenitor cells with migratory characteristics, and also cells that become mature neurons in the olfactory bulb, in the adult human RMS.

However, Arturo Alvarez-Buylla at the University of California-San Francisco said he is not convinced by the findings. "It is an interesting work, but it does not demonstrate neurogenesis and much less migration along the RMS," he said in an Email." Indeed, the findings appear to contradict a 2004 Nature paper he co-authored, which found no migrating neuroblasts in the human SVZ or in the pathway to the olfactory bulb.

Fred Gage, of the Salk Institute for Biological Studies in La Jolla, CA, who was not connected with the study, said he expected the new paper to cite 2004 work by Andr￩anne Bedard and Andr￩ Parent, which also offered evidence of newly generated neurons in the human olfactory bulb. (Indeed, Princeton's Elizabeth Gould said in an Email she was "pleased to see these authors have corroborated and extended the work of Bedard and Parent.") The newest paper uses more technology, focuses more on the RMS, and in general provides more evidence, but "they do come to a very similar conclusion," Gage said in an Email.

Parent himself said he admired the latest findings, but was "disappointed" the authors did not mention his work. "We have the very highest respect for Dr Parent's work and we very much apologize for this oversight," Faull responded.

Questions remain, however, as to what those new neurons can actually do, given that the function of neurogenesis remains somewhat of a mystery. Seizures stimulate neuron increase in the hippocampus, "but we don't know whether [neurogenesis] is trying to compensate" for damaged neurons, Cameron told The Scientist. There's some evidence new neurons may combat depression, she noted, but increasing neuron number would not necessarily be helpful in all conditions.

According to Faull, he and his colleagues have preliminary unpublished evidence that SVZ neural precursors don't just move to the olfactory bulb, but also leave the RMS and head for adjacent regions of the basal ganglia and cerebral cortex. In experimental rat models of diseases like Parkinson's, migratory neuroblasts replace neurons lost in the basal ganglia, suggesting this neural migration could have therapeutic benefits, he said.

Tabitha Powledge
mail@the-scientist.com

Links within this article:

MA Curtis, et al, "Human neuroblasts migrate to the olfactory bulb via a lateral ventricular extension," Sciencexpress, February 15, 2007
'http://www.sciencemag.org

AJS Rayl, "Research turns another 'fact' into myth," The Scientist, February 15, 1999
http://www.the-scientist.com/article/display/18407

R. Lewis, "Neurobiology of rehabilitation," The Scientist, June 30, 2003
http://www.the-scientist.com/article/display/13905

Richard Faull
http://www.health.auckland.ac.nz/anatomy/staff/richard_faull.html

Heather Cameron
http://neuroscience.nih.gov/Lab.asp?Org_ID=411

D. Steinberg, "Stem cells tapped to replenish organs," The Scientist, November 27, 2000
http://www.the-scientist.com/article/display/12148

Arturo Alvarez-Buylla
http://www.ucsf.edu/neurosc/faculty/neuro_alvarez-buylla.html

N Sanai et al, "Unique astrocyte ribbon in adult human brain contains neural stem cells but lacks chain migration," Nature, February 19, 2004.
http://www.the-scientist.com/pubmed/14973487

JF Wilson, "Human neurogenesis," The Scientist, December 11, 2000
http://www.the-scientist.com/article/display/12172

A Bedard and A Parent, "Evidence of newly generated neurons in the human olfactory bulb," Brain Res Dev Brain Res, July 19, 2004.
http://www.the-scientist.com/15246702

Elizabeth Gould
http://weblamp.princeton.edu/_{psych/psychology/research/gould/index.php}

JM Parent et al, "Dentate granule cell neurogenesis is increased by seizures and contributes to aberrant network reorganization in the adult rat hippocampus," J Neurosci, May 15, 1997.
http://www.the-scientist.com/pubmed/9133393

JE Malberg et al, "Chronic antidepressant treatment increases neurogenesis in adult rat hippocampus," J Neurosci, December 15, 2000.
http://www.the-scientist.com/pubmed/11124987

B Steiner, "Enriched environment induces cellular plasticity in the adult substantia nigra and improves motor behavior function in the 6-OHDA rat model of Parkinson's disease," Exp Neurol, June 2006.
http://www.the-scientist.com/pubmed/16360152