One of my least favorite factoids in the neurosciences is the fact that the human brain begins to lose weight after the age of 45 years. I have other parts of me where weight loss would be much more welcome. However, I optimistically enjoy data that suggest that we can continue to produce new neurons in the adult brain.
The existence of neurogenesis in the adult hippocampus has enjoyed substantial support , although neurogenesis involving the neocortex has been much more contentious [2, 3].
Maurice Curtis of the University of Auckland and Peter Eriksson of the Sahlgrensak Academy in Sweden report in Science additional evidence of adult neurogenesis in humans . Their participants were cancer patients who had been given a chemical known as BrdU, which was subsequently incorporated into the DNA of any new cells following its administration. This technique allows physicians to track the growth of tumors, which of course are characterized by rapid and uncontrolled growth.
When the patients eventually died, Curtis, Eriksson, and their colleagues found cells with BrdU in the olfactory bulbs. In addition, the BrdU allowed the researchers to identify new neurons in the ventricular zones. Even more surprisingly, examination of these brains identified tube-like structures linking the ventricular zone with the olfactory bulbs. These tubes, previously observed in rodents but not humans, apparently form a highway used by the newly born cells to migrate to the olfactory bulbs.
Although the number of these new cells born in the adult brain is quite tiny relative to the massive neurogenesis that occurs early in development, I’ll be quite happy to have them join the party.
This image from New Scientist illustrates the pathway discovered by Curtis et al.
 Eriksson PS, Perfilieva E, Bjork-Eriksson T, Alborn AM, Nordborg C, Peterson DA, Gage FH. (1998) Neurogenesis in the adult human hippocampus. Nat Med. Nov;4(11):1313-7.
 Gould E, Reeves AJ, Graziano MS, Gross CG. (1999). Neurogenesis in the neocortex of adult primates. Science. Oct 15;286(5439):548-52.
 Bhardwaj, R. D., Curtis, M. A., Spalding, K. L., Buchholz, B. A., Fink, D., Bjork-Eriksson, T., et al. (2006). From the Cover: Neocortical neurogenesis in humans is restricted to development (Vol. 103, pp. 12564-12568).
 Curtis, M. A., Kam, M., Nannmark, U., Anderson, M. F., Axell, M. Z., Wikkelso, C., et al. (2007). Human Neuroblasts Migrate to the Olfactory Bulb via a Lateral Ventricular Extension (pp. 1136281).