Neurogenesis is the process of neuron generation from stem cells and progenitors.
New neurons generated within the hippocampus of an adult animal can functionally integrate into the neural circuitry [Kee et al., 2007; van Praag et al., 2002]. The dental gyrus size does not increase with age and remains fairly constant [Ben Abdallah et al., 2010]. Age has a large effect on neurogenesis. Neurogenesis decreases dramatically at old age [Kuhn et al., 1996]. The major decline in neuronal cell generation appears to occur before middle age, indicating that it represents a critical turning period of adult hippocompal neurogenesis [Wu et al., 2008]. Neurogenesis is suggested to peaks at puberty and this might be related to the transition from juvenile to adult behaviour [Amrein and Lipp, 2009].
Among seven different species, the rate of decrease (which appears to be exponential) of neurogenesis correlates with species longevity, but not body mass nor basal metabolic rate. This means longer-lived species have a slower rate of decline in proliferation and neurogenesis .
Developmental ages of mice and corresponding human ages are estimated. 3 months is the age at which a mouse becomes an adult (corresponds to approximately 20 years in humans) [Harrison, 2010; http://research.jax.org/faculty/harrison/ger1vLifespan1.html].
Of the total number of cells added to the dentate gyrus postnatally, 69% of these will be added before adulthood, and only 31% of cells will be added during adulthood. Only 8.5% of new cells will be added from middle age onwards. Therefore, for most of adult life, there is little addition of new cells .
It seems like that there is no "critical turning point" for neurogenesis, but rather than a smooth decrease over time and there appears little evidence for neurogenesis to peak postnatally .
Kee, N., Teixeira, C.M., Wang, A.H., Frankland, P.W., 2007. Preferential incorporation of adult-generated granule cells into spatial memory networks in the dentate gyrus. Nat. Neurosci. 10, 355–362.
van Praag, H., Schinder, A.F., Christie, B.R., Toni, N., Palmer, T.D., Gage, F.H., 2002. Functional neurogenesis in the adult hippocampus. Nature 415, 1030 –1034.
Ben Abdallah, N.M., Slomianka, L., Vyssotski, A.L., Lipp, H.P., 2010. Early age-related changes in adult hippocampal neurogenesis in C57 mice. Neurobiol. Aging 31, 151–161.
Wu, C.-W., Chang, Y.-T., Yu, L., ing -Chen, H., Jen, C. J., Wu, S.-Y., Lo, C.-P., Kuo, Y.-M., 2008. Exercise enhances the proliferation of neural stem cells and neurite growth and survival of neuronal progenitor cells in dentate gyrus of middle-aged mice. J. Appl. Physiol. 105, 1585–1594.
Amrein, I., Lipp, H.P., 2009. Adult hippocampal neurogenesis of mammals: evolution and life history. Biol. Lett. 5, 141–144.
Harrison, D.E., 2010. Life span as a biomarker. The Jackson Laboratory, Figure V.3. Available at: http://research.jax.org/faculty/harrison/ger1vLifespan1.html.
Kuhn, H.G., Dickinson-Anson, H., Gage, F.H., 1996. Neurogenesis in the dentate gyrus of the adult rat: age-related decrease of neuronal progenitor proliferation. J. Neurosci. 16, 2027–2033.