An open access review paper: "Several studies suggest that an increase in adult neurogenesis has beneficial effects on emotional behavior and cognitive performance including learning and memory. The observation that aging has a negative effect on the proliferation of neural stem cells has prompted several laboratories to investigate new systems to artificially increase neurogenesis in senescent animals as a means to compensate for age-related cognitive decline. ... recent evidences indicate that the relative abundance of stem cells in certain organs does not necessarily correlate with their impact on organ function. Specifically, the mammalian brain is perhaps the organ with the lowest regenerative potential but the one in which the signs of aging are more manifested. Using the words of the renaissance writer Michel de Montaigne, 'age imprints more wrinkles on the mind than it does on the face' indicating that age-related cognitive decline has the highest impact on the quality of life. To which extent this decline is dependent on neural stem and progenitor cells (together referred to as NSCs) is hard to tell but growing evidences indicate that, despite their negligible numbers, the few resident NSCs that are located in specific brain regions, most notably the subgranular zone of the hippocampus, seem to play a major role in cognitive functions such as learning, memory, and emotional behavior by generating, through intermediate progenitors, neurons that are constantly added to the brain circuitry throughout life. ... the available data strongly suggests that aging almost exclusively acts at the level of NSC proliferation. Yet, the many contradicting results and uncertainties on identifying the exact causes of this 'decreased proliferation' [need] to be fully acknowledged in order to give a rigorous and meaningful direction to this relatively new field. ... The fact that NSCs can efficiently respond to physiological and pathological stimuli to increase neurogenesis indicates that stimulation of endogenous NSCs offers a promising alternative to transplantation approaches that until now were intensely investigated."