Adult neurogenesis is the process in which neurons are generated from neural stem cells in the adult. This process differs from prenatal neurogenesis. In most mammals, new neurons are born throughout adulthood in two regions of the brain: . More attention has been given to the neurogenesis in the dentate gyrus than in the striatum. In rodents , many of the newborn dentate gyrus neurons die shortly after they are born,  but a number of them become functionally integrated into the surrounding brain tissue. Adult neurogenesis is reported to play a role in learning and memory, emotion, stress, depression, response to injury, and other conditions.
Adult Hippocampal Neurogenesis: Regulation and Possible Functional and Clinical Correlates
Neurogenesis in the adult human hippocampus
The formation of new neurons in the adult central nervous system CNS has been recognized as one of the major findings in neuroanatomical research. The hippocampal formation HF , one of the main targets of these investigations, holds a neurogenic niche widely recognized among several mammalian species and whose existence in the human brain has sparked controversy and extensive debate. Many cellular features from this region emphasize that hippocampal neurogenesis suffers changes with normal aging and, among regulatory factors, physical exercise and chronic stress provoke opposite effects on cell proliferation, maturation and survival. Considering the numerous functions attributable to the HF, increasing or decreasing the integration of new neurons in the delicate neuronal network might be significant for modulation of cognition and emotion. The role that immature and mature adult-born neurons play in this circuitry is still mostly unknown but it could prove fundamental to understand hippocampal-dependent cognitive processes, the pathophysiology of depression, and the therapeutic effects of antidepressant medication in modulating behavior and mental health. Neurogenesis is the biological process through which new neurons are formed Altman and Das,
However, a fundamental problem is that it remains unclear as to what extent AHN actually occurs in humans. The answer to this is indispensable when physiological and pathological functions of human AHN are deduced from studies of rodent AHN, but there are controversial data on the extent of human AHN. In this review, studies on AHN performed in rodents and humans will be briefly reviewed, followed by a discussion of the studies in non-human primates. Adult hippocampal neurogenesis AHN is now widely studied in the neuroscience field, because newly born neurons induce large-scale neuronal circuit alterations that are reported to be involved in learning and memory Lledo et al.
Of the neurogenic zones in the adult brain, adult hippocampal neurogenesis attracts the most attention, because it is involved in higher cognitive function, most notably memory processes, and certain affective behaviors. Adult hippocampal neurogenesis is also found in humans at a considerable level and appears to contribute significantly to hippocampal plasticity across the life span, because it is regulated by activity. Adult hippocampal neurogenesis generates new excitatory granule cells in the dentate gyrus, whose axons form the mossy fiber tract that links the dentate gyrus to CA3. It originates from a population of radial glia-like precursor cells type 1 cells that have astrocytic properties, express markers of neural stem cells and divide rarely. They give rise to intermediate progenitor cells with first glial type 2a and then neuronal type 2b phenotype.