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: The subgranular zone (SGZ), part of the dentate gyrus of the hippocampus, where neural stem cells give birth to granule cells (implicated in memory formation and learning). The subventricular zone (SVZ) of the lateral ventricles, which can be divided into three microdomains: lateral, dorsal and medial. Neural stem cells migrate to the olfactory bulb through the rostral migratory stream where they differentiate into interneurons participating in the sense of smell. In humans, however, few if any olfactory bulb neurons are generated after birth. 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 in rodents is reported to play a role in learning and memory, emotion, stress, depression, response to injury, and other conditions. The numbers of neurons born in the human adult hippocampus remains controversial; some studies have reported that in adult humans about 700 new neurons are added in the hippocampus every day, while more recent studies show that adult hippocampal neurogenesis does not exist in humans, or, if it does, it is at undetectable levels. Recent evidence shows that adult neurogenesis is basically extinct in humans. The role of new neurons in human adult brain function thus remains unclear. Mechanism Adult neural stem cells Neural stem cells (NSCs) are the self-renewing, multipotent cells that generate the main phenotypes of the nervous system. Lineage reprogramming (trans-differentiation) Emerging evidence suggests that neural microvascular pericytes, under instruction from resident glial cells, are reprogrammed into interneurons and enrich local neuronal microcircuits. This response is amplified by concomitant angiogenesis. Model organisms of neurogenesis Planarian Planarian are one of the earliest model organisms used to study regeneration with Pallas as the forefather of planarian studies. Planarian are a classical invertebrate model that in recent decades have been used to examine neurogenesis. The central nervous system of a planarian is simple, though fully formed with two lobes located in the head and two ventral nerve cords. This model reproduces asexually producing a complete and fully functioning nervous system after division