Adult neurogenesis

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:^[3]

• The subgranular zone (SGZ), part of the dentate gyrus of the hippocampus,^[4][5] where neural stem cells give birth to granule cells (implicated in memory formation and learning).^[6]
• The subventricular zone (SVZ) of the lateral ventricles, which can be divided into three microdomains: lateral, dorsal and medial.^[7] 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.^[8][9]

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,^[4] but a number of them become functionally integrated into the surrounding brain tissue.^[10][11][12] Adult neurogenesis in rodents is reported to play a role in learning and memory, emotion, stress, depression, response to injury, and other conditions.^[13]

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,^[14] while more recent studies show that adult hippocampal neurogenesis does not exist in humans, or, if it does, it is at undetectable levels.^[15] Recent evidence shows that adult neurogenesis is basically extinct in humans.^[16] The experiments advocating for the presence of adult neurogenesis have focused on how dual antigen retrieval finds that DCX antibodies are staining many cells within the adult human dentate gyrus. This finding is not as clear though as supporters of adult neurogenesis suggest; the dentate gyrus cells stained with DCX have been shown to have a mature morphology, contrasting the idea that novel neurons are being generated within the adult brain.^[17] The role of new neurons in human adult brain function thus remains unclear.