Neurongenesis

Neurongenesis, more commonly spelled neurogenesis, is the process by which new neurons are generated from neural stem and progenitor cells. In the vertebrate brain, most neurons are formed before birth, during embryonic and early postnatal development, but a limited amount continues to be produced in certain regions of the adult brain.

In adult mammals, the principal sites of ongoing neurogenesis are the subgranular zone of the dentate gyrus in the hippocampus and the subventricular zone along the lateral ventricles. In some species, progenitors from the subventricular zone migrate via the rostral migratory stream to the olfactory bulb. In humans, hippocampal neurogenesis persists into adulthood but declines with age, and evidence for widespread neurogenesis outside the hippocampus remains a topic of debate.

Lineage and development follow a general progression: neural stem cells give rise to intermediate progenitors, which differentiate into neuroblasts and immature neurons that migrate, differentiate, and integrate into existing neural circuits. These steps are guided by the neural niche and a complex array of signals, growth factors, and transcriptional programs, with maturation culminating in functional, synaptically integrated neurons.

Regulation of neurongenesis is influenced by environmental and physiological factors. Physical exercise and environmental enrichment tend to enhance hippocampal neurogenesis, while stress, aging, and certain hormonal influences can suppress it. Growth factors such as brain-derived neurotrophic factor (BDNF) and vascular endothelial growth factor (VEGF), as well as some pharmacological agents including antidepressants, modulate rates of neurogenesis.

The functional significance of adult neurogenesis, especially in humans, remains under investigation. In animals, it is associated with certain forms of learning and memory and with mood regulation; disruptions are linked to mood disorders and injury responses in experimental models. Ongoing research seeks to clarify its relevance to human cognition and disease.