Neurogenesis (birth of neurons) is the process by which neurons are generated from neural stem cells and progenitor cells. Most active during pre-natal development, neurogenesis is responsible for populating the growing brain with neurons. Recently neurogenesis was shown to continue in several small parts of the brain of mammals: the hippocampus and the subventricular zone. Studies have indicated that the hormone testosterone in vertebrates, and the prohormone ecdysone in insects, have an influence on the rate of neurogenesis. Occurrence in adults New neurons are continually born throughout adulthood in predominantly two regions of the brain: Many of the newborn cells die shortly after they are born, but a number of them become functionally integrated into the surrounding brain tissue. Role in learning Effects of stress Some studies have hypothesized that learning and memory are linked to depression, and that neurogenesis may promote neuroplasticity.