It has long been believed that once dead, neurons cannot be restored. so, once they are lost, they are lost forever. but, of late, neurogenesis has been proved to be possible.

It has classically been believed that neurogenesis—the capability for production of neurons from undifferentiated, proliferative progenitor cells—is confined to the development period that precedes birth in mammals. According to this traditional view, after pathological insults that result in neuronal death, the number of neurons is permanently reduced. However, recent evidence has indicated that a small number of neuronal precursor cells, capable of dividing and then differentiating into neurons, may exist in the forebrain of adult mammals, including humans. These rare precursor cells reside in the subventricular zone. For example, here is some evidence for postnatal neurogenesis in the dentate gyrus of the hippocampus, and it has been suggested that the rate of generation of new neurons in this critical region can be accelerated in an enriched environment.