By Dr Ananya Mandal, MD
The term neurogenesis is made up of the words “neuro” meaning “relating to nerves” and “genesis” meaning the formation of something. The term therefore refers to the growth and development of neurons. This process is most active while a baby is developing in the womb and is responsible for the production of the brain’s neurons.
The development of new neurons continues during adulthood in two regions of the brain. Neurogenesis takes place in the subventricular zone (SVZ) that forms the lining of the lateral ventricles and the subgranular zone that forms part of the dentate gyrus of the hippocampus area. The SVZ is the site where neuroblasts are formed, which migrate via the rostral migratory stream to the olfactory bulb. Many of these neuroblasts die shortly after they are generated. However, some go on to be functional in the tissue of the brain.
Previously, neuroanatomists such as Santiago Ramon Cajal, believed the nervous system was a fixed system that was not capable of regeneration but in 1962, the first evidence of adult neurogenesis was demonstrated by Joseph Altman who also identified the rostral migratory stream in 1969. These findings were largely ignored by the scientific community until the 1980s when research reignited interested in the topic by showing that neurogenesis occurs in rats and birds. In the early 1990s, adult neurogenesis was also demonstrated in non-human primates and humans.
The actual function of adult neurogenesis has not yet been clearly determined. Some evidence suggests that the process is key to functions such as learning and memory. Studies have shown that new neurons increase memory capacity, reduce the overlap between different memories and also add information regarding time to memories. Other studies have shown that the learning process itself is also linked to the survival of neurons.
Another important discovery is the role of the neurosteroid allopregnanolone in aiding neurogenesis in the brain. Levels of allopregnanolone start to decline in the elderly and in patients with Alzheimer’s disease.
Reviewed by Sally Robertson, BSc