It is worth noting that the question of adult neurogenesis—or the formation of new neurons—remains a topic of debate among scientists, and certain controversies persist. However, many studies now converge on the idea that adult neurogenesis occurs in specific parts of the brain and can be modulated by various environmental and behavioral factors. Other studies, however, limit neurogenesis to embryonic development and early childhood. A 2019 study appears to bring clarity to this controversy, demonstrating that the formation of new neurons can indeed take place in certain regions of the adult brain. This rigorous research was conducted under carefully controlled methodological conditions, highlighting the shortcomings of earlier studies and explaining the inconsistency of previous findings.
The primary brain region where adult neurogenesis has been observed is the hippocampus, an area involved in memory and learning. Neurogenesis in the hippocampus is a complex process that includes the proliferation of neural stem cells, their differentiation into neurons, and their integration into existing neural circuits.
Several factors influence adult neurogenesis, such as physical activity, a stimulating environment, stress, sleep, and even certain hormonal factors. For example, regular physical exercise has been associated with increased neurogenesis, whereas chronic stress may have negative effects.
Understanding adult neurogenesis has significant implications for mental health, as it may be linked to processes like learning, memory, and brain plasticity. One critical question is the role of neurogenesis in cognitive function and aging. Does it persist into older age—beyond 70 or even 90? Does it play a role in the development of neurodegenerative diseases?
In this new study, brain tissue was collected from individuals aged 43 to 87. Despite their advanced age, the researchers observed clear evidence of new neuron formation in the hippocampus—specifically in the dentate gyrus—and an absence of neurogenesis in other hippocampal regions. The researchers also examined adult neurogenesis in patients with Alzheimer’s disease at various stages. They identified a correlation between disease progression and the level of production and maturation of new neurons in the hippocampus, opening new avenues for understanding the disease.
If this study ends more than 20 years of controversy, it also raises new questions about the mechanisms and functional roles of adult neurogenesis. What is the proportion of new neurons produced in the hippocampus? How do they integrate into preexisting hippocampal circuits? Do they replace older neurons, or are they added to them? Is the same process observed in other brain areas?
Key References:
- Moreno-Jiménez EP et al. Adult hippocampal neurogenesis is abundant in neurologically healthy subjects and drops sharply in patients with Alzheimer’s disease. Nat Med. 2019;25(4):554–560.
- Spalding KL et al. Dynamics of hippocampal neurogenesis in adult humans. Cell. 2013;153(6):1219–1227.