Hey Brain Rewirers,
Welcome to this week's newsletter! We’re diving deep into neurogenesis and the potential impact this has on health and disease.
Neurogenesis
If you missed the last newsletter, here’s a little recap. Neurogenesis is the process by which new neurons are formed in the brain. This process happens constantly in babies and young children; however, it was originally believed that the adult brain did not have the capacity for neurogenesis. That is until neurogenesis was found to be happening in the adult mammalian brain1,2,3.
It was then that researchers questioned whether neurogenesis might be found in the adult human brain. But like everything else, science can be a bit messy sometimes. There is evidence that neurogenesis does happen in the adult human brain4,5, and evidence that it does not6. This conflicting evidence is likely due to limitations in the methods used to determine if neurogenesis occurs, especially in post-mortem human brain7.
How Does Neurogenesis Happen?
If you’ve been here for a while, you may remember a newsletter on ‘mini-brains’ which introduced the idea of stem cells. We can think of these as ‘baby’ cells, since they have potential to become any type of cell. Well, one kind of stem cell exists called the neural stem cell. This stem cell can become a neuron, an astrocyte, or an oligodendrocyte.
The neural stem cell is continually self-renewing and proliferating. However, under the right conditions, it will differentiate into a neural progenitor cell, then into a neuroblast, then into an immature neuron, and finally into a mature neuron9 (Figure 1).
We know a fair amount about this process since it occurs readily in (human and rodent) babies. And since new neurons come from neural stem cells, we know that if neurogenesis is going to take place, there must be proliferating neural stem cells in the brain. Thus, many studies looking at neurogenesis in adult humans use the presence of neural stem cells or neuroblasts in post-mortem tissue as evidence to suggest it can happen.
Neurogenesis: Where and Why?
Neurogenesis in adult mammalian brains is generally restricted to two regions: the subgranular zone in the dentate gyrus of the hippocampus, and the subventricular zone, which lies along the ventricles in the brain (see Figure 2)10. However, neurogenesis has also been observed in the rodent cortex, striatum, amygdala, and substantia nigra, though to a lesser degree10. Importantly, the neurons generated from the subventricular zone commonly move to the olfactory bulb, which is the centre for smell – one of the main senses that rodents rely on. This poses some questions for how this might translate for humans; whose smell is not one of our main senses. So far, most work in the adult human brain has focused on neurogenesis in the hippocampus.
Regardless, neurogenesis does not equate to a whole-brain repair mechanism. Instead, it is thought to function like synaptic plasticity; as a mechanism to adapt and change, rather than to repair. From animal studies, there is evidence that neurogenesis contributes to some forms of memory and learning11. When neurogenesis was blocked, trace memories were disrupted11. It is thought that neurogenesis in the adult human hippocampus may play a similar role in learning12.
There is also evidence from rodents that neurogenesis from other regions, like the amygdala and striatum, contribute to emotional processing regulation13. In fact, neurogenesis has been shown to play a role in antidepressant-mediated effects in the adult mouse brain14. Neurogenesis has also been promoted by environmental enrichment, exercise, and learning tasks in the adult mammalian brain15,16,17. Although adult neurogenesis does occur in select brain areas in rodents, and likely also in humans, it is critical to note that it is greatly decreased in adults compared to babies18,19. Therefore, it remains unclear to what extent the brain uses this process.
How does this affect us?
If you can just make some new neurons, who cares, right?
Well, baseline neurogenesis cannot maintain function in a normal aging brain, nor when an injury/neurodegeneration has occurred. We know this because cognitive decline in aging is prevalent as are neurodegenerative disorders. Indeed, neurogenesis is promoted in adult rodent brains when an injury occurs, but cannot fully recover function, and comes at a cost20,21. Additionally, it remains unclear what happens to neurogenesis with chronic injury, which occurs in neurodegenerative disease and repeated brain trauma.
Some researchers propose that therapeutics which enhance the neurogenesis process may be beneficial for neurodegenerative diseases22. However, further research needs to be done to understand if a drug could promote the process to the extent that functionality is recovered and if such a large number of new neurons could integrate into pre-existing neural systems sufficiently.
Where does this leave us?
Well, research suggests that adult neurogenesis is important in learning and emotional processing, and there is evidence this happens in humans. It could be another tool in our arsenal for brain adaptation, alongside synaptic plasticity, neuroplasticity, and myelin plasticity. It seems that we should be doing things that would promote our brain health, and the brain's ability for neurogenesis, like exercise. And we probably should avoid doing things that hurt our brain health, like skipping out on sleep, excessive alcohol intake, and unhealthy foods.
Life Updates
If you follow me on Instagram you probably saw that I’ve been on the road! The past few weeks I have been in the US for the launch of Rewire, which has been a completely surreal experience. I cannot thank everyone enough for turning up to events, pre-ordering the book, and showing your love for the beautiful brain.
I was also invited on the Today show (!!!) where I got to share with the whole country some top tips for brain health that can be found in Rewire!
It’s not too late to order the book or find it in your local bookstore if you want to find out more about how to rewire your brain!
Until next week,
Nicole x
P.S. Please leave a comment with topic requests for future newsletters!
References
Neurogenesis in the adult human hippocampus - PubMed (nih.gov)
Mounting evidence suggests human adult neurogenesis is unlikely: Neuron (cell.com)
https://www.sciencedirect.com/topics/neuroscience/neural-stem-cell#
Review Article on adult neurogenesis in humans - ScienceDirect
Neurogenesis in the adult is involved in the formation of trace memories - PubMed (nih.gov)
Running enhances neurogenesis, learning, and long-term potentiation in mice - PMC (nih.gov)
Age-related production of new granule cells in the adult dentate gyrus - PubMed (nih.gov)
https://www.sciencedirect.com/science/article/abs/pii/B9780323904711000189
omg Nicole, I love these kind of topics! Keep going because they are so interesting and you make them feel super easy to understand. One thing I would love you to talk about is sleeping and driving. Not only how much driving is affected by sleep deprivation / not getting enough sleep but how driving is affected when doing it first thing in the morning just after waking up! Thank you so much!