University years

Where is adult neurogenesis happening?

During my first year of Master’s degree in Neurobiology I had to face quite some striking challenges, that started already during the first semester. I actually had to go back to Physics and this time try to really understand it and apply it to what I have always wanted to study: the brain.

As I have already mentioned in a previous post, neurons communicate in between each other via the so called action potential, an electric signal that travels along the neuron up to its terminal end where then it is “converted” into a chemical signal so that it can “jump” from one neuron to another and transmit its information to the right centre of the brain. Therefore, when I actually started studying how the electric signal was moving along the neuron, I realised that there was a lot of Physics behind all this process. To be honest, I should have noticed the first signs already from the beginning, when we had to learn how to calculate the resistance or the capacity of a system and so on, but I was still blindfolded by this new interesting adventure and I just pushed myself to study more and figure everything out.

I already wrote about how I had to study Physics throughout all my high school years and then, during my Bachelor’s, I had a Physics exam (the same topics we had to study in 5 years during high school were now the whole program of a one semester course), and as much as I really tried to put myself into it, I really never liked it, and I swear, I had to take a serious effort trying to enjoy this course called “Membrane Biophysics and Electrophysiology”. I will not lie if I say that my hate for Physics started slowly growing and the way we were taught this course and the following more electrophysiological courses we had during the next semesters made me start hating Electrophysiology and made me realise how I finally knew what I did not want to do in my life: being an electrophysiologist.

But enough with the part of the post where I told what I did not like about my Master’s and let’s move on to one of the courses I found incredibly interesting: “Neurogenesis and Vertebrate Neuromorphology”. This course was focused on the development of the vertebrate central nervous system (CNS), from the embryonic stages up to the adult neurogenesis, a process that I will describe later on in this post for the SRF. Furthermore, after studying how the human system develops, the course put the emphasis on the evolution of the CNS in vertebrate. It was mesmerizing realising that even if the brains of modern hagfishes, lampreys, sharks, amphibians, reptiles and mammals show a gradient of size and complexity that roughly follows the evolutionary sequence, they all contain the same set of basic anatomical components (although many are rudimentary in the hagfish).

Since differences in the CNS among vertebrates is a pretty complex but yet incredibly enthralling topic, I will definitely talk about it more in a future post (also presenting some evidences and fun facts about it). However, for today’s science related fact (SRF) I will talk about adult neurogenesis, the process by which new neurons are formed in the adult brain.

“Science Related Fact” (SRF):

Up until the 1990s, neuroscientists considered the CNS incapable of neurogenesis and unable to regenerate. Scientists believed that neurogenesis, the process by which new neurons are formed in the brain, was restricted only to the stages of embryonic development. Indeed, the mature brain has many specialised areas of function and neurons that differ in structure and connections: for example, the hippocampus alone, a brain region that plays an important role in memory and spatial navigation, has at least 27 different types of neurons. The incredible diversity of neurons in the brain results from regulated neurogenesis and from the 1990s, when stem cells were discovered in parts of the adult brain, adult neurogenesis is accepted to be a normal process that occurs in the healthy brain.

Adult neurogenesis is known to occur in three regions in the mammalian brain, that are the the subgranular zone (SGZ) of the dentate gyrus in the hippocampus, which is a region that is involved in regulating learning and memory, the subventricular zone (SVZ), which is situated throughout the lateral walls of the brain’s lateral ventricles and the amygdala, an area of the brain involved in the perception of emotions such as anger, fear and sadness. The neurons formed in the SVZ migrate to the olfactory bulb, the area of the brain responsible for olfaction. Blocking neurogenesis in the SVZ has been shown to impair cognitive functions, including olfactory memory. Newborn neurons resulting from adult neurogenesis in the hippocampus play crucial roles in regulating mood, memory and spatial learning (see picture below).

Mouse hippocampus with adult neural stem cells labelled in green. From Berg et al., 2019: “A Common Embryonic Origin of Stem Cells for Continuous Developmental and Adult Neurogenesis”.

Research on hippocampus suggests that different factors can modulate adult neurogenesis: for example, researchers found that exercise increases neurogenesis in this area, resulting in the increased production of newborn neurons. Conversely, depression was found to decrease neurogenesis and adult neurogenesis has also been shown to decline with age.  

Neuroscientists are now interested in developing ways to control the brain’s reservoir of neural stem cells to enhance hippocampal neurogenesis. All these researches have the ultimate goal to be able to treat conditions that show a significant decline in neurons, such as age-associated cognitive decline and neurodegenerative diseases including dementias and mental illnesses, by stimulating the production of newborn neurons in order to fill the gaps left by the death of the old ones.

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