Neurogenesis

Neurogenesis Definition

Adult neurogenesis is the generation of new neurons in adults. These neurons arise from progenitor cells or stem cells. This usually happens during embryonic development and early growth, and can continue into adulthood in some insects, fish, and amphibia. It has traditionally been thought that the adult mammalian brain was incapable of neurogenesis, but recent studies have shown this to be untrue.

Evidence for the Concept that Neurons Cannot Replicate

Even though many other types of cells are able to replicate themselves when necessary, it was thought that neurons were different. One reason for this belief is their level of complexity: highly branched dendrites forking off from cell bodies with at least one long axon and many postsynaptic terminals, all currently busy as a functioning unit of an existing network…it’s easy to see that replication of such a cell would be intricate and difficult. Another piece of evidence for lack of neuronal regeneration was the fact that while other tissues can repair themselves, injuries to the brain and spinal cord are irreversible. Lastly, the death of neurons as aging proceeded was thought to contribute to cognitive decline, since they couldn’t be replaced.

Neuron cell diagram
The above diagram shows the complexity of a typical neuron, illustrating the difficulty it would have in undergoing the already-complicated process of cell division (mitosis).

Lab Studies of Neurogenesis

Lab studies of mice, rats, and shrews became a proving ground for not only the existence of, but the importance of, neurogenesis. New tools for imaging and labeling neurons helped researchers prove conclusively that new neurons were born in the brain. The rate of neurogenesis declined as rats aged, but it was discovered that environmental factors could slow or alter this degeneration. Stress was shown to be a factor that lessened neurogenesis, while exercise and cognitive enrichment actually enhanced it. In 2005, researchers saw that older mice who ran regularly on wheels had a higher rate of neurogenesis than sedentary mice.

Adult Neurogenesis

The reality of adult hippocampal neurogenesis has been proven through ground-breaking studies over the past 50 years. There are two areas of the adult mammalian brain in which neural regeneration has been proven to occur, one being the hippocampus. This tiny seahorse-shaped region buried deep in the brain is a center of memory, vital in the conversion of short-term to long-term memory. The hippocampus is also critical to spatial learning, which contributes to navigational ability in the three-dimensional environment.

The other location in the brain where neurogenesis occurs is the subventricular zone. This area is rich in progenitor and stem cells for new neurons. Studies are still underway to solve the riddle of exactly what happens with new neurons formed here. They do migrate to the olfactory bulb, but their ultimate function is yet to be determined. However, one thing is certain: the existence of stem cells in the adult brain helped overcome the potential difficulties of complex neurons dividing in half to replicate, as many other types of cells do. It’s much easier for a progenitor or stem cell to produce new neurons than for an adult neuron to be copied.

Damage to Hippocampus

The hippocampus is vulnerable to damage in the early stages of Alzheimer’s disease, which may affect the disease’s progression. Since the hippocampus is vital to both memory and navigation, this also helps explain why Alzheimer’s patients struggle with remembering things, as well as why they tend to wander and get lost. Other causes of damage to the hippocampus include periods of low oxygen, or hypoxia. This may occur during a heart attack, near drowning, sleep apnea, carbon monoxide poisoning, or other hazardous events. Damage to the hippocampus can result in anterograde amnesia, in which a person has clear memories of life before a brain injury, but little new memory is able to form.

How to Increase Neurogenesis

Now that it’s an established fact that neurogenesis is possible, many people may wonder if it’s possible to intentionally increase the rate of new neuron production in their brains. The answer is yes! According to Dr. Sandrine Thuret, a neural stem cell researcher at King’s College in London, there are plentiful ways to do this. Exercise, learning, and sex all tend to increase neurogenesis. Certain foods have also been shown to be beneficial, including flavonoids found in dark chocolate and blueberries, omega-3 fatty acids like those found in salmon, and the resveratrol in red wine. Even intermittent fasting, which involves short periods during the week in which meals are skipped or greatly reduced, can boost the formation of new neurons.

Conversely, factors such as stress, sleep deprivation, a diet high in saturated fat, and alcohol are known to decrease neurogenesis. It’s becoming clear that the rate of neurogenesis can be controlled, to some degree, by behavioral and dietary choices. This is currently leading researchers to investigate the possibilities of enhancing neurogenesis as a therapeutic strategy for those suffering hippocampal damage from disease or injury. New information and strategies could also combat cognitive decline due to aging.

Quiz

1. Which of these is not a reason why it was thought that neurons were lost forever once they died?
A. They are very complex.
B. They originate from stem cells.
C. Injuries to nerve tissue are irreparable.
D. Cognitive decline during aging was attributed to dying neurons that weren’t replaced.

Answer to Question #1
B is correct. The other three responses all give reasons why neurons were thought to be irreplaceable.

2. What does the hippocampus function in?
A. Muscle coordination.
B. Regulation of breathing and heart rate.
C. Monitoring endocrine system.
D. Learning and memory.

Answer to Question #2
D is correct. Muscle coordination is controlled by the cerebellum, while the medulla oblongata regulates breathing and heart rate. The almond-sized hypothalamus directly monitors and controls the pituitary gland, which is the master control gland of the entire endocrine system.

3. Which of these is not a way to increase neurogenesis?
A. Exercise
B. Intermittent fasting
C. Saturated fat
D. Omega-3 fatty acids

Answer to Question #3
C is correct. Diets high in saturated fat have been shown to decrease neurogenesis, most likely due to increased serum corticosterone, a corticoid hormone that regulates the body’s response to stress.

References

  • Arturo Alvarez-Buylla, A., Manuel Garcı́a-Verdugo, J. M. (2002) Neurogenesis in Adult Subventricular Zone. Journal of Neuro Science. Retrieved from jneurosci.org.
  • Gage, F. H. (2002). Neurogenesis in the Adult Brain. The Journal of Neuroscience. Retrieved from http://www.jneurosci.org/content/22/3/612
  • Neurogenesis: An Overview. (2016). Retrieved from http://www.brainfacts.org/brain-basics/brain-development/articles/2016/neurogenesis-an-overview-072116/
  • Thuret, S. You can grow new brain cells. Here’s how. TED talk. Oct. 2015 Retrieved from https://www.ted.com/talks/sandrine_thuret_you_can_grow_new_brain_cells_here_s_how/transcript?language=en
  • Yangling Mu, Y., Gage, F. H. (2011) Adult hippocampal neurogenesis and its role in Alzheimer’s disease. Retrieved from https://molecularneurodegeneration.biomedcentral.com/articles/10.1186/1750-1326-6-85
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