The Days of our Neurons Part II: Olfactory Neurogenesis

Janani Rajan
Edited by Keshav Ratra
Graphic Designs by Josephine Wang and Olivia Tang

Neurogenesis is an intricate process that results in the creation of new neurons and neuroglia. Most neurogenesis occurs in the developing embryo, and this topic is covered in detail in the previous blog post: The Days of our Neurons Part I: Prenatal Neurogenesis. In fact, it was long thought that after childhood, neurogenesis would stop occurring, and as a person ages, their neurons and neuroglia would slowly die out. However, this is not the case. Neurogenesis persists even during adulthood, and occurs in three major locations: The subgranular zone (which supplies the dentate gyrus of the hippocampus with new neurons), the subventricular zone (which supplies the olfactory bulb with new interneurons), and the olfactory neuroepithelium, which generates and supplies the olfactory bulb with excitatory sensory neurons. The locations that concern olfactory neurogenesis will be discussed in this blog post.

Olfactory Neurogenesis occurs primarily in two organs: the vomeronasal (Jackabson’s) organ, and the main olfactory epithelium. The vomeronasal organ (VNO) is the peripheral sensory organ of the olfactory nervous system, while the main olfactory epithelium is a specialized tissue containing olfactory receptor cells. While both of them are a part of the olfactory sensory system, they still have differing functions. The main olfactory epithelium detects volatile odors, which are common chemical contaminants. Some examples of volatile odors are smells from food, floral scents, and even unpleasant odors from garbage, or skunks. Meanwhile, the VNO detects pheromones. These are chemicals that humans and animals produce through various behaviors, that change the behavior or trigger a certain response from an individual of the same species. For example, animals can produce pheromones when predators are nearby in order to alert nearby animals of the same species, signaling them to move away from the area. Similarly, when animals urinate on pieces of land, this delineates their territory, signaling other animals that they cannot cross it. The vomeronasal organ primarily detects odor information that concerns social organization, as well as reproductive status.

One important thing to note about the vomeronasal organ is that its functional existence in humans is highly debated. The functional existence of a vomeronasal organ (VNO) in animals is scientifically accepted. There is little doubt that animals use pheromones for social behavior including mating. However, there is much controversy regarding a functional VNO in humans. The anatomy of the anterior nose suggests that humans retained some of the anatomies for a functional VNO from their ancestors, apes. Furthermore, one of the strongest pieces of evidence for functional VNO in humans is that women in college dorms synchronize their menstrual cycle which is believed to be from the armpit sweat from other women. This is well established in the scientific community.

Both the vomeronasal organ and olfactory epithelium have neurological similarities. For example, they both contain mature and immature sensory neurons. While immature sensory neurons are still developing from stem cells, mature sensory neurons are bipolar neurons with long dendrites and intricate cilia, where odors are detected and transduced. These neurons specialize in transducing chemical signals, and contain long axons which form glutamatergic synapses. These synapses contain the neurotransmitter glutamate (the primary excitatory neurotransmitters of the brain) packaged into vesicles in the presynaptic membrane, and glutamate receptors in the postsynaptic membrane. The glutamatergic synapses are made between these neurons and mitral cells in the olfactory bulb, which carry olfactory information from the olfactory bulb into the rest of the brain.

Throughout adulthood, basal cells constantly generate sensory neurons in the olfactory epithelium and in the vomeronasal organ. Basal cells are a part of the outer layer of the skin, called the epidermis, and there are two types of basal cells involved in this process: horizontal basal cells and globose basal cells. Globose basal cells are thought to be the precursors for neurons during ongoing neurogenesis, which is the neurogenesis required to consistently repopulate the olfactory epithelium and the vomeronasal organ. Additionally, they are also the precursors for neurogenesis after mild injuries. Meanwhile, the horizontal basal cells are neural stem cells that respond to severe injury. They are the precursors for the neurons during neurogenesis after severe injury, in order to repair any damage sustained by either the olfactory epithelium or the vomeronasal organ. These basal cells first differentiate into progenitors for neurons, which then differentiate into Neurogenin-1 and NeuroD1+, two types of immediate neuron precursors. These precursors then form mature olfactory receptor neurons that are localized in the olfactory epithelium and vomeronasal organs.

One important thing to note about neurogenesis in the olfactory epithelium is that neurogenesis in the olfactory epithelium is simply the turnover of the population of immature sensory neurons. This means that all the immature sensory neurons in the olfactory epithelium are constantly replaced by newer immature sensory neurons, and it is less common for mature sensory neurons to be replenished. Another interesting fact about olfactory neurogenesis is that it rarely leads to tumors. This means that this neurogenesis is carefully regulated, to avoid the overproduction of neurons from basal cells.

Overall, olfactory neurogenesis is a very careful process that allows humans and other animals to have a more refined sense of smell. As individuals grow, and as their body surface increases, olfactory neurogenesis is necessary throughout life in order for the olfactory system to keep up the pace along with the rest of the body’s growth. Furthermore, neurogenesis must be used to respond to lesions and repair any damage done to olfactory organs. Since the olfactory organ is almost completely exposed to the environment, it is more susceptible to damage than other organs, which is why olfactory neurogenesis is needed lifelong. However, as discussed earlier, olfactory neurogenesis is not the only neurogenesis that occurs throughout an individual’s life. Neurogenesis persists in the dentate gyrus of the hippocampus as well, in order for memories to be formed and stored throughout a person’s life. Hippocampal neurogenesis will be discussed further in the third and final blog post of this series: Neurogenesis Part III: Hippocampal Neurogenesis.

Approved by Dr. Charles Pidgeon.

This blog is protected by US Copyright law and is owned by Dr. Charles Pidgeon.

References:

  1. Brann, Jessica H, and Stuart J Firestein. “A Lifetime of Neurogenesis in the Olfactory System.” Frontiers in Neuroscience, Frontiers Media S.A., 26 June 2014, www.ncbi.nlm.nih.gov/pmc/articles/PMC4071289/.
  2. Derby, Charles D. “Why Have Neurogenesis in Adult Olfactory Systems? The Presidential Symposium at the 2006 AChemS Conference.” Validate User, 2 Apr. 2007, academic.oup.com/chemse/article/32/4/361/386242.
  3. Feierstein, Claudia E. “Linking Adult Olfactory Neurogenesis to Social Behavior.” Frontiers, Frontiers, 15 Nov. 2012, www.frontiersin.org/articles/10.3389/fnins.2012.00173/full.

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