Context: Loss of the sense of smell (and taste) is one of the more recently identified symptoms of Covid-19, and is now recognised as such by the World Health Organization (WHO).
- A new study in mice has explored why this symptom appears in some Covid-19 patients.
How loss of smell happens?
- There are two kinds of cells in the nose as the likely first entry points for the virus. These are goblet cells (which produce mucus) and ciliated cells (which help sweep mucus to the throat so it can be swallowed). These occur in the respiratory epithelium.
- The new study looked specifically at the olfactory epithelium.
- SARS-CoV-2, the virus that causes Covid-19, hijacks two human proteins to invade cells.
- One is the ACE2 “receptor” on the cell surface (it opens the door for the virus) while the other is called TMPRSS2, which the virus uses to replicate its genetic material.
- The more entry proteins a host cell has, the easier it is for the virus to bind, enter and infect that cell.
- Using the Human Cell Atlas database, the study looked at data from different tissues of non-infected people. It found that these two proteins had the highest presence in goblet and ciliated cells.
- Within the olfactory epithelium, which is a tissue lining the nasal cavity that is involved in smell, the “sustentacular cells” had the highest level of SARS-CoV-2 receptors.
- The sense of smell in Covid-19 patients appears to be lost, because the sustentacular cells assist neurons in sensing odours, probably by processing odour-binding proteins.
The Human Cell Atlas (HCA) is a global, scientist-led collaboration to map and characterize all cells in a healthy human body: cell types, numbers, locations, relationships, and molecular components.
Once complete, it will be a fundamental resource for scientists, allowing them to better understand how healthy cells work, and what goes wrong when disease strikes.
Significance of study:
- More accurate diagnostic tests: Identifying these cells could help in the development of more accurate diagnostic tests. Future studies should examine whether sustentacular cells can pass the virus to neurons, which could provide SARS-CoV-2 a route to infect the brain.
- Why elderly are prone to Covid-19 infection: The researchers also found that larger amounts of the proteins are made in older mice than in younger ones.
- The high levels of entry proteins in the nasal epithelium may explain why older humans are more likely to become infected with the novel coronavirus than younger humans.
Why were mice chosen for experiment?
- Mice are common experimental animals in laboratory research of biology and psychology fields primarily because they are mammals, and also because they share a high degree of homology with humans.
Image source: Boston University
Biology facts about the Respiratory Tract
- Respiration continually brings air from the environment in contact with the delicate cells in our lungs to provide oxygen and to expel carbon dioxide, which is a byproduct of metabolism.
- Inspired air enters the nose or mouth, passes through the pharynx (throat) and the larynx (voice box) and then enters the tracheo-bronchial tree.
- The trachea bifurcates into the right and left main stem bronchi, which branch again and again into increasingly smaller conduits called bronchioles.
Cells Lining the Respiratory Tract: The inspired air can also carry chemical and particulate contaminants which can be harmful, and several mechanisms have evolved to mitigate the effects of these contaminants. Nasal hairs and mucus can trap dust and other particulate matter.
- Respiratory epithelial cells line the respiratory tract from trachea to bronchi into bronchioles and alveolar sacs.
- Their primary functions are to moisten, protect the airway tract from potential pathogens, infections and tissue injury, and facilitate gas exchange.
- It primarily consists of three main cell types – cilia cells, goblet cells, and basal cells.
- The ciliated cells facilitate the movement of mucus across the airway tract.
- The goblet cells produce and secrete mucous to trap pathogens and debris within the airway tract.
- Basal cells respond to injury of the airway and subsequently differentiate to restore a healthy epithelial cell layer.
Alveolar epithelial cells (AEC)
- They line the small, spongy sacs called alveoli that are found throughout the lung. They are involved in gas exchange with microvascular endothelial cells that surround the alveoli.
- Alveoli carry oxygen to the blood from the respiratory tract and take CO2 away from the blood back out of the airway tract.
- AEC II contributes to lung defense and have been the subject of numerous studies due to their regenerative potential.
Respiratory endothelial cells function at the lung-blood barrier, where they surround the alveolar sacs and facilitate O2/CO2 transfer.
- Dysfunction of respiratory endothelium has been tied to acute lung injury and acute respiratory stress syndrome.
Pulmonary vein – carries the oxygenated blood away from lung to the heart
Pulmonary artery – carries deoxygenated blood to the lung