This is a possible explanation for the rapid improvement in olfactory function in many cases, since the ORNs themselves are only indirectly affected. This assumption supports the knowledge gained in the meantime that SARS-CoV‑2 enters the cell via the surface receptor angiotensin-converting enzyme (ACE) 2, which is mainly expressed by the supporting cells. Since SARS-CoV‑2 patients rarely complain of additional rhinitic symptoms, pathophysiological considerations suggest that the olfactory dysfunction is not primarily a conductive problem, but instead one of damage to the olfactory mucosa or a neuronal affection. The sudden onset of olfactory dysfunction is more typical of postinfectious olfactory dysfunction. In addition, it was noted relatively early in the pandemic that olfactory impairment is an early symptom and this is typically noticed around the fourth day of illness.
With regard to COVID-19-associated olfactory disorders, it is noteworthy that significant improvement often occurs two to four weeks. As a consequence, olfaction may be completely absent, a condition that is called “anosmia.” Depending on the cause of the olfactory disorder, the symptoms may be temporary or constant. Old age, infections, chronic sinonasal diseases, craniocerebral trauma, or neurodegenerative diseases represent the most common causes of olfactory impairment. They provide electrical insulation and extend from the periphery, after passing through the approximately 1–2-mm openings of the lamina cribrosa, to the central nervous system up to the OB. The lamina propria of the olfactory mucosa also contains olfactory ensheathing cells, a type of glial cell that surrounds ORNs. In the context of COVID-19-associated olfactory disorders, they appear to play a central role. Supporting cells maintain the ionic balance, among other functions. The task of the microvillar cells is not yet clear, but they also may serve sensory functions. Due to its mitotic activity and differentiation ability, the olfactory mucosa has a lifelong regenerative capacity, which is unique for a human sensory epithelium. Basal cells can be differentiated into horizontal and globose basal cells (GBCs ). However, in addition to ORNs, the olfactory mucosa also contains basal cells, which in turn are precursor cells for ORNs, supporting cells, and microvillar cells.
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