**2. The olfactory epithelium**

This chapter briefly introduces the structures and functions of the nose and olfactory pathway since they form the basis of dysfunctions of the olfactory system and nasal disorders. The nasal passages start with the nostrils or nares separated by a septum. The nasal passages include the vestibule which is the most anterior part of the nasal cavity. The nasal cavity is enclosed by an elastic cartilage and lined by a stratified squamous, keratinized epithelium. The back part of the nasal cavity is lined by the respiratory epithelium, which is a pseudostratified, ciliated, and columnar epithelium. Likewise, this respiratory epithelium is found further down the airways including the trachea and bronchi. Our organ of smell is a specialized epithelium, the olfactory epithelium, which is also a pseudostratified, ciliated, and columnar epithelium. This epithelium covers the superior nasal concha and presents as the olfactory area. Each nasal cavity has an olfactory area in the roof of the nose. The term olfactory mucosa describes the olfactory epithelium and the underlying connective tissue (lamina propria). The olfactory mucosa contains several

cell types. The olfactory receptor cells in the epithelium are bipolar nerve cells. Their oval nuclei are located in the central one third of the olfactory epithelium. These cells detect smell [9]. The axons of olfactory receptor cells form the olfactory nerve, cranial nerve I. The axons traverse the cribriform plate of the ethmoid bone and project to the ipsilateral olfactory bulb where they target central neurons. Olfactory receptor cells are surrounded and cushioned by the supporting cells. The supporting cells (sustentacular cells) have their nuclei in the upper one third of the epithelium. They have cigar-shaped, elongated nuclei. Olfactory receptor cells are equipped with radiating cilia, whereas the supporting cells have microvilli at their apical surface. The basal cells have their nuclei in the lower one third of the epithelium at the base of the epithelium. They are precursor cells and actively divide after birth to replace olfactory receptor cells. The life span of olfactory receptor cells is 30–60 days. They undergo continuous replacement through the basal stem cell population [10]. Bowman's glands in the connective tissue secrete mucus to prevent constant olfactory stimulation. Bowman's glands have a duct to the surface of the olfactory epithelium. Their secretion produces a fluid environment around the olfactory cilia and may clear the cilia, facilitating the access of new odor substances. In addition, the mucus creates the ionic milieu around the cilia and contains an odorant-binding protein to trap odorants and to bring them to cilia.

The olfactory epithelium in the nose is part of the respiratory system. The primary function of the respiratory system is respiration, that is, the system provides the gas exchange between air and blood, so blood becomes oxygenated. The part of the system involved in gas exchange is the lungs. Another part of the system is a branching system of airways that brings air to and from lungs via the respiratory movements of thoracic walls and diaphragm. This part carries out a second function of the system, which is a somewhat minor function, namely, it humidifies the air, cleans the air, and warms the air. It works more like an air-conditioning system. Along the same line, we can divide the system into two principal regions. The conducting portion includes the parts of the respiratory system that are responsible for supplying the lungs with air: nasal cavities with olfactory areas, nasopharynx, larynx and epiglottis, trachea, bronchi, bronchioles, and terminal bronchioles. The respiratory portion is the site of gas exchange and includes the respiratory bronchioles, alveolar ducts and sacs, and alveoli. The olfactory epithelium is much thicker than the respiratory epithelium, which is found in the nose and the respiratory tract, whereas the olfactory epithelium is found only on the roof of nasal cavity. The respiratory epithelium contains goblet cells that secret a mucus which covers the epithelium and traps dust particles. Goblet cells are absent from the olfactory epithelium.
