Section of the olfactory mucous membrane.
Plan of olfactory neurons.
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 3 square centimeters (on each side) and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.
Layers of olfactory epithelium
Olfactory epithelium consists of four distinct cell types:
- Olfactory cells
- Supporting cells
- Basal cells
- Brush cells
The olfactory cells of the epithelium are bipolar neurons which congregate to form the olfactory nerve. The olfactory nerves go through the cribriform plate and terminate on the dendrites of the mitral cells located in the glomeruli of the olfactory bulb. The apical poles of these neurons are covered with non-motile cilia, with the plasma membrane containing odorant-binding proteins acting as olfactory receptors. The incoming odorants are made soluble by the serous secretion from Bowman's glands, located in the lamina propria of the mucosa.
Analogous to neural glial cells, the supporting cells (a.k.a. Sustentacular cells) of the olfactory epithelium function as metabolic and physical support for the olfactory cells. Histologically, the supporting cells are pseudostratified ciliated columnar epithelium. The nuclei of supporting cells are more apically located than those of the other olfactory epithelial cells.
Resting on the basal lamina of the olfactory epithelium, basal cells are stem cells capable of division and differentiation into either supporting or olfactory cells. The constant divisions of the basal cells leads to the olfactory epithelium being replaced every 2–4 weeks.
Basal cells can be divided on the basis of cellular anatomy histological markers into two populations: the horizontal basal cells which line the olfactory epithelium and the slightly more superficial globose basal cells. Horizontal basal cells are now thought to be the primary stem cell population supplying new cells in this system., although this is subject to some debate with some scientists maintaining that the globose basal cells are the true stem cells.
A microvilli-bearing columnar cell with basal surface in contact with afferent nerve endings, specialised for transduction of general sensation. Nerve fibres are terminal branches of trigeminal nerve (cranial nerve V), rather than the olfactory nerve, as afferent olfactory signals are.
Bowman's (olfactory) glands
Tubuloalveolar serous secreting glands lying in the lamina propria of the mucosa. These glands deliver a proteinaceous secretion via ducts onto the surface of the mucosa. The role of the secretions are to trap and dissolve odiferous substances for the bipolar neurons. Constant flow from the Bowman's glands allows old odors to be constantly washed away.
The olfactory epithelium can be damaged by inhalation of toxic fumes, physical injury to the interior of the nose, and possibly by the use of some nasal sprays. Because of its regenerative capacity, damage to the olfactory epithelium can be temporary but in extreme cases, injury can be permanent, leading to anosmia.
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