The
olfactory epithelium is a specialized
epithelialIn biology and medicine, an epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue...
tissue inside the
nasal cavityThe nasal cavity is a large fluid filled space above and behind the nose in the middle of the face.- Function :The nasal cavities conditions the air to be received by the areas of the respiratory tract and noses...
that is involved in
smellOlfaction refers to the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
. In humans, it measures about 1 square centimetre (on each side) and lies on the roof of the nasal cavity about about 7 cm above and behind the nostrils (Moran, 1982). The olfactory epithelium is the part of the
olfactory systemThe olfactory system is the sensory system used for olfaction, or the sense of smell. Most mammals and reptiles have two distinct parts to their olfactory system: a main olfactory system and an accessory olfactory system. The main olfactory system detects volatile, airborne substances, while the...
directly responsible for detecting
odorAn odor or odour is caused by one or more volatilized chemical compounds, generally at a very low concentration, that humans or other animals perceive by the sense of olfaction. Odors are also called smells, which can refer to both pleasant and unpleasant odors...
s.
Layers of Olfactory Epithelium
Olfactory epithelium consists of three distinct types of cells:
1. Olfactory cells
2. Supporting cells
3. Basal cells
Olfactory Cells
The olfactory cells of the epithelium are bipolar neurons which congregate to form the
olfactory nerveThe olfactory nerve, or cranial nerve I, is the first of twelve cranial nerves. The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity...
(cranial nerve I). The apical poles of these neurons are ciliated and coated with a serous secretion from Bowman's glands located in the
lamina propriaThe lamina propria is a constituent of the moist linings known as mucous membranes or mucosa, which line various tubes in the body ....
of the
olfactory mucosaThe olfactory mucosa is located in the upper region of the nasal cavity and is made up of the olfactory epithelium and the underlying lamina propria, connective tissue containing fibroblasts, blood vessels, Bowman's glands and bundles of fine axons from the olfactory neurons .The mucus protects the...
.
Supporting Cells
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 tall columnar cells featuring microvilli and a prominent
terminal webThe terminal web is a filamentous structure found at the apical surface of epithelial cells that possess microvilli. It is comprised primarily of filaments of spectrin and Myosin II. The Myosin II molecules in the terminal web endow this structure with contractile activities....
. The nuclei of supporting cells are more apically located than those of the other olfactory epithelial cells.
Basal 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 (Schwob, 2002). 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.
Pathology
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
anosmiaAnosmia is a lack of functioning olfaction, or in other words, an inability to perceive smells. It can be either temporary or permanent. A related term, hyposmia, refers to decreased ability to smell, while hyperosmia refers to an increased ability to smell. Some people may be anosmic for one...
.
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