| Olfactory epithelium | |
|---|---|
 Section of the olfactory mucous membrane. | |
 Plan of olfactory neurons. | |
| Details | |
| Precursor | Olfactory placode andneural crest |
| System | Olfactory system |
| Nerve | Olfactory nerve |
| Identifiers | |
| TH | H3.11.07.0.01001 |
| FMA | 64803 |
| Anatomical terminology | |
Theolfactory epithelium is a specializedepithelial tissue inside thenasal cavity that is involved insmell. In humans, it measures5 cm2 (0.78 sq in)[1] and lies on the roof of the nasal cavity about 7 cm (2.8 in) above and behind the nostrils.[2] The olfactory epithelium is the part of theolfactory system directly responsible for detectingodors.
Olfactory epithelium consists of four distinct cell types:[3]
Theolfactory receptor neurons are sensory neurons of the olfactory epithelium. They arebipolar neurons and their apical poles express odorant receptors onnon-motile cilia at the ends of thedendritic knob,[4] which extend out into the airspace to interact with odorants. Odorant receptors bind odorants in the airspace, which are made soluble by the serous secretions fromolfactory glands located in the lamina propria of the mucosa.[5] The axons of the olfactory sensory neurons congregate to form theolfactory nerve (CN I). Once the axons pass through thecribriform plate, they terminate and synapse with the dendrites ofmitral cells in theglomeruli of theolfactory bulb.
Analogous to neuralglial cells, the supporting cells are non-neural cells in the olfactory epithelium that are located in the apical layer of the pseudostratified ciliated columnar epithelium. There are two types of supporting cells in the olfactory epithelium:sustentacular cells and microvillar cells. The sustentacular cells function as metabolic and physical support for the olfactory epithelium. Microvillar cells are another class of supporting cells that are morphologically and biochemically distinct from the sustentacular cells, and arise from a basal cell population that expresses thec-KIT cell surface protein.[6]
Resting on or near thebasal lamina of the olfactory epithelium, basal cells are stem cells capable of division and differentiation into either supporting or olfactory cells. While some of these basal cells divide rapidly, a significant proportion remain relatively quiescent and replenish olfactory epithelial cells as needed. This leads to the olfactory epithelium being replaced every 6–8 weeks.[7]
Basal cells can be divided on the basis of their cellular and histological features into two populations: the horizontal basal cells, which are slowly dividing reserve cells that express p63; and globose basal cells, which are a heterogeneous population of cells consisting of reserve cells, amplifying progenitor cells, and immediate precursor cells.[8]
Abrush cell is a microvilli-bearing columnar cell with its basal surface in contact with afferent nerve endings of thetrigeminal nerve (CN V) and is specialized for transduction of general sensation.
Tubuloalveolar serous secreting glands lying in the lamina propria of theolfactory 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 olfactory glands allows old odors to be constantly washed away.[5]
The olfactory epithelium derives from two structures duringembryonic development: thenasal placodes, which were long believed to be its sole origin; andneural crest cells, whose contributions have been identified more recently throughfate mapping studies.[9]
The embryonic olfactory epithelium consists of fewer cell types than in the adult, including apical and basalprogenitor cells, as well as immatureolfactory sensory neurons.[9] Early embryonic neurogenesis relies mostly on the apical cells, while later stage embryonicneurogenesis and secondary neurogenesis in adults relies on basal stem cells.[10] Theaxons of the immatureolfactory sensory neurons, along with a mixed population ofmigratory cells, including immatureolfactory ensheathing cells andgonadotropin-releasing hormone neurons form a "migratory mass" that travels towards theolfactory bulb.[9][10] At the end of the embryonic stage, the epithelium develops into apseudostratified columnar epithelium and begins secondary neurogenesis.[9]
Neurogenic placodes are transient, focal aggregations ofectoderm located in the developmental region of the futurevertebratehead, and give rise tosensory organs.[11] Early cranial sensory placodes are marked by expression ofSix1, part of the Six family of transcription factors that regulate thepreplacodal specification.[12] Theolfactory placode forms as two thickenings of non-neural region ofembryonic ectoderm.[13] In mice, the olfactory placode derives from an anterior portion of theneural tube, ~9-9.5 days into development and not long after the closure of theneural plate.[9] Development of the olfactory placode requires the presence underlyingneural crest-derivedmesenchymal tissue.[12] The specification of the olfactory placode tissue involves signaling of multiplegene networks, beginning with signals frombone morphogenetic proteins (BMP),retinoic acid (RA), andfibroblast growth factor (FGF), specificallyFGF8.[14] The resulting regulated downstream expression oftranscription factors, such asPax6,Dlx3,Sox2, and others, within the presumptive olfactory placode are crucial for sub-regionalization within the future olfactory epithelium and is responsible for the diversity of cells that compose the future epithelium.[9][12][15]
Similar to the other embryonic placodes, the olfactory placode gives rise to both neural and non-neural structures, ultimately resulting in the formation of the nasal epithelium.[16] The specification of neural versus non-neural tissue involves signals both within the olfactory placode, and between the olfactory placode and the underlyingmesenchymal compartment.[12] Continued signaling by BMP, FGF, and RA, themorphogens that initially induced placode formation, collectively coordinate the patterning of olfactory placode tissue into the future distinct cell types that make up the olfactory epithelium.[16] The cell types derived from the olfactory placode include:[17]
However, there is significant evidence for an additionalneural crest-origin for many of these cell types as well.[13]
Olfaction results from the proper development and interaction of the two components of the primaryolfactory pathway: the olfactory epithelium and theolfactory bulb.[18] The olfactory epithelium containsolfactory sensory neurons, whoseaxonsinnervate the olfactory bulb. In order for olfactory sensory neurons to function properly, they must expressodorant receptors and the propertransduction proteins onnon-motile cilia that extend from thedendritic knob in addition to projecting their axons to the olfactory bulb.[19]
The cells of the olfactory epithelium, including olfactory sensory neurons, begin todifferentiate soon after the induction of theolfactory placode. Once the olfactory sensory neurons differentiate, they express odorant receptors, which transduceodorant information from the environment to thecentral nervous system and aids in the development of the odorant map.[20] The differentiated olfactory sensory neurons extendpioneering axons, which followguidance cues released by the underlyingmesenchyme, as well as otherchemotrophic cues released from thetelencephalon.[10] As development of the olfactory pathway progresses, more axons innervate the olfactory bulb, which develops from the rostral-most region of telencephalon. The organization and subsequent processing of odorant information is possible due to the convergence of olfactory sensory neuron axons expressing the same odorant receptors onto the sameglomerulus at the olfactory bulb.[21]
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 toanosmia.
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