Olfactory epithelium

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Olfactory epithelium
Section of the olfactory mucous membrane.
Plan of olfactory neurons.
MeSH A04.531.520.573
Code TH H3.
Anatomical terminology
A schematic drawing of the components of the embryonic olfactory epithelium.

The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 9 cm2 (3 centimeters by 3 centimeters) and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils.[1] The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.

Development of the Olfactory Epithelium[edit]


The olfactory epithelium derives from two structures during embryonic development: the olfactory placode, which was long believed to be its sole origin; and neural crest cells, whose contributions have been identified more recently through fate-mapping studies.[2]

The embryonic olfactory epithelium consists of fewer cell types than in the adult, including apical and basal progenitor cells, as well as immature olfactory receptor neurons.[2] Early embryonic neurogenesis relies mostly on the apical cells, while later stage embryonic neurogenesis and secondary neurogenesis in adults relies on basal stem cells.[3] The axons of the immature olfactory receptor neurons, along with a mixed population of migratory cells, including immature olfactory ensheathing cells (OECs) and gonadotropin-releasing hormone (GnRH) neurons form a “migratory mass” that travels towards the olfactory bulb.[2][3] At the end of the embryonic stage, the epithelium develops into a pseudostratified columnar epithelium and begins secondary neurogenesis.[2]

Olfactory Placode[edit]

Embryonic morphogen signaling on olfactory placode and neural crest cells contributes to formation of neurogenic and non-neurogenic tissue of the olfactory epithelium.

The olfactory placode (OP) forms as two thickenings of non-neural region of embryonic ectoderm.[4] In mice, the OP derives from an anterior portion of the neural tube, ~9-9.5 days into development and not long after the closure of the neural plate.[2] Development of the OP requires the presence underlying neural crest-derived mesenchymal tissue.[5] The specification of the OP tissue involves signaling of multiple gene networks, beginning with signals from bone morphogenetic proteins (BMP), retinoic acid (RA), and fibroblast growth factor (FGF).[6] The resulting regulated downstream expression of homeotic transcription factors, such as Pax6, Dlx3, Sox2, and others, within the presumptive OP are crucial for sub-regionalization within the future olfactory epithelium (OE) and is responsible for the diversity of cells that compose it.[2][5][7]

Similar to the other embryonic placodes, it gives rise to both neural and non-neural structures ultimately resulting in the formation of the nasal epithelium.[8] The specification of neural versus non-neural tissue involves signals both within the OP, and between the OP and the underlying mesenchymal compartment.[5] Continued signaling by BMP, FGF, and RA, the morphogens that initially induced placode formation, collectively coordinate the patterning of OP tissue into its future distinct cell types that make up the OE.[8] The cell types derived from the olfactory placode include:[9]

However, there is significant evidence for an additional neural crest-origin for many of these cell types as well.[4]

Olfactory sensory neurons (OSNs) express odorant receptors. The axons of OSNs expressing the same odorant receptors converge onto the same glomerulus at the olfactory bulb, allowing for the organization of olfactory information.

Olfactory sensory neuron development[edit]

Olfaction is possible due to the proper development and interaction of the two components of the primary olfactory pathway: the olfactory epithelium and the olfactory bulb.[10] The olfactory epithelium contains olfactory sensory neurons (OSNs), whose axons innervate the olfactory bulb. In order for OSNs to function properly, they must express odorant receptors at the surface of their dendritic knob and project their axons to the olfactory bulb.

The cells of the olfactory epithelium, including OSNs, begin to differentiate soon after the induction of the olfactory placode. Once the OSNs differentiate, they express odorant receptors, which transduce odorant information from the environment to the central nervous system and aids in the development of the odorant map.[11] The differentiated OSNs extend pioneering axons, following guidance cues released by the underlying mesenchyme, in addition to other chemotrophic cues released from the telencephalon.[3] As development of the olfactory pathway progresses, more axons innervate the olfactory bulb and OSNs expressing the same odorant receptors converge on the same olfactory glomerulus, generating an odorant map for organizing olfactory information.[12]

Layers of the Olfactory Epithelium[edit]

Olfactory epithelium consists of four distinct cell types:[13]

  • Olfactory cells
  • Supporting cells
  • Basal cells
  • Brush cells

Olfactory cells[edit]

The olfactory cells of the epithelium are bipolar olfactory receptor 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 olfactory glands, located in the lamina propria of the mucosa.[14]

Supporting cells[edit]

Analogous to neural glial cells, the supporting 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.

Basal cells[edit]

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.[15] Horizontal basal cells are now thought to be the primary stem cell population supplying new cells in this system.,[16] although this is subject to some debate with some scientists maintaining that the globose basal cells are the true stem cells.

Brush cells[edit]

A brush cell is 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.

Olfactory glands[edit]

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 olfactory glands allows old odors to be constantly washed away.[14]

Pathology of the Olfactory Epithelium[edit]

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.

Additional images[edit]

See also[edit]


  1. ^ Moran, David T.; Rowley Jc, 3rd; Jafek, BW; Lovell, MA (1982), "The fine structure of the olfactory mucosa in man", Journal of Neurocytology, 11 (5): 721–746, doi:10.1007/BF01153516, PMID 7143026 
  2. ^ a b c d e f Suzuki, Jun; Osumi, Noriko (2015-01-01). "Neural crest and placode contributions to olfactory development". Current Topics in Developmental Biology. 111: 351–374. doi:10.1016/bs.ctdb.2014.11.010. ISSN 1557-8933. PMID 25662265. 
  3. ^ a b c Treloar, Helen B.; Miller, Alexandra M.; Ray, Arundhati; Greer, Charles A. (2010-01-01). Menini, Anna, ed. The Neurobiology of Olfaction. Frontiers in Neuroscience. Boca Raton (FL): CRC Press/Taylor & Francis. ISBN 9781420071979. PMID 21882426. 
  4. ^ a b Forni, Paolo E.; Wray, Susan (2012-10-01). "Neural crest and olfactory system: new prospective". Molecular Neurobiology. 46 (2): 349–360. doi:10.1007/s12035-012-8286-5. ISSN 1559-1182. PMC 3586243Freely accessible. PMID 22773137. 
  5. ^ a b c Moody, Sally A.; LaMantia, Anthony-Samuel (2015-01-01). "Transcriptional regulation of cranial sensory placode development". Current Topics in Developmental Biology. 111: 301–350. doi:10.1016/bs.ctdb.2014.11.009. ISSN 1557-8933. PMC 4425424Freely accessible. PMID 25662264. 
  6. ^ Maier, Esther C.; Whitfield, Tanya T. (2014-12-01). "RA and FGF signalling are required in the zebrafish otic vesicle to pattern and maintain ventral otic identities". PLoS genetics. 10 (12): e1004858. doi:10.1371/journal.pgen.1004858. ISSN 1553-7404. PMC 4256275Freely accessible. PMID 25473832. 
  7. ^ Bhattacharyya, Sujata; Bronner-Fraser, Marianne (2008-12-01). "Competence, specification and commitment to an olfactory placode fate". Development (Cambridge, England). 135 (24): 4165–4177. doi:10.1242/dev.026633. ISSN 0950-1991. PMID 19029046. 
  8. ^ a b Maier, Esther C.; Saxena, Ankur; Alsina, Berta; Bronner, Marianne E.; Whitfield, Tanya T. (2014-05-01). "Sensational placodes: neurogenesis in the otic and olfactory systems". Developmental Biology. 389 (1): 50–67. doi:10.1016/j.ydbio.2014.01.023. ISSN 1095-564X. PMC 3988839Freely accessible. PMID 24508480. 
  9. ^ Farbman, A. I. (1994-02-01). "Developmental biology of olfactory sensory neurons". Seminars in Cell Biology. 5 (1): 3–10. ISSN 1043-4682. PMID 8186394. 
  10. ^ Ravi, Namasivayam; Sanchez-Guardado, Luis; Lois, Carlos; Kelsch, Wolfgang (2017-03-01). "Determination of the connectivity of newborn neurons in mammalian olfactory circuits". Cellular and molecular life sciences: CMLS. 74 (5): 849–867. doi:10.1007/s00018-016-2367-y. ISSN 1420-9071. PMID 27695873. 
  11. ^ Valle-Leija, Pablo (2015-01-01). "Odorant receptors signaling instructs the development and plasticity of the glomerular map". Neural Plasticity. 2015: 975367. doi:10.1155/2015/975367. ISSN 1687-5443. PMC 4320882Freely accessible. PMID 25688305. 
  12. ^ Nishizumi, Hirofumi; Sakano, Hitoshi (2015-06-01). "Developmental regulation of neural map formation in the mouse olfactory system". Developmental Neurobiology. 75 (6): 594–607. doi:10.1002/dneu.22268. ISSN 1932-846X. PMID 25649346. 
  13. ^ Ross, MH, Histology: A Text and Atlas, 5th Edition. Philadelphia: Lippincott, Williams and Wilkins, 2006. page 615-616.
  14. ^ a b Ross, MH, Histology: A Text and Atlas, 5th Edition. Philadelphia: Lippincott, Williams and Wilkins, 2006. page 616.
  15. ^ Schwob, James E. (2002), "Neural Regeneration and the Peripheral Olfactory System", The Anatomical Record, 269 (1): 33–49, doi:10.1002/ar.10047, PMID 11891623 
  16. ^ Leung, C.T.; Coulombe, P.A.; Reed, R.R. (2007). "Contribution of olfactory neural stem cells to tissue maintenance and regeneration". Nat Neurosci. 10 (6): 673–4. doi:10.1038/nn1882. PMID 17468753. 

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