olfactory epithelium

Olfactory epithelium consists of four distinct cell types:Ross, MH, Histology: A Text and Atlas, 5th Edition. Philadelphia: Lippincott, Williams and Wilkins, 2006. page 615-616.

• Olfactory sensory neurons
• Supporting cells
• Basal cells
• Brush cells

{{Main|Olfactory receptor neuron}}

The olfactory receptor neurons are sensory neurons of the olfactory epithelium. They are bipolar neurons and their apical poles express odorant receptors on non-motile cilia at the ends of the dendritic knob, {{Cite web|url=https://www.yeastgenome.org/go/GO:0098788|title=Dendritic knob|website=yeast genome.org|access-date=28 July 2019}} 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 from olfactory glands located in the lamina propria of the mucosa.Ross, MH, Histology: A Text and Atlas, 5th Edition. Philadelphia: Lippincott, Williams and Wilkins, 2006. page 616. The axons of the olfactory sensory neurons congregate to form the olfactory nerve (CN I). Once the axons pass through the cribriform plate, they terminate and synapse with the dendrites of mitral cells in the glomeruli of the olfactory bulb.

Analogous to neural glial 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 the c-KIT cell surface protein.{{Cite journal|last1=Goss|first1=Garrett M.|last2=Chaudhari|first2=Nirupa|last3=Hare|first3=Joshua M.|last4=Nwojo|first4=Raphael|last5=Seidler|first5=Barbara|last6=Saur|first6=Dieter|last7=Goldstein|first7=Bradley J.|date=2016-03-01|title=Differentiation potential of individual olfactory c-Kit+ progenitors determined via multicolor lineage tracing|journal=Developmental Neurobiology|volume=76|issue=3|pages=241–251|doi=10.1002/dneu.22310|issn=1932-846X|pmc=4662645|pmid=26016700}}

Resting on or near the basal 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.{{Cite journal|last1=Purves|first1=Dale|last2=Augustine|first2=George J.|last3=Fitzpatrick|first3=David|last4=Katz|first4=Lawrence C.|last5=LaMantia|first5=Anthony-Samuel|last6=McNamara|first6=James O.|last7=Williams|first7=S. Mark|date=2001-01-01|title=The Olfactory Epithelium and Olfactory Receptor Neurons|publisher=Sinauer Associates |url=https://www.ncbi.nlm.nih.gov/books/NBK10896/|language=en}}

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.{{Cite journal|last1=Schwob|first1=James E.|last2=Jang|first2=Woochan|last3=Holbrook|first3=Eric H.|last4=Lin|first4=Brian|last5=Herrick|first5=Daniel B.|last6=Peterson|first6=Jesse N.|last7=Hewitt Coleman|first7=Julie|date=2017-03-01|title=Stem and progenitor cells of the mammalian olfactory epithelium: Taking poietic license|journal=Journal of Comparative Neurology|language=en|volume=525|issue=4|pages=1034–1054|doi=10.1002/cne.24105|pmid=27560601|pmc=5805156|issn=1096-9861}}

A brush cell is a microvilli-bearing columnar cell with its basal surface in contact with afferent nerve endings of the trigeminal nerve (CN V) and is specialized for transduction of general sensation.

Tubuloalveolar serous secreting glands lying in the lamina propria of the olfactory 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.

File:Embryonic olfactory epithelium.svg, OEC = olfactory ensheathing cell]]

The olfactory epithelium derives from two structures during embryonic development: the nasal placodes, which were long believed to be its sole origin; and neural crest cells, whose contributions have been identified more recently through fate mapping studies.{{Cite book|last1=Suzuki|first1=Jun|last2=Osumi|first2=Noriko|date=2015-01-01|title=Neural crest and placode contributions to olfactory development|journal=Current Topics in Developmental Biology|volume=111|pages=351–374|doi=10.1016/bs.ctdb.2014.11.010|issn=1557-8933|pmid=25662265|isbn=9780124077591}}

The embryonic olfactory epithelium consists of fewer cell types than in the adult, including apical and basal progenitor cells, as well as immature olfactory sensory neurons. Early embryonic neurogenesis relies mostly on the apical cells, while later stage embryonic neurogenesis and secondary neurogenesis in adults relies on basal stem cells.{{Cite book|url=https://www.ncbi.nlm.nih.gov/books/NBK55972/|title=The Neurobiology of Olfaction|last1=Treloar|first1=Helen B.|last2=Miller|first2=Alexandra M.|last3=Ray|first3=Arundhati|last4=Greer|first4=Charles A.|date=2010-01-01|publisher=CRC Press/Taylor & Francis|isbn=9781420071979|editor-last=Menini|editor-first=Anna|series=Frontiers in Neuroscience|location=Boca Raton (FL)|pmid=21882426}} The axons of the immature olfactory sensory neurons, along with a mixed population of migratory cells, including immature olfactory ensheathing cells and gonadotropin-releasing hormone neurons form a "migratory mass" that travels towards the olfactory bulb. At the end of the embryonic stage, the epithelium develops into a pseudostratified columnar epithelium and begins secondary neurogenesis.

File:Wikipedia project fig upload.pngNeurogenic placodes are transient, focal aggregations of ectoderm located in the developmental region of the future vertebrate head, and give rise to sensory organs.{{Cite journal|last=Whitlock|first=Kathleen E.|author-link = Kathleen Whitlock|date=2004-01-01|title=A new model for olfactory placode development|journal=Brain, Behavior and Evolution|volume=64|issue=3|pages=126–140|doi=10.1159/000079742|issn=0006-8977|pmid=15353905|s2cid=46531167}} Early cranial sensory placodes are marked by expression of Six1, part of the Six family of transcription factors that regulate the preplacodal specification. The olfactory placode forms as two thickenings of non-neural region of embryonic ectoderm.{{Cite journal|last1=Forni|first1=Paolo E.|last2=Wray|first2=Susan|date=2012-10-01|title=Neural crest and olfactory system: new prospective|journal=Molecular Neurobiology|volume=46|issue=2|pages=349–360|doi=10.1007/s12035-012-8286-5|issn=1559-1182|pmc=3586243|pmid=22773137}} In mice, the olfactory placode 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. Development of the olfactory placode requires the presence underlying neural crest-derived mesenchymal tissue.{{Cite book|last1=Moody|first1=Sally A.|last2=LaMantia|first2=Anthony-Samuel|date=2015-01-01|title=Transcriptional regulation of cranial sensory placode development|journal=Current Topics in Developmental Biology|volume=111|pages=301–350|doi=10.1016/bs.ctdb.2014.11.009|issn=1557-8933|pmc=4425424|pmid=25662264|isbn=9780124077591}} The specification of the olfactory placode tissue involves signaling of multiple gene networks, beginning with signals from bone morphogenetic proteins (BMP), retinoic acid (RA), and fibroblast growth factor (FGF), specifically FGF8.{{Cite journal|last1=Maier|first1=Esther C.|last2=Whitfield|first2=Tanya T.|date=2014-12-01|title=RA and FGF signalling are required in the zebrafish otic vesicle to pattern and maintain ventral otic identities|journal=PLOS Genetics|volume=10|issue=12|pages=e1004858|doi=10.1371/journal.pgen.1004858|issn=1553-7404|pmc=4256275|pmid=25473832 |doi-access=free }} The resulting regulated downstream expression of transcription factors, such as Pax6, 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.{{Cite journal|last1=Bhattacharyya|first1=Sujata|last2=Bronner-Fraser|first2=Marianne|date=2008-12-01|title=Competence, specification and commitment to an olfactory placode fate|journal=Development|volume=135|issue=24|pages=4165–4177|doi=10.1242/dev.026633|issn=0950-1991|pmid=19029046|doi-access=free}}

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.{{Cite journal|last1=Maier|first1=Esther C.|last2=Saxena|first2=Ankur|last3=Alsina|first3=Berta|last4=Bronner|first4=Marianne E.|last5=Whitfield|first5=Tanya T.|date=2014-05-01|title=Sensational placodes: neurogenesis in the otic and olfactory systems|journal=Developmental Biology|volume=389|issue=1|pages=50–67|doi=10.1016/j.ydbio.2014.01.023|issn=1095-564X|pmc=3988839|pmid=24508480}} The specification of neural versus non-neural tissue involves signals both within the olfactory placode, and between the olfactory placode and the underlying mesenchymal compartment. Continued signaling by BMP, FGF, and RA, the morphogens 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. The cell types derived from the olfactory placode include:{{Cite journal|last=Farbman|first=A. I.|date=1994-02-01|title=Developmental biology of olfactory sensory neurons|url=https://pubmed.ncbi.nlm.nih.gov/8186394/|journal=Seminars in Cell Biology|volume=5|issue=1|pages=3–10|issn=1043-4682|pmid=8186394|doi=10.1006/scel.1994.1002}}

• Neural: olfactory sensory neurons, LHRH-secreting neurons, and ganglion cells
• Non-neuronal: basal cells, olfactory supporting cells, ciliated cells, Bowman’s glands, Schwann cells, submucosal glands, and brush cells

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

File:Olfactory Sensory Neurons innervating Olfactory Glomeruli.jpgOlfaction results from the proper development and interaction of the two components of the primary olfactory pathway: the olfactory epithelium and the olfactory bulb.{{Cite journal|last1=Ravi|first1=Namasivayam|last2=Sanchez-Guardado|first2=Luis|last3=Lois|first3=Carlos|last4=Kelsch|first4=Wolfgang|date=2017-03-01|title=Determination of the connectivity of newborn neurons in mammalian olfactory circuits|journal=Cellular and Molecular Life Sciences|volume=74|issue=5|pages=849–867|doi=10.1007/s00018-016-2367-y|issn=1420-9071|pmid=27695873|s2cid=36297243|pmc=11107630}} The olfactory epithelium contains olfactory sensory neurons, whose axons innervate the olfactory bulb. In order for olfactory sensory neurons to function properly, they must express odorant receptors and the proper transduction proteins on non-motile cilia that extend from the dendritic knob in addition to projecting their axons to the olfactory bulb.{{Cite journal|last1=Purves|first1=Dale|last2=Augustine|first2=George J.|last3=Fitzpatrick|first3=David|last4=Katz|first4=Lawrence C.|last5=LaMantia|first5=Anthony-Samuel|last6=McNamara|first6=James O.|last7=Williams|first7=S. Mark|date=2001-01-01|title=The Transduction of Olfactory Signals|publisher=Sinauer Associates |url=https://www.ncbi.nlm.nih.gov/books/NBK11039/|language=en}}

The cells of the olfactory epithelium, including olfactory sensory neurons, begin to differentiate soon after the induction of the olfactory placode. Once the olfactory sensory neurons 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.{{Cite journal|last=Valle-Leija|first=Pablo|date=2015-01-01|title=Odorant receptors signaling instructs the development and plasticity of the glomerular map|journal=Neural Plasticity|volume=2015|pages=975367|doi=10.1155/2015/975367|issn=1687-5443|pmc=4320882|pmid=25688305|doi-access=free }} The differentiated olfactory sensory neurons extend pioneering axons, which follow guidance cues released by the underlying mesenchyme, as well as other chemotrophic cues released from the telencephalon. 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 same glomerulus at the olfactory bulb.{{Cite journal|last1=Nishizumi|first1=Hirofumi|last2=Sakano|first2=Hitoshi|date=2015-06-01|title=Developmental regulation of neural map formation in the mouse olfactory system|journal=Developmental Neurobiology|volume=75|issue=6|pages=594–607|doi=10.1002/dneu.22268|issn=1932-846X|pmid=25649346|s2cid=24497222}}

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.

Image:Riechschleimhaut.svg|Composition of the Olfactory receptor neuron (captions in German)

Image:Olfactorisch epitheel varken olfactory epithelium pig.jpg|olfactory epithelium pig

• Phantosmia
• List of distinct cell types in the adult human body

{{Commons category|Olfactory epithelium}}

{{Clear}}

{{Reflist}}

• [https://thesis.library.caltech.edu/2200/ Embryonic origin of the olfactory sensory system: fate map, lineage analysis and specification of the avian olfactory placode] and [https://books.google.com/books?id=HnhqQwAACAAJ]
• {{cite journal|last=Katoh|first=Hiroyuki |author2=Shibata, Shinsuke |author3=Fukuda, Kimiko |author4=Sato, Momoka |author5=Satoh, Etsuko |author6=Nagoshi, Narihito |author7=Minematsu, Takeo |author8=Matsuzaki, Yumi |author9=Akazawa, Chihiro |author10=Toyama, Yoshiaki |author11=Nakamura, Masaya |author12=Okano, Hideyuki |title=The dual origin of the peripheral olfactory system: placode and neural crest|journal=Molecular Brain|date=2011|volume=4|issue=1|pages=34|doi=10.1186/1756-6606-4-34|pmid=21943152|pmc=3215936 |doi-access=free }}

{{Olfactory system}}

{{Epithelium and epithelial tissue}}

{{Authority control}}

{{DEFAULTSORT:Olfactory Epithelium}}

Category:Olfactory system

Category:Otorhinolaryngology