Respiratory epithelium#Structure

File:Psuedostratified epithelium animated.gif

File:Respiratory Tract Cells.png

The respiratory epithelium lining the upper respiratory airways is classified as ciliated pseudostratified columnar epithelium.Mescher AL, "Chapter 17. The Respiratory System" (Chapter). Mescher AL: Junqueira's Basic Histology: Text & Atlas, 12e: {{cite web |url=http://www.accessmedicine.com/content.aspx?aID=6182422 |title=AccessMedicine | the Respiratory System: Introduction |access-date=2015-02-24 |url-status=dead |archive-url=https://web.archive.org/web/20130603013209/http://www.accessmedicine.com/content.aspx?aid=6182422 |archive-date=2013-06-03 }}. This designation is due to the arrangement of the multiple cell types composing the respiratory epithelium. While all cells make contact with the basement membrane and are, therefore, a single layer of cells, their nuclei are not aligned in the same plane. Hence, it appears as though several layers of cells are present and the epithelium is called pseudostratified (falsely layered). The respiratory mucosa transitions to simple ciliated cuboidal epithelium and finally to simple squamous epithelium in the alveolar ducts and alveoli.{{Cite web|url=https://www.nih.gov|title=Bronchi, Bronchial Tree & Lungs|website=nih.gov|access-date=17 September 2019}}

The cells in the respiratory epithelium are of five main types: a) ciliated cells, b) goblet cells, c) brush cells, d) airway basal cells, and e) small granule cells (NDES){{cite book |last1=Mescher |first1=Anthony L. |title=Junqueira's basic histology: text and atlas |date=2018 |location=[New York] |isbn=978-1-26-002618-4 |page=350 |edition=Fifteenth}} Goblet cells become increasingly fewer further down the respiratory tree until they are absent in the terminal bronchioles; club cells take over their role to some extent here.{{cite book |last1=Weinberger |first1=Steven |last2=Cockrill |first2=Barbara |last3=Mandel |first3=Jess |title=Principles of pulmonary medicine |date=2019 |publisher=Elsevier |isbn=9780323523714 |page=67 |edition=Seventh}} Another important cell type is the pulmonary neuroendocrine cell. These are innervated cells that only make up around 0.5% of the respiratory epithelial cells. The ciliated cells are columnar epithelial cells with specialized ciliary modifications. The ciliated cells make up between 50 and 80 per cent of the epithelium.{{cite journal |last1=Yaghi |first1=A |last2=Dolovich |first2=MB |title=Airway Epithelial Cell Cilia and Obstructive Lung Disease. |journal=Cells |date=11 November 2016 |volume=5 |issue=4 |pages=40 |doi=10.3390/cells5040040 |pmid=27845721|pmc=5187524 |doi-access=free }}

Between the ciliated cells are numerous microvilli, attached as tufts to brush cells sometimes referred to as pulmonary brush cells; these are also known as the tuft cells of the gastrointestinal tract, or intestinal tuft cells,{{Cite book|title=Spencer's Pathology of the Lung|last=Hasleton|first=Philip|publisher=McGraw-Hill|year=1996|isbn=0071054480|pages=[https://archive.org/details/lepotagerdevers00andrgoog/page/10 10]|url=https://archive.org/details/lepotagerdevers00andrgoog/page/10}} although there is a difference between the two types: the brush cells lack the terminal web that lies under the microvilli of the tuft cells.{{cite journal |last1=Reid |first1=L |last2=Meyrick |first2=B |last3=Antony |first3=VB |last4=Chang |first4=LY |last5=Crapo |first5=JD |last6=Reynolds |first6=HY |title=The mysterious pulmonary brush cell: a cell in search of a function. |journal=American Journal of Respiratory and Critical Care Medicine |date=1 July 2005 |volume=172 |issue=1 |pages=136–9 |doi=10.1164/rccm.200502-203WS |pmid=15817800|pmc=2718446 }} Although their function is not yet fully understood, it has been suggested that they exhibit a virulence associated clearance role, activating mucociliary clearance by releasing acetylcholine.{{cite journal |last1=Perniss |first1=Alexander |last2=Lui |first2=Shuya |last3=Boonen |first3=Brett |last4=Zufall |first4=Frank |last5=Buffe |first5=Bernd |last6=Kummer |first6=Wolfgang |title=Chemosensory Cell-Derived Acetylcholine Drives Tracheal Mucociliary Clearance in Response to Virulence-Associated Formyl Peptides |journal=Cell Immunity |date=2020-04-14 |volume=52 |issue=2 |pages=683–699.e11 |doi=10.1016/j.immuni.2020.03.005 |doi-access=free|pmid=32294408 }}

The respiratory epithelium functions to moisten and protect the airways. It acts as a physical barrier to pathogens, as well as their removal in the mechanism of mucociliary clearance.

The ciliated cells are the primary components in the mucociliary clearance mechanism. Each epithelial cell has around 200 cilia that beat constantly at a rate of between 10 and 20 times per second. The direction of their beat is targeted towards the pharynx, either upwards from the lower respiratory tract or downwards from the nasal structures.{{Cite book|title=Guyton and Hall Textbook of Medical Physiology|last=Hall|first=John|year=2011|isbn=9781416045748|pages=473|publisher=Saunders/Elsevier }}

Goblet cells, so named because they are shaped like a wine goblet, are columnar epithelial cells that contain membrane-bound mucous granules and secrete mucus as part of the airway surface liquid (ASL), also known as the epithelial lining fluid, the composition of which is tightly regulated; the mucus helps maintain epithelial moisture and traps particulate material and pathogens moving through the airway. and determines how well mucociliary clearance works.Stanke F The Contribution of the Airway Epithelial Cell to Host Defense. Mediators Inflamm. 2015;2015:463016. {{PMID|26185361}} [https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4491388/ PMC 4491388]U.S. EPA. Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2016 Final Report). U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-15/068, 2016. [https://www.federalregister.gov/articles/2016/01/28/2016-01548/integrated-science-assessment-for-oxides-of-nitrogen-health-criteria Federal Register Notice Jan 28, 2016] Free download available at [https://cfpub.epa.gov/ncea/isa/recordisplay.cfm?deid=310879 Report page at EPA website].

The basal cells are small, nearly cuboidal that differentiate into the other cell types found within the epithelium. Basal cells respond to injury of the airway epithelium, migrating to cover a site denuded of differentiated epithelial cells, and subsequently differentiating to restore a healthy epithelial cell layer. The differentiated epithelial cells can also dedifferentiate into stem cells and contribute to the repairing of the barrier.{{cite journal |last1=Hiemstra |first1=PS |last2=McCray PB |first2=Jr |last3=Bals |first3=R |title=The innate immune function of airway epithelial cells in inflammatory lung disease. |journal=The European Respiratory Journal |date=April 2015 |volume=45 |issue=4 |pages=1150–62 |doi=10.1183/09031936.00141514 |pmid=25700381|pmc=4719567 }}

Club cells carry out similar functions in the more distal airways.

Certain parts of the respiratory tract, such as the oropharynx, are also subject to the abrasive swallowing of food. To prevent the destruction of the epithelium in these areas, it changes to stratified squamous epithelium, which is better suited to the constant sloughing and abrasion. The squamous layer of the oropharynx is continuous with the esophagus.{{Citation needed|date=March 2010}}

The respiratory epithelium has a further role of immunity for the lungs - that of glucose homeostasis.{{cite journal |last1=Baker |first1=EH |last2=Baines |first2=DL |title=Airway Glucose Homeostasis: A New Target in the Prevention and Treatment of Pulmonary Infection. |journal=Chest |date=February 2018 |volume=153 |issue=2 |pages=507–514 |doi=10.1016/j.chest.2017.05.031 |pmid=28610911|s2cid=13733461 |url=http://openaccess.sgul.ac.uk/108898/1/Chest%20review%20resubmission%2012-5-17clean%20version.docx }} The glucose concentration in the airway surface liquid is held at a level of around 12 times lower than that of the blood sugar concentration. The tight junctions act as a barrier that restricts the passage of glucose across the epithelium into the airway lumen. Some glucose passes through, where it diffuses into the airway surface liquid to be kept at its reduced level by pulmonary glucose transport, and metabolism.{{cite journal |last1=Garnett |first1=JP |last2=Baker |first2=EH |last3=Baines |first3=DL |title=Sweet talk: insights into the nature and importance of glucose transport in lung epithelium. |journal=The European Respiratory Journal |date=November 2012 |volume=40 |issue=5 |pages=1269–76 |doi=10.1183/09031936.00052612 |pmid=22878875|doi-access=free }} However, airway inflammation decreases the effectiveness of the tight junctions making the barrier more permeable to glucose. Higher levels of glucose promote the proliferation of bacteria by providing glucose as a source for carbon for them. Increased levels of glucose in the airway surface liquid is associated with respiratory diseases, and hyperglycemia.

Long-term irritation of the epithelial cells can cause the overproduction of mucus, known as mucus hypersecretion. Mucus hypersecretion results in the productive cough of chronic bronchitis.{{cite book |title=Global Initiative for Chronic Obstructive Lung Disease - GOLD |date=2018 |pages=15 |url=https://goldcopd.org/wp-content/uploads/2018/11/GOLD-2019-v1.7-FINAL-14Nov2018-WMS.pdf |access-date=10 November 2019}}

Pulmonary neuroendocrine cells have been associated with a range of chronic lung disorders. They are also the originating cells of small-cell lung cancer.{{cite journal |last1=Garg |first1=A |last2=Sui |first2=P |last3=Verheyden |first3=JM |last4=Young |first4=LR |last5=Sun |first5=X |title=Consider the lung as a sensory organ: A tip from pulmonary neuroendocrine cells. |journal=Current Topics in Developmental Biology |date=2019 |volume=132 |pages=67–89 |doi=10.1016/bs.ctdb.2018.12.002 |pmid=30797518|isbn=9780128104897 |s2cid=73489416 }}

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2304 Pseudostratified Epithelium.jpg|Cross-section of pseudostratified columnar epithelium

2308 The Trachea-b.jpg|Second cross-section

{{Epithelial types}}

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Category:Respiratory system

Category:Epithelium