Editing Cilium (section)

==Types==
===Non-motile cilia===
In animals, non-motile '''primary cilia''' are found on nearly every type of cell, blood cells being a prominent exception.<ref name="HHMIB2005"/> Most cells only possess one, in contrast to cells with motile cilia, an exception being [[Olfactory epithelium#Olfactory sensory neurons|olfactory sensory neurons]], where the [[odorant receptors]] are located, which each possess about ten cilia. Some cell types, such as retinal photoreceptor cells, possess highly specialized primary cilia.<ref name="Wolfrum">{{cite journal |last1=Wolfrum |first1=U |last2=Schmitt |first2=A |title=Rhodopsin transport in the membrane of the connecting cilium of mammalian photoreceptor cells. |journal=Cell Motility and the Cytoskeleton |date=June 2000 |volume=46 |issue=2 |pages=95–107 |doi=10.1002/1097-0169(200006)46:2<95::AID-CM2>3.0.CO;2-Q |pmid=10891855}}</ref>

Although the primary cilium was discovered in 1898, it was largely ignored for a century and considered a [[vestigial]] organelle without important function.<ref>{{Cite journal|last=Satir|first=Peter|date=2017|title=CILIA: before and after|journal=Cilia|volume=6|pages=1|doi=10.1186/s13630-017-0046-8|issn=2046-2530|pmc=5343305|pmid=28293419 |doi-access=free }}</ref><ref name="HHMIB2005" />  Recent findings regarding its physiological roles in chemosensation, signal transduction, and cell growth control, have revealed its importance in cell function. Its importance to human biology has been underscored by the discovery of its role in a diverse group of diseases caused by the [[Anterior segment dysgenesis|dysgenesis]] or dysfunction of cilia, such as [[polycystic kidney disease]],<ref name="pmid18264930">{{cite journal | vauthors = Wagner CA | title = News from the cyst: insights into polycystic kidney disease | journal = Journal of Nephrology | volume = 21 | issue = 1 | pages = 14–16 | year = 2008 | pmid = 18264930 | url = http://www.jnephrol.com/Article.action?cmd=navigate&urlkey=Public_Details&t=JN&uid=9A13E591-2E27-441B-8356-23BF86D9CFB0 }}{{Dead link|date=July 2019 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> [[congenital heart disease]],<ref name="pmid17548739">{{cite journal | vauthors = Brueckner M | title = Heterotaxia, congenital heart disease, and primary ciliary dyskinesia | journal = Circulation | volume = 115 | issue = 22 | pages = 2793–95 | date = June 2007 | pmid = 17548739 | doi = 10.1161/CIRCULATIONAHA.107.699256 | s2cid = 14405881 | doi-access =  }}</ref> [[mitral valve prolapse]],<ref name="pmid 32380895">{{cite journal |vauthors=Toomer KA et al. | title = Primary cilia defects causing mitral valve prolapse | journal = Sci. Transl. Med. | volume = 11 | issue = 493| pages = eaax0290 | year = 2019 | pmid = 31118289 | doi = 10.1126/scitranslmed.aax0290| pmc = 7331025 | doi-access = free }}</ref>  and retinal degeneration,<ref>{{Cite journal|last1=Chen|first1=Holly Y.|last2=Kelley|first2=Ryan A.|last3=Li|first3=Tiansen|last4=Swaroop|first4=Anand|date=2020-07-31|title=Primary cilia biogenesis and associated retinal ciliopathies|journal=Seminars in Cell & Developmental Biology|volume=110|pages=70–88|doi=10.1016/j.semcdb.2020.07.013|issn=1096-3634|pmid=32747192|pmc=7855621|doi-access=free}}</ref> called [[ciliopathy|ciliopathies]].<ref name="badano2006">{{cite journal | vauthors = Badano JL, Mitsuma N, Beales PL, Katsanis N | title = The ciliopathies: an emerging class of human genetic disorders | journal = Annual Review of Genomics and Human Genetics | volume = 7 | pages = 125–48 | year = 2006 | pmid = 16722803 | doi = 10.1146/annurev.genom.7.080505.115610 }}</ref><ref name="Reiter 533–547">{{Cite journal|last1=Reiter|first1=Jeremy F.|last2=Leroux|first2=Michel R.|date=September 2017|title=Genes and molecular pathways underpinning ciliopathies|journal=Nature Reviews. Molecular Cell Biology|volume=18|issue=9|pages=533–547|doi=10.1038/nrm.2017.60|issn=1471-0080|pmc=5851292|pmid=28698599}}</ref> The primary cilium is now known to play an important role in the function of many human organs.<ref name="HHMIB2005">{{cite journal | last = Gardiner | first = Mary Beth | name-list-style = vanc | title = The Importance of Being Cilia | journal = HHMI Bulletin | volume = 18 | issue = 2 | date = September 2005 | url = http://www.hhmi.org/sites/default/files/Bulletin/2005/September/sept2005_fulltext.pdf | access-date = 26 July 2008 }}</ref><ref name="Singla 629–633"/> Primary cilia on pancreatic [[beta cell]]s regulate their function and energy metabolism. Cilia deletion can lead to islet dysfunction and [[type 2 diabetes]].<ref name="Hegyi">{{cite journal |last1=Hegyi |first1=P |last2=Petersen |first2=OH |title=The exocrine pancreas: the acinar-ductal tango in physiology and pathophysiology. |journal=Reviews of Physiology, Biochemistry and Pharmacology |date=2013 |volume=165 |pages=1–30 |doi=10.1007/112_2013_14 |pmid=23881310|isbn=978-3-319-00998-8 }}</ref>

Cilia are assembled during the [[G1 phase|G<sub>1</sub> phase]] and are disassembled before mitosis occurs.<ref>{{cite journal | vauthors = Pan J, Snell W | title = The primary cilium: keeper of the key to cell division | journal = Cell | volume = 129 | issue = 7 | pages = 1255–57 | date = June 2007 | pmid = 17604715 | doi = 10.1016/j.cell.2007.06.018 | s2cid = 17712155 | doi-access = free }}</ref><ref name="Patel"/> Disassembly of cilia requires the action of [[aurora kinase A]].<ref name="entrez">{{cite journal|vauthors=Pugacheva EN, Jablonski SA, Hartman TR, Henske EP, Golemis EA|date=June 2007|title=HEF1-dependent Aurora A activation induces disassembly of the primary cilium|journal=Cell|volume=129|issue=7|pages=1351–63|doi=10.1016/j.cell.2007.04.035|pmc=2504417|pmid=17604723}}</ref> 
The current scientific understanding of primary cilia views them as "sensory [[Cell (biology)|cellular]] antennae that coordinate many cellular signaling pathways, sometimes coupling the signaling to ciliary motility or alternatively to cell division and differentiation."<ref name="Satir2008">{{cite journal | vauthors = Satir P, Christensen ST | title = Structure and function of mammalian cilia | journal = Histochemistry and Cell Biology | volume = 129 | issue = 6 | pages = 687–93 | date = June 2008 | pmid = 18365235 | pmc = 2386530 | doi = 10.1007/s00418-008-0416-9 }}</ref>
The cilium is composed of subdomains{{clarify|date=October 2020}} and enclosed by a plasma membrane continuous with the plasma membrane of the cell. For many cilia, the [[basal body]], where the cilium originates, is located within a membrane invagination called the ciliary pocket. The cilium membrane and the basal body microtubules are connected by distal appendages (also called transition fibers). Vesicles carrying molecules for the cilia dock at the distal appendages. Distal to the transition fibers form a transition zone where entry and exit of molecules is regulated to and from the cilia.  Some of the signaling with these cilia occur through ligand binding such as [[Hedgehog signaling]].<ref>{{Cite journal|last1=Wong|first1=Sunny Y.|last2=Reiter|first2=Jeremy F.|date=2008|title=The primary cilium at the crossroads of mammalian hedgehog signaling|journal=Current Topics in Developmental Biology|volume=85|pages=225–260|doi=10.1016/S0070-2153(08)00809-0|issn=0070-2153|pmc=2653622|pmid=19147008}}</ref> Other forms of signaling include [[G protein-coupled receptor]]s including the [[somatostatin receptor 3]] in neurons.<ref>{{cite journal | vauthors = Wheway G, Nazlamova L, Hancock JT | title = Signaling through the Primary Cilium | journal = Frontiers in Cell and Developmental Biology | volume = 6 | pages = 8 | year = 2018 | pmid = 29473038 | pmc = 5809511 | doi = 10.3389/fcell.2018.00008 | doi-access = free }}</ref>
[[File:Blausen 0766 RespiratoryEpithelium.png|thumb|Illustration depicting motile cilia on [[respiratory epithelium]].]]

====Modified non-motile cilia====
[[Kinocilia]] that are found on hair cells in the inner ear are termed as specialized primary cilia, or modified non-motile cilia. They possess the 9+2 axoneme of the motile cilia but lack the inner dynein arms that give movement. They do move passively following the detection of sound, allowed by the outer dynein arms.<ref name="Wang">{{cite journal |last1=Wang |first1=D |last2=Zhou |first2=J |title=The Kinocilia of Cochlear Hair Cells: Structures, Functions, and Diseases. |journal=Frontiers in Cell and Developmental Biology |date=2021 |volume=9 |pages=715037 |doi=10.3389/fcell.2021.715037 |pmid=34422834|pmc=8374625 |doi-access=free }}</ref><ref name=Takeda>{{cite journal |last1=Takeda |first1=Sen |last2=Narita |first2=Keishi |title=Structure and function of vertebrate cilia, towards a new taxonomy |journal=Differentiation |date=February 2012 |volume=83 |issue=2 |pages=S4–S11 |doi=10.1016/j.diff.2011.11.002|pmid=22118931 }}</ref>

===Motile cilia===
[[File:Bronchiolar epithelium 4 - SEM.jpg|thumb|[[Trachea]]l respiratory epithelium showing cilia and much smaller [[microvilli]] on non-ciliated cells in [[Scanning electron microscope|scanning electron micrograph]].]]
[[Mammal]]s also have '''motile cilia''' or '''secondary cilia''' that are usually present on a cell's surface in large numbers (multiciliate), and beat in coordinated [[Metachronal rhythm|metachronal waves]].<ref name="Lewin2007">{{cite book|author=Benjamin Lewin|title=Cells|url=https://books.google.com/books?id=2VEGC8j9g9wC&pg=PA359|year=2007|publisher=Jones & Bartlett Learning|isbn=978-0-7637-3905-8|page=359}}</ref> Multiciliated cells are found [[respiratory epithelium|lining the respiratory tract]] where they function in [[mucociliary clearance]] sweeping mucus containing debris away from the [[lung]]s.<ref name="2012-Enuka">{{cite journal | vauthors = Enuka Y, Hanukoglu I, Edelheit O, Vaknine H, Hanukoglu A | title = Epithelial sodium channels (ENaC) are uniformly distributed on motile cilia in the oviduct and the respiratory airways | journal = Histochemistry and Cell Biology | volume = 137 | issue = 3 | pages = 339–53 | date = March 2012 | pmid = 22207244 | doi = 10.1007/s00418-011-0904-1 | s2cid = 15178940 }}</ref> Each cell in the respiratory epithelium has around 200 motile cilia.<ref name=Horani/>

In the [[reproductive tract]], [[smooth muscle]] contractions help the beating of the cilia in moving the [[egg cell]] from the ovary to the uterus.<ref name="2012-Enuka" /><ref name="Panelli"/>
In the [[ventricular system|ventricles of the brain]] ciliated [[ependymal cells]] circulate the [[cerebrospinal fluid]].

The functioning of motile cilia is strongly dependent on the maintenance of optimal levels of [[Periciliary liquid layer|periciliary fluid]] bathing the cilia. [[Epithelial sodium channel]]s (ENaCs) are specifically expressed along the entire length of cilia in the respiratory tract, and [[fallopian tube]] or ''[[oviduct]]'' that apparently serve as sensors to regulate the periciliary fluid.<ref name="2012-Enuka" /><ref name="2016-Hanukoglu" >{{cite journal | vauthors = Hanukoglu I, Hanukoglu A | title = Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases | journal = Gene | volume = 579 | issue = 2 | pages = 95–132 | date = April 2016 | pmid = 26772908 | pmc = 4756657 | doi = 10.1016/j.gene.2015.12.061 }}</ref>

====Modified motile cilia====
Motile cilia without the central pair of singlets (9+0) are found in early embryonic development. They are present as nodal cilia on the nodal cells of the [[primitive node]]. Nodal cells are responsible for the [[left-right asymmetry]] in [[bilaterians|bilateral animals]].<ref name="Desgrange"/> While lacking the central apparatus there are [[dynein arm]]s present that allow the nodal cilia to move in a spinning  fashion. The movement creates a current flow of the extraembryonic fluid across the [[primitive node|nodal surface]] in a leftward direction that initiates the left-right asymmetry in the developing embryo.
<ref name="Horani"/><ref name="Larsen"/>

Motile, multiple, 9+0 cilia are found on the epithelial cells of the [[choroid plexus]]. Cilia also can change structure when introduced to hot temperatures and become sharp. They are present in large numbers on each cell and move relatively slowly, making them intermediate between motile and primary cilia. In addition to 9+0 cilia that are mobile, there are also solitary 9+2 cilia that stay immobile found in hair cells.<ref name=Takeda/>

====Nodal cilia====
[[File:Nodal cilia.jpg|thumb|[[Scanning electron micrograph]] of nodal cilia on a [[mouse]] embryo]]
'''Nodal cells''' have a single cilium called a monocilium. They are present in the very early [[Embryonic development|development of the embryo]] on the [[primitive node]]. There are two areas of the node with different types of '''nodal cilia'''. On the central node are motile cilia, and on the peripheral area of the node the nodal cilia are modified motile.<ref name="Larsen">{{cite book |last1=Schoenwolf |first1=Gary C. |title=Larsen's human embryology |date=2015 |location=Philadelphia, PA |isbn=9781455706846 |page=64 |edition=Fifth}}</ref>

The motile cilia on the central cells rotate to generate the leftward flow of extracellular fluid needed to initiate the left-right asymmetry.<ref name="Larsen"/>

===Cilia versus flagella===
{{See also|Flagellum#Terminology}}
The motile cilia on [[sperm cell]]s and many [[protozoan]]s enables swimming through liquids and are traditionally referred to as "[[flagella]]".<ref name="Haimo_JCB198112"/> As these protrusions are structurally identical to motile cilia, attempts at preserving this terminology include making a distinction by morphology ("flagella" are typically longer than ordinary cilia and have a different undulating motion)<ref name="Alberts1"/> and by number.<ref name=pmid20145000>{{cite journal |last1=Lindemann |first1=CB |last2=Lesich |first2=KA |title=Flagellar and ciliary beating: the proven and the possible. |journal=Journal of Cell Science |date=15 February 2010 |volume=123 |issue=Pt 4 |pages=519–28 |doi=10.1242/jcs.051326 |pmid=20145000 |s2cid=18673550 |doi-access=}}</ref>

===Microorganisms===
[[Ciliate]]s are [[eukaryotic]] [[microorganism]]s that possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. A ''[[Paramecium]]'' for example is covered in thousands of cilia that enable its swimming. These motile cilia have been shown to be also sensory.<ref name="Valentine">{{cite journal |last1=Valentine |first1=M |last2=Van Houten |first2=J |title=Using ''Paramecium'' as a Model for Ciliopathies. |journal=Genes |date=24 September 2021 |volume=12 |issue=10 |page=1493 |doi=10.3390/genes12101493 |pmid=34680887|pmc=8535419 |doi-access=free }}</ref>