Editing Cilium (section)

===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>