View source for Cilium


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[[Image:Bronchiolar epithelium 3 - SEM.jpg|thumb|250px|right|[[SEM]] micrograph of the cilia projecting from [[respiratory epithelium]] in the lungs]]

==Overview==
A '''cilium''' (plural ''cilia'') is an [[organelle]] found in [[eukaryote|eukaryotic]] [[cell (biology)|cell]]s. Cilia are tail-like ''projections'' extending approximately 5–10 [[micrometre|micrometer]]s outwards from the cell body. 

There are two types of cilia: ''motile cilia'', which constantly beat in a single direction, and ''non-motile cilia'', which typically serve as sensory organelles. Along with [[flagella]], they make up a group of organelles known as [[undulipodia]].

== Types and distribution ==
Cilia are rare in plants, occurring most notably in [[cycad]]s. [[Ciliate]]s possess motile cilia exclusively and use them for either locomotion or to simply move liquid over their surface. Some ciliates bear groups of cilia that are fused together into large mobile projections called '''cirri''' (''singular'', '''cirrus''').

Larger eukaryotes, such as mammals, have motile cilia as well. Motile cilia are rarely found alone, usually present on a cell's surface in large numbers and beating in coordinated waves. In [[human]]s, for example, motile cilia are found in the lining of the [[vertebrate trachea|trachea]] (windpipe), where they sweep mucus and dirt out of the lungs. In [[female]] [[mammals]], the beating of cilia in the Fallopian tubes moves the [[ovum]] from the [[ovary]] to the [[uterus]].

In contrast to motile cilia, non-motile cilia usually occur one per cell. The outer segment of the rod [[photoreceptor cell]] in the human eye is connected to its cell body with a specialized non-motile cilium. The dendritic knob of the [[olfactory]] neuron, where the odorant receptors are located, is also carrying non-motile cilia (about 10 cilia / dendritic knobs). Aside from these specialized examples, almost all mammalian cells have a single non-motile ''primary cilium''. Although the primary cilium has historically been one of the oldest cellular organelles to be studied (at least since 1898), only a small group of devotees have followed it until its importance began to become clear in the late 1990. Recent findings regarding its physiological roles in chemical sensation, signal transduction, and control of cell growth, have led scientists to acknowledge its importance in cell function, with the discovery of its role in diseases not previously recognized to involve the dysgenesis and dysfunction of cilia, such as [[polycystic kidney disease]]<ref name="pmid18264930">{{cite journal |author=Wagner CA |title=News from the cyst: insights into polycystic kidney disease. |journal=J. Nephrol. |volume=21 |issue=1 |pages=14–6 |year=2008 |pmid=18264930 |doi=}}</ref> and [[congenital heart disease]]<ref name="pmid17548739">{{cite journal |author=Brueckner M |title=Heterotaxia, congenital heart disease, and primary ciliary dyskinesia. |journal=Circulation |volume=115 |issue=22 |pages=2793–5 |year=2007 |pmid=17548739 |doi=10.1161/CIRCULATIONAHA.107.699256 |url=http://circ.ahajournals.org/cgi/pmidlookup?view=long&pmid=17548739}}</ref>.

== Assembly and maintenance ==
To grow a cilium, the building blocks of the cilia such as [[tubulins]] and other partially assembled axonemal proteins are added to the ciliary tips which point away from the cell body. In most species bi-directional motility called [[intraflagellar transport]] or [[IFT]] plays an essential role to move these building materials from the cell body to the assembly site. IFT also carries the disassembled material to be recycled from the ciliary tip back to the cell body. By regulating the equilibrium between these two IFT proceses, the length  of cilia can be maintained dynamically.

Exceptions where IFT is not present include ''[[Plasmodium falciparum]]'' which is one of the species of ''[[Plasmodium]]'' that cause [[malaria]] in humans. In this parasite, cilia assemble in the cytoplasm.<ref>[http://www.pandasthumb.org/archives/2007/06/of_cilia_and_si.html Of cilia and silliness (more on Behe) - The Panda's Thumb<!-- Bot generated title -->]</ref>

== Cilium-related disease ==
[[Image:Eukaryotic cilium diagram en.svg|thumb|300px]]
Ciliary defects can lead to several human diseases. Genetic mutations compromising the proper functioning of cilia can cause chronic disorders such as [[primary ciliary dyskinesia]] (PCD). In addition, a defect of the primary cilium in the renal tube cells can lead to [[polycystic kidney disease]] (PKD). In another genetic disorder called [[Bardet-Biedl syndrome]] (BBS), the mutant gene products are the components in the basal body and cilia.

Lack of functional cilia in mammalian [[Fallopian tubes]] can cause [[ectopic pregnancy]]. A fertilized [[ovum]] may not reach the [[uterus]] if the cilia are unable to move it there. In such a case, the ovum will implant in the Fallopian tubes, causing a [[tubal pregnancy]], the most common form of ectopic pregnancy.

Since the flagellum of human sperm is actually a modified cilium, ciliary dysfunction can also be responsible for male infertility.<ref name="pmid16850538">{{cite journal |author=Ichioka K, Kohei N, Okubo K, Nishiyama H, Terai A |title=Obstructive azoospermia associated with chronic sinopulmonary infection and situs inversus totalis. |journal=Urology |volume=68 |issue=1 |pages=204.e5–7 |year=2006 |pmid=16850538 |doi=}}</ref>

Of interest, there is an association of primary ciliary dyskinesia with left-right anatomic abnormalities such as [[situs inversus]] (a combination of findings known as [[Kartagener's syndrome]]) and other heterotaxic defects. These left-right anatomic abnormalities can also result in [[congenital heart disease]]<ref name="pmid17515466">{{cite journal |author=Kennedy MP, Omran H, Leigh MW, ''et al'' |title=Congenital heart disease and other heterotaxic defects in a large cohort of patients with primary ciliary dyskinesia. |journal=Circulation |volume=115 |issue=22 |pages=2814–21 |year=2007 |pmid=17515466 |doi=10.1161/CIRCULATIONAHA.106.649038 |url=http://circ.ahajournals.org/cgi/pmidlookup?view=long&pmid=17515466}}</ref>. In fact, it has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals.<ref name="pmid12888012">{{cite journal |author=McGrath J, Brueckner M |title=Cilia are at the heart of vertebrate left-right asymmetry. |journal=Curr. Opin. Genet. Dev. |volume=13 |issue=4 |pages=385–92 |year=2003 |pmid=12888012 |doi= |url=http://linkinghub.elsevier.com/retrieve/pii/S0959437X03000911}}</ref>

==References==
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==External links==
* [http://www.hhmi.org/bulletin/sept2005/pdf/Cilia.pdf Brief summary of importance of cilia to many organs in human physiology]
* [http://www.ciliaproteome.org The Ciliary Proteome Web Page at Johns Hopkins] 
{{organelles}}
{{Epithelial tissue}}

[[Category:Cell movement]]
[[Category:Organelles]]

[[ca:Cili]]
[[cs:Řasinka]]
[[da:Fimrehår]]
[[de:Cilie]]
[[et:Ripsmed (organellid)]]
[[es:Cilio]]
[[eo:Cilio]]
[[fr:Cil vibratile]]
[[is:Bifhár]]
[[it:Ciglia (cellule)]]
[[lt:Blakstienėlė]]
[[nl:Trilhaar]]
[[ja:繊毛]]
[[no:Flimmerhår]]
[[nn:Flimmerhår]]
[[pl:Rzęska]]
[[pt:Cílio]]
[[ru:Реснички]]
[[sr:Трепље]]
[[fi:Cilium]]
[[sv:Cilier]]
[[th:ซีเลีย]]
[[tr:Sil]]

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