Editing Cilium (section)

== Structure==
[[File:Eukaryotic cilium diagram en.svg|thumb|300px|Eukaryotic motile cilium]]
A cilium is assembled and built from a [[basal body]] on the cell surface. From the basal body, the ciliary rootlet forms ahead of the transition plate and transition zone where the earlier microtubule triplets change to the microtubule doublets of the axoneme.

===Basal body===
The foundation of the cilium is the basal body, a term applied to  the mother centriole when it is associated with a cilium. Mammalian basal bodies consist of a barrel of nine triplet microtubules, subdistal appendages and nine strut-like structures, known as distal appendages, which attach the basal body to the membrane at the base of the cilium. Two of each of the basal body's triplet microtubules extend during growth of the axoneme to become the doublet microtubules.

===Ciliary rootlet===
The ciliary rootlet is a cytoskeleton-like structure that originates from the basal body at the proximal end of a cilium. Rootlets are typically 80-100&nbsp;nm in diameter and contain cross striae distributed at regular intervals of approximately 55-70&nbsp;nm. A prominent component of the rootlet is [[rootletin]] a coiled coil rootlet protein coded for by the [[Rootletin|''CROCC'' gene]].<ref name="uniprot">{{cite web |title=Rootelin |url=https://www.uniprot.org/uniprot/Q5TZA2 |access-date=28 March 2022}}</ref>

===Transition zone===
To achieve its distinct composition, the proximal-most region of the cilium consists of a '''transition zone''', also known as the '''ciliary gate''', that controls the entry and exit of proteins to and from the cilium.<ref>{{Cite journal|last1=Garcia|first1=Galo|last2=Raleigh|first2=David R.|last3=Reiter|first3=Jeremy F.|date=23 April 2018|title=How the Ciliary Membrane Is Organized Inside-Out to Communicate Outside-In|journal=Current Biology|volume=28|issue=8|pages=R421–R434|doi=10.1016/j.cub.2018.03.010|issn=1879-0445|pmc=6434934|pmid=29689227|bibcode=2018CBio...28.R421G }}</ref><ref>{{Cite journal|last1=Garcia-Gonzalo|first1=Francesc R.|last2=Reiter|first2=Jeremy F.|date=2017-02-01|title=Open Sesame: How Transition Fibers and the Transition Zone Control Ciliary Composition|journal=Cold Spring Harbor Perspectives in Biology|volume=9|issue=2|pages=a028134|doi=10.1101/cshperspect.a028134|issn=1943-0264|pmc=5287074|pmid=27770015}}</ref><ref>{{Cite journal|last1=Gonçalves|first1=João|last2=Pelletier|first2=Laurence|date=April 2017|title=The Ciliary Transition Zone: Finding the Pieces and Assembling the Gate|journal=Molecules and Cells|volume=40|issue=4|pages=243–253|doi=10.14348/molcells.2017.0054|issn=0219-1032|pmc=5424270|pmid=28401750}}</ref> At the transition zone, Y-shaped structures connect the ciliary membrane to the underlying axoneme. Control of selective entry into cilia may involve a sieve-like function of transition zone. Inherited defects in components of the transition zone cause ciliopathies, such as Joubert syndrome. Transition zone structure and function is conserved across diverse organisms, including vertebrates, ''[[Caenorhabditis elegans]]'', ''[[Drosophila melanogaster]]'' and ''[[Chlamydomonas reinhardtii]]''. In mammals, disruption of the transition zone reduces the ciliary abundance of membrane-associated ciliary proteins, such as those involved in [[Hedgehog signaling pathway|Hedgehog signal transduction]], compromising Hedgehog-dependent embryonic development of digit number and central nervous system patterning.

===Axoneme===
Inside a cilium is a [[microtubule]]-based [[cytoskeleton|cytoskeletal core]] called the [[axoneme]].  The axoneme of a primary cilium typically has a ring of nine outer microtubule doublets (called a [[9+0 axoneme]]),  and the axoneme of a motile cilium has, in addition to the nine outer doublets,  two central microtubule singlets (called a [[9+2 axoneme]]). This is the same axoneme type of the [[flagellum]]. The axoneme in a motile cilium acts as a scaffold for the inner and outer [[dynein arm]]s that move the cilium, and provides tracks for the [[microtubule motor]] [[motor protein|proteins]] of kinesin and dynein.<ref name="HHMIB2005"/><ref>{{cite journal | vauthors = Rosenbaum JL, Witman GB | title = Intraflagellar transport | journal = Nature Reviews. Molecular Cell Biology | volume = 3 | issue = 11 | pages = 813–25 | date = November 2002 | pmid = 12415299 | doi = 10.1038/nrm952 | s2cid = 12130216 }}</ref><ref>{{cite journal | vauthors = Scholey JM | title = Intraflagellar transport motors in cilia: moving along the cell's antenna | journal = The Journal of Cell Biology | volume = 180 | issue = 1 | pages = 23–29 | date = January 2008 | pmid = 18180368 | pmc = 2213603 | doi = 10.1083/jcb.200709133 }}</ref> The transport of ciliary components is carried out by [[intraflagellar transport]] (IFT) which is similar to the [[axonal transport]] in a [[axon|nerve fibre]]. Transport is bidirectional and [[Motor protein#Cytoskeletal motor proteins|cytoskeletal motor proteins]] kinesin and dynein transport ciliary components along the microtubule tracks; kinesin in an anterograde movement towards the ciliary tip and dynein in a retrograde movement towards the cell body. The cilium has its own ciliary membrane enclosed within the surrounding [[cell membrane]].<ref>{{cite journal | vauthors = Rohatgi R, Snell WJ | title = The ciliary membrane | journal = Current Opinion in Cell Biology | volume = 22 | issue = 4 | pages = 541–46 | date = August 2010 | pmid = 20399632 | pmc = 2910237 | doi = 10.1016/j.ceb.2010.03.010 }}</ref>