Depending on their function, cilia can be broadly subdivided into motile cilia and primary cilia (non-motile cilia).

Motile cilia and flagella are characterized by a typical or ?9+2? architecture, with nine outer microtubule doublets and a central pair of microtubules. Motile cilia are present, for example, in the mammalian trachaea (the hair-like structures in panel a) and flagella in sperm, where they function in sperm motility.

Primary cilia have a sensory role, typically appearing as single organelles on the apical surface of cells, and lack the central pair of microtubules (?9+0? cilia). For example, primary cilia are present in the epithelium of kidney tubules and in IMCD3 cells (inner medullary collecting duct cells).

In some organs, motile cilia and primary cilia are thought to cooperate to propagate morphogenetic signals. For example, in the embryonic node, a specialized group of motile cilia is required for the production of a leftward flow, which generates a morphogenetic gradient that is detected by flanking sensory cilia.

Structure

Cilia and flagella comprise a microtubular backbone (the ciliary axoneme), which is surrounded by plasma membrane and is nucleated and organized by the basal body, which is a cylindrical structure that consists of nine microtubule triplets and is located underneath the cell membrane.

The architecture of the microtubule backbone distinguishes motile cilia, in which nine doublets of microtubules surround a central pair (9+2), from primary cilia, in which the central pair of microtubules is absent (9+0).

Types

 primary cilia (non-motile cilia)
 motile cilia and flagella

Function

Motile cilia and flagella appeared very early in evolution to provide unicellular organisms with motility in water. Adaptation to non-aquatic life in plants resulted in the almost complete elimination of these organelles, except for gametic transport in some phylogenetic groups.

In contrast, cilia and flagella were retained and employed for a wide variety of functions requiring fluid movement in complex multicellular animals.

The function of the primary cilium as a highly conserved sensory organelle in evolution. The functions of cilia in diverse processes such as left-right axis pattern formation, cerebrospinal fluid flow, - sensory reception, mucociliary clearance and renal physiology indicate that cilia have been adapted as versatile tools for many biological processes.

Pathology

 motile cilia diseases
 primary ciliary dyskinesia

• Kartagener syndrome
• situs inversus totalis (MIM.270100)

 primary cilia diseases (non-motile cilia diseases)

• cystic kidney diseases (CKDs)
  • Bardet-Bield symdrome
  • polycystic kidney diseases (PKDs)
    • ADPKD and ARPKD

See also

 stereocilia
 ciliary proteins

References

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