AskDefine | Define cilium

Dictionary Definition



1 a hairlike projection from the surface of a cell; provides locomotion in free-swimming unicellular organisms
2 any of the short curved hairs that grow from the edges of the eyelids [syn: eyelash, lash] [also: cilia (pl)]

User Contributed Dictionary



  1. A hairlike organelle projecting from a eukaryotic cell (such as unicellular organism or one cell of a multicelled organism). These structures serve either for locomotion by moving or as sensors.

Related terms


hairlike organelle
  • Czech: řasinka, brva
  • Finnish: värekarva, ripsi
  • Swedish: cilie

Extensive Definition

A cilium (plural cilia) is an organelle found in eukaryotic cells. Cilia are tail-like projections extending approximately 5–10 micrometers 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 cycads. Ciliates 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 humans, for example, motile cilia are found in the lining of the 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 comparison 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 1990s. 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 and congenital heart disease.

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.

Cilium-related disease

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.
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. In fact, it has been shown that proper cilial function is responsible for the normal left-right asymmetry in mammals.


External links

cilium in Catalan: Cili
cilium in Czech: Řasinka
cilium in Danish: Fimrehår
cilium in German: Cilie
cilium in Estonian: Ripsmed (organellid)
cilium in Spanish: Cilio
cilium in Esperanto: Cilio
cilium in French: Cil vibratile
cilium in Icelandic: Bifhár
cilium in Italian: Ciglia (cellule)
cilium in Lithuanian: Blakstienėlė
cilium in Dutch: Trilhaar
cilium in Japanese: 繊毛
cilium in Norwegian: Flimmerhår
cilium in Norwegian Nynorsk: Flimmerhår
cilium in Polish: Rzęska
cilium in Portuguese: Cílio
cilium in Russian: Реснички
cilium in Serbian: Трепље
cilium in Finnish: Cilium
cilium in Swedish: Cilier
cilium in Thai: ซีเลีย
cilium in Turkish: Sil
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