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cilium

  (sĭl'ē-əm) pronunciation
n., pl. -i·a (-ē-ə).
  1. A microscopic hairlike process extending from the surface of a cell or unicellular organism. Capable of rhythmical motion, it acts in unison with other such structures to bring about the movement of the cell or of the surrounding medium.
  2. An eyelash.
  3. Botany. One of the hairs along the margin or edge of a structure, such as a leaf, usually forming a fringe.

[Latin, eyelid.]


 
 

Short, eyelashlike filament that is numerous on tissue cells of most animals. Capable of beating in unison, cilia perform a variety of functions, including providing the means of locomotion for some protozoans, moving mammalian ova (eggs) through oviducts, generating water currents to carry food and oxygen past the gills of clams, and cleaning debris from mammalian respiratory systems. Like a flagellum, a cilium has a central core consisting of two central microtubules surrounded by an outer ring of nine double fibres. Ciliary outgrowth is controlled by the basal body, located just inside the cell surface at the base of the cilium. Beneath the surface of some cells is a network of microtubular bundles that may coordinate ciliary beating.

For more information on cilium, visit Britannica.com.

 

Pl. cilia [L.] see cilia.

 
(plural: cilia)

One of the hairs along the margin or edge of a structure, such as a leaf, usually forming a fringe.

 
Wikipedia: cilium
SEM micrograph of the cilia projecting from respiratory epithelium in the lungs
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SEM micrograph of the cilia projecting from respiratory epithelium in the lungs

A cilium (plural cilia) is an organelle found in eukaryotic cells. Cilia are thin, 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. Protozoans (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 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". Though the primary cilium has historically been ignored by scientists, recent findings regarding its physiological roles in chemical sensation, signal transduction, and control of cell growth, have led scientists to re-evaluate its importance.

Assembly and maintenance

Cross-section of two motile cilia, showing the "9+2" structure
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Cross-section of two motile cilia, showing the "9+2" structure

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 intraciliary/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.[1]

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.

References

  1. ^ http://www.pandasthumb.org/archives/2007/06/of_cilia_and_si.html

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Copyrights:

Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved.  Read more
Britannica Concise Encyclopedia. Britannica Concise Encyclopedia. © 2006 Encyclopædia Britannica, Inc. All rights reserved.  Read more
Veterinary Dictionary. Saunders Comprehensive Veterinary Dictionary 3rd Edition. Copyright © 2007 by D.C. Blood, V.P. Studdert and C.C. Gay, Elsevier. All rights reserved.  Read more
Gardener's Dictionary. Taylor's Dictionary for Gardeners, by Frances Tenenbaum. Copyright © 1997 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.  Read more
Wikipedia. This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Cilium" Read more

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