Cilium

As cilia or cilium (from Latin cilium, eyelid n ) is called slim - tubular, 5-10 microns long and about 250 nm thick cell processes. They contain a skeleton of microtubules, called the axoneme, and form a separate cell compartment. The axoneme is a tube made ​​up of nine double microtubules which may contain a central microtubules. There is a difference

The only currently (2012 ) known exception is the

To avoid the generally accepted, but no longer true by this discovery in the desired direction designation, motile cilia ', to be used in this article, the term Kinozilie.

The interest of researchers has long been almost exclusively the conspicuous by their mobility cilia, while indeed saw the inconspicuous primary cilia in many cases, but considered unimportant. Only since then showed from about 1990 that the development of the animal organism and the function of many of his organs critically depends on the presence of healthy primary cilia, they are intensively investigated.

Cilia occur in many eukaryotes. In the animal kingdom they are found on almost all cell types, but rarely in plants, especially in flowering plants, but for example in cycads. In nematodes and arthropods only primary cilia are found in some nerve cells. Protozoa such as ciliates have cilia only.

Structure and movement

Each cilium is surrounded externally by a plasma membrane. Inside there is a bundle of fibers, the so-called axoneme which is formed of microtubules. These are 2 2- pattern arranged according to the so -called 9 ×: In the middle of the cilium are two separate Zentraltubuli which nine Doppeltubuli ( doublets ) are surrounded, each consisting of an A - and B- tubule. The Zentraltubuli are of a type vagina (central sheath) surrounding it.

Are pairs of arm-like structures ( dynein arms ) that suggest that the B- tubule of the adjacent doublet at each A- tubule. The arranged in a circle doublets are connected by a ring Nexinbindeglieder each other and by the so-called Radiärspeichen directly with the two Zentraltubuli. The microtubules originate from a basal body ( kinetosome ), which is the educational center of the cilium.

For the curvature itself in need of energy, ATP-dependent shifts of the microtubules inside the cilia are responsible. The motion mechanism is called a sliding mechanism ( " sliding filament mechanism" ) described. In this case, the Dyneinarm which is always anchored to the A tube, with its pointed contact with the tube B of the adjacent doublets forth, and causes the ATP-dependent displacements of the outer Tubulindubletts past each other, so that a bend is generated. Nexin, a highly extensible protein that keeps neighboring doublets together during sliding.

Cilia are often formed in large numbers on the cell surface of single-celled or multicellular organisms. Flagella and cilia are scientifically under the generic term Undulipodien (singular Undulipodium ) summarized, based on the same design principle.

Some ciliates have groups of cilia that have connection with each other. These extensions are called Cirrus.

Cilia often be confused with the microvilli. However, these are not formed by microtubules, but of actin filaments, are of different origin and usually not movable. Microvilli, not cilia, are, for example, in the intestines of mammals, where they serve to increase the surface area. Moves, the digested food through peristalsis. Another example of the wrong cilia are the hair cells in the inner ear. The designated earlier than stereocilia stimulus- receiving extensions on the hair cells are microvilli and today are therefore also referred to as Stereovilli. A cilium exists in human hair cell only in the plant and degenerate during development.

It should not be confused with the cilia flagella of bacteria. These are substantially smaller, composed entirely of protein ( flagellin ), not surrounded by a membrane. Even the way they work is an entirely different principle (rotation similar to a ship's screw) is based.

Function and occurrence

Kinocilia

Cilia are rarely found alone, but usually in rows or fields in greater numbers on a cell. The coordinated, oar -like beating of the cilia moving serves the following functions:

• The progression of the cell as the ciliates, numerous larval stages of smaller, wasserbewohnender animals or the sperm of higher animals
• The inducing swirls of food particles or
• The transport of particles and fluids within an organism, such as the ciliated epithelium in the airways for the transport of mucus and pollutants from the lungs out or the transport of the ovum in the fallopian tube.

Eyelashes are so to speak, flexible miniature oars, striking in contrast to flagella uniplanar ( in a plane ). Level and direction of impact are determined for each cilium. During the forceful proposal, the cilium is stretched. The slower setback occurs curved with a bend shaft from the Zilienbasis runs until the Zilienspitze, whereby the eyelash is returned under low water resistance back to its original position. This may involve coincidental curve will pass through the room.

Each batting a Zilienreihe beats from the previous by a fraction later. This is called metachronous movement. The collective motion is wavy, like a billowing in the wind cornfield.

The beat frequency of cilia can be depending on the environmental conditions 5-20 Hz. There are factors which may speed up the frequency, such as some drugs or heat. However, other factors inhibit the frequency and even lead to the stoppage such as nicotine or a bacterial infection.

Non - moving cilia

In contrast to mobile, " motile " cilia exist and immovable, "non- motile " cilia. So the central doublet is missing - Usually only one such cilia per cell, which are formed after the 9 × 2 0 scheme exists.

Almost all cells of vertebrates possess a single cilium nichtmotile, the so-called "primary cilium ", which has long been neglected in research. These primary cilia often form sensory antennas for the cells. For these nichtmotilen cilia to specialized structures have formed; For example, the outer segment of photoreceptor cells in the eye through a specialized cilium, called Verbindungscilium, connected to the cell body. The end of the olfactory nerve cells with the olfactory receptors is a nichtmotiles cilium.