Cilia

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As cilia or cilium ( lat. Cilium , eyelash ') refers to a special type of cell process in cells of eukaryotic organisms. The cylindrical, 5-10 µm long and about 250 nm slender protuberance of the cell membrane contains, in addition to the cytoplasm, a tubular skeleton made up of bundles of microtubules , the so-called axoneme . This framework consists of nine circularly arranged microtubule doublets (9 × 2), in the middle of which two central microtubules can lie (9 × 2 + 2) or not (9 × 2 + 0). One differentiates accordingly

  • Secondary cilia with central microtubules , blueprint 9 × 2 + 2 , which are mostly actively movable, motile .
Secondary cilia under the electron microscope : the structure 9 × 2 + 2 becomes visible in cross section.
These are also called cilia , cilia or eyelashes and are grouped together with the similarly structured, somewhat longer flagella of the eukaryotes as undulipodia ("wave-beating feet"). While individual cells move on their own through movements of their flagella, connected epithelial cells use their cilia to move fluids in the bronchi or an egg in the fallopian tube over the epithelial surface in coordinated action.
• The intrinsic mobility of secondary cilia is only made possible by equipping them with additional proteins, which connect the microtubule pairs with one another (nexin band, radial spokes) and move them against each other (dynein arms). If these are missing, cilia with a 9 × 2 + 2 pattern are also not motile, such as the odor receptor- bearing cilia in the olfactory cells .
  • Primary cilia without central microtubules , blueprint 9 × 2 + 0 , which as a rule can only be moved passively.
Primary cilia are also the nodal cilia of a mouse embryo (electron microscope image)
It is possible that all cells in the human body each carry one of these primary cilia, which plays an important role during embryonic development and the differentiation of tissues and can serve as a mechano or chemosensor.
• However, a (primary) structure without centrally located microtubules does not in principle exclude active mobility. The nodal cilia - named after the embryonic primitive knot (Latin nodus , knot), a formation during the gastrula phase of embryonic development in mammals, where this type of cilia was discovered - for example, are 9 × 2 + 0 built and yet motile . Their activity creates an asymmetrical periembryonic flow, which leads to the expression of signal proteins (such as Nodal ) on different sides and through it causes the lateralization of internal organs.

For a long time, the research interest focused almost exclusively on the kinetic cilia, which are conspicuous due to their mobility, while the inconspicuous primary cilia were seen but little paid attention to. However, since it became apparent from around 2000 that the development of the animal organism and the function of many of its organs is crucially dependent on the presence of healthy primary cilia, these have been intensively investigated, or the diseases associated with them as ciliopathies , such as Kartagener's syndrome in humans.

Cilia are found in many eukaryotes . In the animal kingdom they can be found on almost all cell types, but rarely in plants, especially not in flowering plants, for example in cycads . In roundworms and arthropods , only primary cilia are found in some nerve cells . Protozoa such as ciliates only have secondary kinocilia.

Secondary cilia

Structure and occurrence

Each cilia is surrounded by a plasma membrane on the outside . Inside the cytoplasm, as a special formation of the cytoskeleton, lies the so-called axoneme from bundles of fine tubulin hollow fibers, the microtubules . In the case of secondary cilia, these are arranged according to the 9 × 2 + 2 pattern: In the middle of the cilia are two individual central tubules (singlets) in a sheath-like covering (central sheath). These two central single tubules are circularly surrounded by nine double tubules (doublets), each of which consists of an A-tubule and a B-tubule fused to it.

Scheme of a secondary cilia

On each A-tubule there are pairs of arm-like structures ( dynein arms ) that are directed towards the B-tubule of the neighboring doublet. The doublets arranged in a circle are connected to one another in a ring-shaped manner by nexin connectors as well as to the enclosed central tubules by so-called radial spokes.

All twenty microtubules emanate from a basal body , the formation center of the cilia at the base of the cilia. Fibers of the transition zone anchor this kinetosome in the cell membrane and separate the membrane-covered cilia with the axoneme from the rest of the cell body in such a way that one can speak of compartmentalization .

The beat of a secondary cilia as an active movement is brought about by the energy-consuming tension of the microtubules inside the cilia. The movement process can be described as a sliding filament mechanism. Arms made of dynein , always anchored to the A tubule, make contact with their tips to the B tubule of the next neighboring doublet and can use this connection to cause an ATP- dependent shift of the doublets against each other, so that a bend is created. Nexin , a highly stretchable protein, holds neighboring doublets together during this gliding process.

Such cinema cilia are often formed in large numbers on the cell surface of unicellular or multicellular organisms . Scourges and cilia are also grouped under the generic term Undulipodien (singular Undulipodium ), due to the same construction principle.

Cilia of the bronchial epithelium , next to it some microvilli are visible (bottom right in the picture)

Some ciliates have groups of cilia that are connected to each other. Such groupings of appendages are also called cirrus (Latin for locke).

Occasionally, cilia are not clearly distinguished from microvilli . However, these do not have a structure made of microtubules, but are equipped with actin filaments , also of other origin and usually only passively mobile. Microvilli, not cilia, are found in the intestines of mammals , for example , where they serve to increase surface area. The pulp is moved here via the peristalsis of muscle cells. Another example of cell processes erroneously referred to as cilia are the formation of hair cells in the inner ear . The stimulus-absorbing appendages of these sensory cells, formerly known as stereocilia , are also microvilli and are therefore now called stereovilli . In the human hair cell, a cilia only exists in the embryonic system and degenerates during development.

The cilia should not be confused with the flagella of bacteria . These are much slimmer, consist entirely of protein (flagellin) and are not surrounded by a membrane. Their mode of operation is also based on a completely different principle (namely a rotation similar to a ship's propeller).

Movement and function

Kinocilia are rarely found alone, but mostly in large numbers and often in rows or fields on a cell. The coordinated, oar-like beating of the mobile cilia serves the following functions:

The powerful forward stroke is followed by a slower, more curved backward movement of the cilia

Kinocilia or eyelashes are, so to speak, flexible miniature oars that, in contrast to flagella, strike uniplanar (in one plane). The level and direction of lay are fixed for each ceilings. During the powerful suggestion, the cilia is almost stretched. The slower kickback occurs in a curved manner, with a bending wave running from the base of the cilia to the tip of the cilia and the cilia being returned to its original position with little resistance from the surrounding medium. A curve in space can be run through at the same time.

Each cilia in a row of cilia strikes a fraction later than the previous one. This is called a metachronic movement. The collective movement is undulating, comparable to a cornfield swaying in the wind.

The beat frequency of a cilia can be between 5 and 20 Hz , depending on the environmental conditions . There are factors that can speed up the frequency, such as heat or some medication. Other factors, however, inhibit the frequency or even lead to a standstill, such as nicotine or a bacterial infection.

Primary cilia

Representation of the basal apparatus of a cilium with the kinetosome

In contrast to the mostly actively movable, "motile" secondary cilia, primary cilia are mostly only passively movable. As a rule, there is only one such “non-motile” cell per cell, which is formed according to the 9 × 2 + 0 scheme - the central pair is therefore missing.

Almost all cells of vertebrates have a single non-motile cilium, also called the “primary cilium”, which has long been neglected in research. These primary cilia often represent a sensitive extension of the cell. Specialized structures have also developed from these non-motile cilia; For example, the outer segment of photoreceptor cells in the eye is connected to the inner segment via a specialized cilium, the so-called connecting cilium.

In contrast, the olfactory cells as sensory cells of smell also olfactory vertebrate neurons , each with up to twenty nichtmotile cilia with special olfactory receptors ; however, these special nerve cell processes are structured according to the 9 × 2 + 2 scheme of secondary cilia.

Individual evidence

  1. ↑ a b Renate Lüllmann-Rauch: Pocket textbook histology. 2nd Edition. Thieme Verlag, 2006, ISBN 978-3-13-129242-1 , pp. 550f .
  2. ↑ Stefan Silbernagl: Pocket Atlas Physiology. 8th edition. Thieme Verlag, 2012, ISBN 978-3-13-152538-3 pp . 36f .

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