Cilia: Structure, Function & Diseases

Secondary cilia are freely moving cellular processes found in the ciliated epithelium of the lung. Their movements enable the transport of mucus and fluids. In diseases such as asthma or cystic fibrosis, this transport is impaired by ciliary paralysis.

What are cilia?

Cilia are the technical term for freely movable cellular extensions. These five- to ten-µm-long plasma membrane projections are about 0.25 µm slender and contain cytoplasm. Their skeleton is equipped with an axoneme containing microtubules. All cilia are firmly anchored by fine fibers in the basal body of the lacy cytoplasm. For example, the cilia or kinocilia are cilia. However, cilia are also found in the fallopian tube, testis, or respiratory tract. In addition to primary cilia, there are secondary cilia. They differ in the number of microtubules they contain and in their ability to move. Together with the flagella, cilia are also included under the collective term undulipodium because of their similar construction principle. In ciliates, entire groups of cilia are sometimes called cirrus. Cilia are to be distinguished from microvilli. They occur, for example, in the intestine and do not carry a microtubule scaffold. Also the flagella of bacteria cannot be compared with cilia. They work like a propeller, are significantly smaller than cilia, and are not enclosed in membrane.

Anatomy and structure

Cilia are enclosed externally by plasma membrane. The axoneme sets them apart from the cell body. The axoneme is axial filaments made of the contractile proteins dynein and kinesin. The proteins enable the cilia to move. The microtubules are fine hollow fibers on the axoneme. They are composed of molecular bonds with electrical charge and thus have one positive and one negative tubule. Thus, each microtubule doublet is divided into an A and a B tubule. Each A tubule is equipped with arm-like structures. These structures always align with the B tubule of the neighboring cilium. The microtubules of a cilium are each duplicated. These microtubule doublets of the tubular ciliary skeleton are in a circular arrangement to each other. In the center of this circle, some cilia have two additional central microtubules. These cilia are also called secondary cilia. Cilia without central microtubules are called primary cilia. Inside them lies cytoplasm, which forms the cytoskeleton of the cilia and thus generates the axoneme. The individual microtubule doublets are connected to each other by nexin binding members. In secondary cilia, the decentralized doublets are also cross-linked to the central doublet via radial spokes.

Function and tasks

Secondary cilia are usually capable of active beating or rowing movement. They can stretch and bend by tensing their microtubules. Thus, a sliding mechanism occurs. The bending of the cilium occurs as the arm of the A tubule makes contacts with the B tubule of the neighboring cilium and displaces the tubules of the tubulin doublet against each other. The highly flexible protein nexin holds the cilia’s neighboring tubules together during this displacement. While the cilium is proposing, it is elongated. While the recoils, it is flexed. Secondary cilia are usually arranged in large masses and move in a coordinated manner one behind the other according to the principle just described. This means that the opposite rows of a cilia row each strike a fraction later. This principle of motion is also called metachronous motion. This results in an evenly beating flicker current on the surface of the cilia group, which runs in a wave-like manner. In a warm-blooded animal, the beating frequency of the cilia corresponds to about 20 per second. In humans, the coordinated movements of the secondary cilia generally serve to transport fluid and mucus films in the organism. For example, they transport the ovum in the fallopian tube or mucus in the bronchi. In ciliates, the movement serves the locomotion of the single cell. Also in connection with the sperm of higher animal species, cilia movement is responsible for cell locomotion. Sometimes the movement of secondary cilia also serves to swirl food. Primary cilia are usually not capable of active movement. Primary cilia, unlike secondary cilia, usually do not move, but perform the function of a sensory antenna. Thus, they are found primarily in the visual system and the olfactory system.

Diseases

Various circumstances can paralyze the ciliary movement of the secondary cilia. Especially in relation to the ciliated epithelium of the lung, such paralysis can occur. For example, when the pH falls below 6.4 or exceeds nine, paralysis occurs. Allergic mechanisms can also cause cilia movement to suspend. This happens, for example, in asthma, which causes the cilia in the lungs to momentarily stop beating. In the metabolic disorder cystic fibrosis, such paralysis of the lung cilia also occurs. Physical or mechanical damage to the cilia may also be responsible for paralysis or impaired movement. High temperatures or cold can trigger a physical disorder. Air turbulence, on the other hand, is one of the most common causes of mechanical damage. Ciliary dysfunction is defined by medicine as a general dysfunction of the cilia. Primary ciliary dysfunction can occur, for example, in the context of diseases such as Kartagener’s syndrome. Secondary ciliary dysfunction of the lungs, on the other hand, could occur if the affected person has inhaled pollutants. Chronic paralysis of ciliary movement may cause the ciliated epithelium to convert to squamous epithelium. This means that mucus can no longer be transported out of the lungs. This phenomenon is common in heavy smokers, but the diseases just mentioned may also be related.