CILIA: CHARACTERISTICS, STRUCTURE, FUNCTIONS AND EXAMPLES - BIOLOGY - 2025

The cilia are short filamentary projections present on the surfaces of the plasma membrane of many cell types. These structures are capable of vibratory movements that serve for cellular locomotion and for the creation of currents in the extracellular environment.

Many cells are lined by cilia with a length of approximately 10 µm. In general, the cilia move in a fairly coordinated back-to-front motion. In this way, the cell travels through the fluid or the fluid travels on the surface of the cell itself.

Source: Respectively: Picturepest, Anatoly Mikhaltsov, Bernd Laber, Deuterostome, Flupke59

These prolonged structures in the membrane are constituted mainly by microtubules and are responsible for movement in various types of cells in eukaryotic organisms.

Cilia are characteristic of the group of ciliated protozoa. They are usually present in eumetazoa (except in nematodes and arthropods), where they are generally located in epithelial tissues, forming ciliated epithelia.

characteristics

Eukaryotic cilia and flagella are very similar structures, each with a diameter of approximately 0.25 µm. Structurally they are similar to flagella, however in those cells that present them they are much more numerous than flagella, having an appearance of villi on the cell surface.

The cilium first moves downward and then gradually straightens, giving the impression of a rowing motion.

The cilia move in such a way that each one is slightly out of rhythm with its closest neighbor (metachronic rhythm), producing a constant flow of fluid over the cell surface. This coordination is purely physical.

Sometimes an elaborate system of microtubules and fibers joins the basal bodies, but it has not been proven that they play a coordinating role in ciliary movement.

Many cilia do not appear to function as mobile structures and have been called primary cilia. Most animal tissues have primary cilia including cells in the oviducts, neurons, cartilage, ectoderm of developing extremities, liver cells, urinary ducts, among others.

Although the latter are not mobile, it was observed that the ciliary membrane had numerous receptors and ion channels with sensory function.

Ciliated organisms

Cilia are an important taxonomic character for the classification of protozoa. Those organisms whose main mechanism of locomotion is by means of cilia belong to the "ciliates or ciliates" (Phylum Ciliophora = that carry or present cilia).

These organisms get that name because the cell surface is lined with cilia that beat in a controlled rhythmic manner. Within this group the arrangement of the cilia varies widely and even some organisms lack cilia in the adult, being present in the early stages of the life cycle.

Ciliates are usually the largest protozoa with a length ranging from 10 µm to 3 mm, and they are also the most structurally complex with a wide range of specializations. Cilia are generally arranged in longitudinal and transverse rows.

All ciliates appear to have kin systems, even those that lack cilia at some point. Many of these organisms are free-living and others are specialized symbionts.

Structure

Cilia grow from basal bodies that are closely related to the centrioles. The basal bodies have the same structure as the centrioles that are embedded in the centrosomes.

The basal bodies have a clear role in the organization of the microtubules of the axoneme, which represents the fundamental structure of the cilia, as well as the anchoring of the cilia to the cell surface.

The axoneme is made up of a set of microtubules and associated proteins. These microtubules are arranged and modified in such a curious pattern that it was one of the most surprising revelations of electron microscopy.

In general, microtubules are arranged in a characteristic "9 + 2" pattern in which a central pair of microtubules is surrounded by 9 outer microtubule doublets. This 9 + 2 conformation is characteristic of all forms of cilia from protozoa to those found in humans.

Microtubules extend continuously along the length of the axoneme, which is usually about 10 µm long, but can be as long as 200 µm in some cells. Each of these microtubules has polarity, the minus (-) ends being attached to the “basal body or kinetosome”.

Microtubule characteristics

The microtubules of the axoneme are associated with numerous proteins, which project in regular positions. Some of them work as cross links that contain the microtubule bundles together and others generate the force to generate the movement of the same.

The central pair of microtubules (individual) is complete. However, the two microtubules that make up each of the outer pairs are structurally different. One of them called tubule "A" is a complete microtubule made up of 13 protofilaments, the other incomplete (tubule B) is made up of 11 protofilaments attached to tubule A.

These nine pairs of outer microtubules are connected to each other and to the central pair by radial bridges of the protein “nexin”. Two dynein arms are attached to each "A" tubule, with the motor activity of these ciliary axonemic dyneins responsible for beating the cilia and other structures with the same conformation such as flagella.

Movement of the cilia

Cilia are moved by flexion of the axoneme, which is a complex bundle of microtubules. Clusters of cilia move in unidirectional waves. Each cilium moves like a whip, the cilium is fully extended followed by a phase of recovery from its original position.

The movements of the cilia are basically produced by the sliding of the outer microtubule doublets relative to each other, driven by the motor activity of axonemic dynein. The base of the dynein binds to the A microtubules and the head groups bind to the adjacent B tubules.

Due to the nexin in the bridges that join the external microtubules of the axoneme, the sliding of one doublet over another forces them to bend. The latter corresponds to the basis of the movement of the cilia, a process about which little is still known.

Subsequently, the microtubules return to their original position, causing the cilium to regain its state of rest. This process allows the cilium to arch and produce the effect that, together with the other cilia on the surface, give mobility to the cell or the surrounding environment.

Energy for ciliary movement

Like cytoplasmic dynein, ciliary dynein has a motor domain, which hydrolyzes ATP (ATPase activity) to move along a microtubule toward its minus end, and a charge-bearing region of the tail, which in this case is a contiguous microtubule.

Cilia move almost continuously, and therefore require a large supply of energy in the form of ATP. This energy is generated by a large number of mitochondria that normally abound near the basal bodies, which is where the cilia originate.

Features

Movement

The main function of cilia is to move fluid over the cell surface or propel individual cells through a fluid.

Ciliary movement is vital for many species in functions such as food handling, reproduction, excretion and osmoregulation (for example, in flamboyant cells) and the movement of fluids and mucus over the surface of cell layers. epithelial.

The cilia in some protozoa such as Paramecium are responsible for both the mobility of the organism and the sweep of organisms or particles towards the oral cavity for food.

Breathing and feeding

In multicellular animals, they function in respiration and nutrition, carrying respiratory gases and food particles over the water on the cell surface, as for example in mollusks whose feeding is by filtration.

In mammals the airways are lined by hair cells that push mucus containing dust and bacteria into the throat.

The cilia also help sweep the eggs along the oviduct, and a related structure, the flagellum, propels the sperm. These structures are particularly evident in the fallopian tubes where they move the egg into the uterine cavity.

The hair cells that line the respiratory tract, which cleanse it of mucus and dust. In the epithelial cells that line the human respiratory tract, large numbers of cilia (109 / cm2 or more) sweep layers of mucus, along with trapped dust particles and dead cells, into the mouth, where they are swallowed and eliminated.

Structural abnormalities in the cilia

In humans, some hereditary defects of ciliary dynein cause the so-called Karteneger syndrome or immotile cilia syndrome. This syndrome is characterized by male sterility due to sperm immobility.

In addition, people with this syndrome have a high susceptibility to lung infections due to paralysis of the cilia in the respiratory tract, which fail to clean dust and bacteria that lodge in them.

On the other hand, this syndrome causes defects in the determination of the left-right axis of the body during early embryonic development. The latter was discovered recently and is related to the laterality and location of certain organs in the body.

Other conditions of this type can occur due to the consumption of heroin during pregnancy. Newborns may present with prolonged neonatal respiratory distress due to ultrastructural alteration of the axoneme of the cilia in the respiratory epithelia.

References

1. Alberts, B., Bray, D., Hopkin, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. & Walter, P. (2004). Essential cell biology. New York: Garland Science. 2nd Edition.
2. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberth, K., & Walter, P. (2008). Molecular Biology of the Cell. Garland Science, Taylor and Francis Group.
3. Audesirk, T., Audesirk, G., & Byers, BE (2004). Biology: science and nature. Pearson Education.
4. Cooper, GM, Hausman, RE & Wright, N. (2010). The cell. (pp. 397-402). Marban.
5. Hickman, C. P, Roberts, LS, Keen, SL, Larson, A., I´Anson, H. & Eisenhour, DJ (2008). Integrated Principles of zoology. New York: McGraw-Hill. 14 ^th Edition.
6. Jiménez García, L. J & H. Merchand Larios. (2003). Cellular and molecular biology. Mexico. Editorial Pearson Education.
7. Sierra, AM, Tolosa, MV, Vao, CSG, López, AG, Monge, RB, Algar, OG & Cardelús, RB (2001). Association between heroin use during pregnancy and structural abnormalities of respiratory cilia in the neonatal period. Annals of Pediatrics, 55 (4) : 335-338).
8. Stevens, A., & Lowe, JS (1998). Human histology. Harcourt Brace.
9. Welsch, U., & Sobotta, J. (2008). Histology. Panamerican Medical Ed.