Cilia

Edited By Irshad Anwar | Updated on Jul 02, 2025 05:52 PM IST

Cilia are tiny hair-like projections on the outer surface of a eukaryotic cell. They help in the motility of cells, fluids and particles inside the cell and other organs. Cilia is a topic of the chapter Cell: The Unit of Life in Biology.

What is Cilia?

Cilia are short, bristle-like structures that arise from the surface of eukaryotic cells and have numerous functions, including motility and sensation. These organelles are involved in many biological activities, such as the migration of cells, the movement of fluids and particles over the cell’s surface, and signal transduction.

This Story also Contains

What is Cilia?
Types of Cilia
Cilia Structure and Composition
Cilia Function and Mechanism
Comparison Between Cilia and Flagella
Cilia in Different Organisms and Systems
Cilia Disorders
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Cilia

The organisation arrangement of cilia is comparable to eukaryotic flagella, in an arrangement 9+2 containing microtubules where the axoneme is surrounded by the cell membrane. Cilia can be locomotor, allowing movement of fundamental fluids in respiratory tracts or oviducts, or non-locomotor, also known as primary cilia, which are involved in sensory functions to determine the state of the environment.

Types of Cilia

Based on the locomotory ability,

Motile Cilia

Structure:

As for transportation cilia called the motile cilia, they are made of the axoneme containing a specific pattern called “9+2” microtubules where nine doublets are embraced centrally by a pair. They are connected with the cell by the basal body which is somewhat similar to the centriole in structure.

Movement:

These cilia move in a coordinated manner, or rather their beating move in a wave-like motion to result in movement. This action is necessary for functions such as mucus and debris movement in the respiratory tracts and pushing eggs along the oviducts.

Non-motile (Primary) Cilia

Structure:

Primary cilia possess a “9+0” microtubular arrangement in the axoneme and do not have the central pair that is seen in motile cilia. They are also inserted on a basal body but, they are often shorter in size and number as compared to motile cilia.

Function as sensory organelles:

While motile cilia have a function to move, primary cilia do not have the function to move. However, they are meant to act as mechanosensitive and chemosensitive organelles, collecting information on mechanical and chemical contexts the cell is located in and being involved in signaling pathways relating to cell division and specialization.

Motile and non-motile cilia

Cilia Structure and Composition

The cilia structure and composition are given below:

Axoneme

"9+2" microtubule arrangement in motile cilia:

Motile cilia have a different axoneme structure known as “9+2”; this is a ring comprised of nine sets of microtubules and two central microtubules. It has to be that way for the rhythmic and synchronised beating of motile cilia in a waving kind of method.

"9+0" microtubule arrangement in primary cilia:

Primary or nonmotile cilia also have a ‘9+0’ structure implying that while there are nine microtubules that form nine detachments, there are no microtubules in the centre. It is in this respect that the structure supports their requirement as sensory organelles and not for motion.

Basal Body

A Cilium is built from microtubules that are nucleated at the basal body, which the structure resembles a centriole but its function is to anchor the cilium to the cell membrane.

Astral: It is involved in arranging microtubules into the axoneme and supports the emergence of the cilium from the cell membrane.

Microtubules

Microtubules that consist of a subset of tubulin proteins, which includes alpha-tubulin and beta-tubulin, are present in motile and primary cilia. These proteins go on to form a coiled structure that is critical in creating the cylindrical shape of cilia, which provides support and stability to the organelle.

Dynein Arms

Dynein arms are membrane-bound proteins anchored to ciliary microtubule doublets in organisms that bear cilia. They utilize ATP to create power, hence leading to the moving of the microtubules past each other. Due to this sliding movement, the cilium bends and as its sections beat in a coordinated motion, they perform the tasks of a wave.

Basal body, Hook, Axoneme, and Dyenin

Cilia Function and Mechanism

The cilia function and mechanism is listed below-

Locomotion

In single-celled organisms such as Paramecium, for instance, the motile cilia form all over the cell surface and undulate in a coordinated fashion to enable swimming in water. Thus, the kind of movement that is possible with the help of this organelle enables Paramecium to move throughout its milieu, escape the jaws of its predators, and find its food.

Fluid Movement

In complex organisms, the motile cilia are vital in moving a given fluid over a given epithelial layer. For instance, in the human respiratory tract model, cilia have a rhythmic beating motion that is involved in the movement of mucus that contains dust and pathogens out of the lungs and towards the throat so that they can be either coughed out or swallowed.

Sensory Functions

Primary cilia are involved in mechanosensation, that is, they control the reception of signals from the environment and their transduction to the cell. They are involved in several signalling pathways including the Gli protein signalling, or the Hedgehog protein signalling, which is involved in cell differentiation and organ development, as well as in maintaining biological balance. Disorder in the primary cilia involved in various diseases underscores their role in cellular signalling and structure.

Comparison Between Cilia and Flagella

Following shows the comparison between cilia and flagella

Feature	Cilia	Flagella
Length	Shorter (5-10 µm)	Longer (10-200 µm)
Number per Cell	Numerous, often hundreds per cell	Few, typically 1-8 per cell
Movement	Coordinated, wave-like beating	Rotational (prokaryotes) or wave-like (eukaryotes)
Microtubule Arrangement	"9+2" in motile cilia, "9+0" in primary cilia	"9+2" in eukaryotic flagella, different in prokaryotes
Function	Locomotion, fluid movement, sensory roles	Locomotion, sometimes sensory functions
Examples	Respiratory tract cilia, cilia in Paramecium	Sperm flagella, bacterial flagella
Anchoring Structure	Basal body	Basal body (eukaryotes), motor complex (prokaryotes)
Energy Source	ATP (via dynein arms)	ATP (eukaryotes), proton motive force (prokaryotes)
Presence in Cells	Found in eukaryotic cells	Found in both prokaryotic and eukaryotic cells
Beating Pattern	Synchronised, often metachronal rhythm	Unsynchronised, propeller-like (prokaryotes), or wave-like (eukaryotes)

Cilia in Different Organisms and Systems

The Cilia in Different Organisms and Systems are listed below-

Protists

In the genera of protists including Paramecium and Tetrahymena the cilia are generally distributed all over the cell body and are involved in movement as well as the feeding process. These cilia move in cyclic motions and enable the organisms to swim in water and guide food particles to their oral groove for consumption.

Human Body

In the human respiratory tract, the epithelial cells are covered by motile cilia which act as a piece of vital transport machinery to move mucus and any trapped particles from the lungs. Coordinated beating of these cilia carries the mucus in an upward fashion towards the throat effectively trapping and removing pathogens and debris in the airways.

In the female reproductive system, cilia imbedded in the lining of the fallopian tubes assist in moving the egg from the ovary to the uterus. In the male reproductive system, cilia help to move sperms in their way in the reproductive system to have fertilization.

The primary cilia are immotile and have an important part in cells’ sensory functions in several organs and tissues. In kidneys, it can be seen that primary cilia sense the flow of fluid and then they are involved in responding to cells to maintain the functioning of the kidney. Concerning the eyes, the primary cilia are involved in photoreceptor cells which are so vital in the processes of sighting light and vision.

Cilia Disorders

The ciliary disorders are listed below-

Ciliopathies

Ciliopathies are associated with malfunctioning cilia and there exist various diseases associated with the improper functioning of cilia.

Primary ciliary dyskinesia is a disorder whereby movements of cilia are reduced; and leads to recurrent chest infections, and reduced fertility, hearing difficulty. PKD is associated with defects in primary cilia within the kidneys’ cells; this results in the formation of cysts that are filled with fluid, and renal failure is the end outcome.

In chronic bronchitis as well as in cystic fibrosis a person experiences loss of cilia which leads to thick build-up of mucus and repeated infections. If the cilia that line the bronchial tubes get damaged, they cannot help move the mucus out thus leading to obstruction of the airway and inflammation.

In cystic fibrosis, mucus is tenacious, thick, and effective to ciliary movement that results in severe disorders affecting the respiratory and digestive system due to mucus deposition and high incidence of infections.

Also Read-

Centrosome	Endoplasmic reticulum
Semi autonomous Organelles	Golgi Apparatus
Endomembrane System	Lysosomes

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Frequently Asked Questions (FAQs)

1. What are cilia and their functions?

Cilia are also straw-like structures that are present at the outer side of the cell. They perform many tasks like the movement of an organism from one place to another, the movement of fluids, and some type of sensation. These are frayed cilia that beat in a coordinated manner to either transport cells or to move fluid over a tissue surface for instance in the respiratory tract. In contrast, the primary cilia are stereocilia that function as sensory organelles forming a part of neural information transducers.

2. How do motile cilia differ from primary cilia?

Special about motile cilia is that they are capable of moving in a coordinated, bending to produce a flowing motion or moving a cell through its milieu. They usually have the “9 plus 2” microtubule organisation and are affiliated with a basal body. Contrary to this, primary cilia are not capable of movement, although they are considered sensitive structures. It is centriole is a ‘9+0’ microtubule and is included in signal transduction and cellular communication.

3. What proteins make up cilia?

Cilia are, mostly, built from tubulin proteins that are a part of the microtubules of the axoneme, which is the main structural component of cilia. Other proteins are also required for ciliary functioning;; these include dynein arms, kinesin, and various motor proteins. Also, cilia have proteins involved in signalling and sensation, including receptors and various signalling molecules.

4. How do cilia contribute to cell movement and fluid flow?

Cilia are involved in cell transport and moving fluids by coordination of beating or waving of cilia. The motility of cilia is coordinated where the coordinated whip-like movement produces propulsion and hence the cell can swim or move fluid over the cell surface. This movement is useful in cases like the movement of mucus coat in the respiratory tract and the transport of eggs or sperm in the reproduction tract. Furthermore, primary cilia have the capability of registering the flow of fluid as well as mechanical forces with impacts on cell behaviour and tissue organization.

5. What are some examples of ciliopathies?

Ciliopathies are a category of heterogeneous diseases originating from mutations in cilia proteins. Some examples include; primary ciliary dyskinesia, which is a disorder that affects the movement of cilia, particularly in the respiratory tract, thus resulting in chronic bacterial infections and reproductive complications. Another example is polycystic kidney disease where aberrations in the function of the primary cilia are known to play a role in the formation of cysts containing water in the kidneys which stage ends up resulting in kidney failure. Other ciliopathies include Bardet-Biedl syndrome, Meckel-Gruber syndrome, and Joubert syndrome; all these disorders have clinical features due to the dysfunction of cilia.

6. What is ciliary dyskinesia and what are its effects?

Ciliary dyskinesia is a genetic disorder where cilia don't function properly. This can lead to various health issues, including chronic respiratory infections, infertility, and in some cases, organs developing on the wrong side of the body (situs inversus). It highlights the importance of properly functioning cilia in multiple body systems.

7. What is ciliopathy?

Ciliopathy is a term for genetic disorders caused by defects in the structure or function of cilia. These can affect multiple organ systems and lead to a wide range of symptoms, including retinal degeneration, kidney disease, obesity, and developmental abnormalities, underscoring the widespread importance of cilia in the body.

8. How do environmental factors affect ciliary function?

Environmental factors like temperature, pH, and the presence of toxins can significantly impact ciliary function. For example, cigarette smoke can paralyze cilia in the respiratory tract, reducing their ability to clear mucus and pathogens. Understanding these effects is crucial for maintaining respiratory health.

9. How do cilia contribute to vision?

In the retina, photoreceptor cells have a modified cilium called the connecting cilium. This structure is crucial for transporting proteins between the inner and outer segments of the photoreceptor. Defects in this ciliary transport can lead to retinal degeneration and blindness.

10. How do cilia contribute to the spread of infections?

While cilia generally help protect against infections by clearing pathogens, some microorganisms have evolved to exploit ciliary function. For example, certain respiratory viruses can impair ciliary beating, reducing mucus clearance and facilitating their spread. Understanding this interplay is crucial for developing strategies to combat respiratory infections.

11. How do cilia move?

Cilia move through the sliding of microtubules within the axoneme. This is powered by motor proteins called dyneins, which use ATP energy to create a bending motion. The coordinated action of many dynein arms along the length of the cilium produces a whip-like movement.

12. What is the role of calcium in ciliary movement?

Calcium ions play a crucial role in regulating ciliary beat frequency and pattern. Changes in intracellular calcium concentration can alter the activity of dynein motor proteins, thereby modulating ciliary movement. This calcium-dependent regulation allows cells to respond to various stimuli by adjusting ciliary activity.

13. What is the relationship between cilia and centrosomes?

The basal body at the base of a cilium is structurally similar to a centriole, which is part of the centrosome. In fact, in many cells, the centriole migrates to the cell surface and becomes the basal body when a cilium forms. This highlights the dual role of these structures in cell division and ciliary function.

14. What is intraflagellar transport and why is it important for cilia?

Intraflagellar transport (IFT) is a bidirectional movement of proteins along the length of cilia and flagella. It's crucial for the assembly and maintenance of these structures, as it carries building materials to the tip and removes waste products. Disruptions in IFT can lead to ciliary dysfunction and associated diseases.

15. How do cilia contribute to the function of the female reproductive system?

In the fallopian tubes, cilia play a vital role in moving the egg from the ovary towards the uterus. Their coordinated beating creates a current that helps transport the egg, which is crucial for successful fertilization and implantation.

16. What is the structure of a cilium?

A cilium consists of a core structure called the axoneme, which is made up of nine pairs of microtubules arranged in a circle around two central microtubules (9+2 arrangement). This is surrounded by an extension of the cell membrane. The base of the cilium contains a basal body, which anchors it to the cell.

17. What is the glycocalyx and how does it relate to cilia?

The glycocalyx is a carbohydrate-rich layer on the cell surface, including on cilia. On cilia, it plays roles in protection, sensing, and potentially in regulating the diffusion of signaling molecules. Understanding the ciliary glycocalyx is important for comprehending how cilia interact with their environment.

18. What is ciliary resorption and why does it occur?

Ciliary resorption is the process by which cells disassemble their cilia. This typically occurs before cell division, as the basal body is needed to form the mitotic spindle. It's a regulated process involving protein modifications and intraflagellar transport. Studying ciliary resorption provides insights into both cell cycle regulation and ciliary assembly.

19. How do cilia contribute to the sense of taste?

Taste receptor cells in taste buds possess primary cilia that play a role in chemosensation. These cilia express certain taste receptors and signaling components, contributing to the detection of some taste modalities. This highlights the diverse sensory roles of cilia beyond their well-known functions in smell and vision.

20. How do cilia contribute to the function of the choroid plexus?

The choroid plexus, which produces cerebrospinal fluid (CSF), is lined with ciliated cells. These cilia help in circulating and mixing the CSF, which is crucial for delivering nutrients to the brain and removing waste products. Proper ciliary function in the choroid plexus is essential for maintaining brain health.

21. How do cilia contribute to the function of the respiratory system?

In the respiratory system, cilia line the airways and work together in a coordinated wave-like motion called the "mucociliary escalator." This action helps to trap and move mucus containing dust, bacteria, and other particles up and out of the lungs, protecting them from infection and damage.

22. How do cilia aid in the sensation of smell?

In the nasal cavity, olfactory neurons have cilia that extend into the mucus layer. These cilia contain olfactory receptors that bind to odor molecules, initiating the process of smell perception. The large surface area provided by numerous cilia increases the sensitivity of smell detection.

23. How do cilia in the inner ear contribute to hearing and balance?

In the inner ear, specialized cilia called stereocilia (which are actually more similar to microvilli) on hair cells bend in response to sound waves or head movements. This bending opens ion channels, converting mechanical stimuli into electrical signals that the brain interprets as sound or positional information.

24. What is the evolutionary significance of cilia?

Cilia are evolutionarily ancient structures, present in the last common ancestor of all eukaryotes. Their conservation across diverse organisms highlights their fundamental importance. The ability of cilia to sense and respond to environmental cues likely played a crucial role in the evolution of complex multicellular life.

25. What role do cilia play in the development of left-right asymmetry in vertebrates?

During early embryonic development, specialized cilia in the node (a structure in the embryo) create a leftward fluid flow. This flow is crucial for establishing left-right asymmetry in the body, determining the correct positioning of organs like the heart, liver, and spleen.

26. What are cilia and how do they differ from flagella?

Cilia are short, hair-like projections on the surface of cells. They differ from flagella in length and number - cilia are shorter and more numerous, while flagella are longer and typically fewer in number. Both are made of microtubules and help in cell movement or moving substances across cell surfaces.

27. What is the difference between cilia and microvilli?

While both are cellular projections, cilia and microvilli have distinct structures and functions. Cilia contain a 9+2 microtubule arrangement and are involved in movement or sensing. Microvilli are smaller, more numerous projections that increase the cell's surface area for absorption, as seen in the intestinal epithelium.

28. How do cilia contribute to male fertility?

In males, sperm cells have a specialized cilium that forms the flagellum, essential for sperm motility. Additionally, cilia in the efferent ducts of the testes help move sperm from the testes to the epididymis. Defects in either of these ciliary structures can lead to male infertility.

29. What is the role of cilia in Chlamydomonas, and why is it a model organism for studying cilia?

Chlamydomonas, a single-celled green alga, uses two flagella (which are essentially long cilia) for movement. It's an excellent model organism for studying cilia because its flagella are easily accessible for biochemical and genetic studies. Research on Chlamydomonas has provided many insights into ciliary structure, assembly, and function.

30. How do planaria use cilia, and what can we learn from them?

Planaria, flatworms known for their regenerative abilities, use cilia for locomotion and to create water currents for feeding. Studying planaria provides insights into both ciliary function and regeneration. Their ability to regenerate cilia-bearing structures makes them valuable models for understanding ciliary assembly and patterning.

31. How do primary cilia differ from motile cilia?

Primary cilia are typically non-motile and serve as sensory organelles, detecting chemical or mechanical signals from the environment. They usually occur one per cell. Motile cilia, on the other hand, can move and are often present in large numbers on cell surfaces, working together to create fluid movement or cell locomotion.

32. How do cilia differ in unicellular and multicellular organisms?

In unicellular organisms like Paramecium, cilia often cover the entire cell surface and are primarily used for locomotion and feeding. In multicellular organisms, cilia are typically specialized for specific functions in different tissues, such as moving fluids in the respiratory tract or sensing in neurons.

33. How do cilia contribute to brain function?

In the brain, many neurons possess a single, non-motile primary cilium. These cilia act as cellular antennae, detecting chemical signals in the cerebrospinal fluid. They play roles in neurodevelopment, adult neurogenesis, and potentially in some aspects of cognition. Ciliary dysfunction in the brain has been linked to various neurological disorders.

34. How do cilia contribute to the function of the kidney?

In the kidney, primary cilia on epithelial cells lining the tubules act as flow sensors. They bend in response to fluid flow, triggering calcium influx and signaling pathways that regulate cell function and gene expression. This mechanosensation is crucial for maintaining proper kidney function and development.

35. How do cilia contribute to embryonic development?

During embryonic development, cilia play crucial roles in various processes. They're involved in establishing left-right asymmetry, patterning the neural tube, and in signaling pathways like Hedgehog that are essential for proper development. Ciliary defects can lead to severe developmental abnormalities.

36. What is the relationship between cilia and the cell cycle?

The presence of cilia is closely tied to the cell cycle. Most cells resorb their cilia before entering mitosis, as the centrioles (which form the basal bodies of cilia) are needed for spindle formation. After cell division, cilia can reassemble. This cycle of assembly and disassembly is important for both cell division and ciliary signaling functions.

37. What is the role of cilia in Hedgehog signaling?

Primary cilia are essential for vertebrate Hedgehog signaling, a pathway crucial for embryonic development and adult tissue homeostasis. The cilium acts as a signaling center where key components of the pathway are concentrated. Disruption of ciliary function can lead to aberrant Hedgehog signaling and associated developmental disorders.

38. What is the relationship between cilia and polycystic kidney disease?

Polycystic kidney disease (PKD) is strongly linked to ciliary dysfunction. The primary cilia in kidney tubules act as flow sensors and are involved in cell signaling. Mutations in genes encoding ciliary proteins can lead to PKD, characterized by the formation of fluid-filled cysts in the kidneys.

39. How do cilia contribute to cerebrospinal fluid flow?

Cilia lining the ventricles of the brain play a role in circulating cerebrospinal fluid (CSF). Their coordinated beating helps create and direct CSF flow, which is crucial for delivering nutrients, removing waste, and maintaining proper pressure in the brain. Dysfunction of these cilia can contribute to hydrocephalus.

40. What is the role of cilia in pancreatic function?

Primary cilia on pancreatic cells are involved in sensing extracellular signals and regulating important cellular processes. They play roles in insulin secretion, cell proliferation, and possibly in the development of pancreatic cancer. Understanding ciliary function in the pancreas is important for addressing pancreatic diseases.

41. How do cilia contribute to wound healing?

Cilia play several roles in wound healing. They're involved in cell migration, a key process in wound closure. Additionally, ciliary signaling pathways like Hedgehog are important in tissue repair and regeneration. Understanding these roles could lead to new strategies for promoting wound healing.

42. What is the relationship between cilia and cancer?

The relationship between cilia and cancer is complex. Some ciliary proteins act as tumor suppressors, and loss of cilia is observed in many cancers. However, cilia can also promote cancer in some contexts by facilitating certain signaling pathways. This dual role makes cilia an interesting target for cancer research.

43. What is the role of cilia in bone development and homeostasis?

Primary cilia on bone cells (osteoblasts, osteocytes, and chondrocytes) act as mechanosensors, detecting mechanical forces and fluid flow. This mechanosensation is crucial for proper bone development, remodeling, and maintenance. Ciliary defects can lead to skeletal abnormalities and conditions like osteoarthritis.

44. How do cilia in the vascular system contribute to blood flow sensing?

Endothelial cells lining blood vessels possess primary cilia that act as flow sensors. These cilia bend in response to blood flow, triggering signaling cascades that regulate vascular tone and remodeling. This mechanosensing is important for maintaining vascular health and adapting to changes in blood flow.

45. What is the role of cilia in adipose tissue?

Primary cilia on adipocytes (fat cells) are involved in fat storage and metabolism. They play roles in adipocyte differentiation and in sensing hormones like insulin. Ciliary dysfunction in adipose tissue has been linked to obesity and metabolic disorders, highlighting a previously underappreciated role of cilia in metabolism.

46. What is the relationship between cilia and planar cell polarity?

Planar cell polarity (PCP) refers to the coordinated orientation of cells within a tissue plane. Cilia play a role in establishing and maintaining PCP, which is crucial for processes like directional cell movement and oriented cell division. The interplay between cilia and PCP is important in development and tissue homeostasis.

47. How do environmental toxins affect ciliary function?

Various environmental toxins can impair ciliary function. For example, heavy metals can disrupt ciliary beating in the respiratory tract, while certain pesticides can affect ciliary signaling pathways. Understanding these effects is important for assessing environmental health risks and developing protective strategies.

48. What is the role of cilia in the immune system?

While not typically associated with immune function, cilia do play roles in immunity. In the respiratory system, they're part of the first line of defense against pathogens. Additionally, some immune cells possess non-motile primary cilia that may be involved in sensing and signaling processes relevant to immune responses.

49. How do cilia contribute to heart development?

During embryonic development, cilia play crucial roles in heart formation. They're involved in left-right patterning, which is essential for proper heart looping. Cilia also participate in signaling pathways that regulate cardiomyocyte differentiation and proliferation. Ciliary defects can lead to congenital heart abnormalities.

50. What is the relationship between cilia and circadian rhythms?

Primary cilia have been implicated in the regulation of circadian rhythms. They appear to be involved in the cellular response to daily light-dark cycles and in the synchronization of cellular clocks. This connection highlights the diverse roles of cilia in cellular physiology beyond their mechanical functions.

51. How do cilia contribute to liver function?

Cholangiocytes, the epithelial cells lining bile ducts in the liver, possess primary cilia that extend into the bile duct lumen. These cilia act as sensors, detecting changes in bile flow and composition. They play roles in regulating bile production and flow, and their dysfunction has been linked to various liver diseases.

52. What is the role of cilia in skin homeostasis?

Primary cilia on skin cells, including keratinocytes and dermal fibroblasts, are involved in various aspects of skin biology. They play roles in