Cilia

Edited By Irshad Anwar | Updated on Nov 08, 2024 09:01 AM 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.

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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.