Cilia is an organelle on the membrane of most of the eukaryotic cells and is also known as cilium (singular). It is absent in bacterial and archaeal cells. The cilia function is to provide locomotion and propelling of cell and food particles. The shape of the cilia is slender, and it is a thread-like structure that arises from the surface of the cell membrane. There are two types of Cilia, including motile and non-motile. In this article, we will study the cilia, its structure, types, and Functions.

What is Cilia?

Cilia are microscopic, hair-like structures, which are protruding from the cell membrane of the eukaryotic cell. There are various functions in which Cilia helps including locomotion, signal transmission, sensing of signals and propelling of food particles, etc. There are two types of Cilia, and they are known as motile and non-motile Cilia. Cilia assist in several tasks, including signal transmission, food particle propulsion, motility, and signal perception. Cilia are classified as either motile or non-motile depending on their kind.

What is Cilia and Flagella?

They are made of microtubules and wrapped in a membrane, flagella and cilia are similar hair-like structures that are present on the surface of cells but have distinct purposes. In contrast to flagella, which are longer and fewer and mainly employed for cell propulsion, cilia are usually shorter and more numerous and help move fluid or particles over cell surfaces or during sensory reception.

Structure of Cilia

Cilia is composed of basal body, ciliary rootlet, transition zone and axoneme.

Basal Body

• The first of cilium structure is basal body which is considered to be the foundation of Cilia.
• Mammalian cell Cilia consists of subdistal appendages, nine triplet microtubule and distal appendages.
• Distal appendages attaches the basal body of Cilia to the base of Cilia.

Ciliary Rootlet

• It is a cytoskeleton type of structure and have diameter of 80 – 100nm, which protruding from the basal body of Cilia at the proximal end.
• Ciliary rootley is made up of protein known as rootlet protein, which is encoded by CROCC gene.

Transition Zone

• Transition zone is also known as ciliary gate which is responsible of entry and exit of protein from Cilia.
• Ciliary gate consists of Y shaped structure which connects it to the axoneme. Ciliary gate only allow selected protein entry.

Axoneme

• Core of Cilia is made up of microtubule known as axoneme.
• In a motile cilium, the axoneme serves as a framework for the inner and outer dynein arms that propel the cilium and as a pathway for the kinesin and dynein microtubule motor proteins.

Types of Cilia

There are basically four types of Cilia which includes

1. Non-motile Cilia
2. Modified non-motile Cilia
3. Motile Cilia
4. Modified motile Cilia

Non-motile Cilia

• These are also known as primary Cilia and are discovered in 1898.
• These type of Cilia found on every animal cell except blood cells.
• These are present on olfactory neurons, retinal photoreceptor cells.
• Function of non-motile Cilia are chemosensation, control of cell growth and signal transduction.

Modified non-motile Cilia

• Modified non-motile Cilia are also known as Kinocilia.
• Modified non-motile Cilia are present in the inner ear hair cells.
• They have 9+2 axoneme of motile Cilia but these Cilia do not have dynein arm that provide motility to the Cilia.

Motile Cilia

• They are found in tracheal respiratory tract.
• These type of Cilia is also present in mammalians respiratory tract.
• In respiratory tract about 200 Cilia are present on every respiratory epithelium cells.
• They help in cleaning and sweeping of mucus.
• These also aid in motion of egg cells from ovaries to uterus.
• In brain ventricle ciliated cell aid in proper circulation of CSF.

Also Read: Human Respiratory System

Modified motile Cilia

• When motile Cilia do not have central apparatus are known as modified motile Cilia.
• They are present on nodel cells as nodel Cilia.
• Modified motile Cilia have dynein arm which allows it to move.
• These are present on choroid plexus epithelial cells.

Nodel Cilia

• These are also known as monocilium due to having only single cilium.
• They are present on early development stage of embryo.

Cilia Function in Cell

There are so many functions of Cilia that depends upon the type of Cilia. Some of the functions are listed below

1. Movement of body fluids: This function is performed by motile Cilia. Motile Cilia is present of various of cells. For an instance respiratory cell, Cilia help in sweeping and movement of mucous.
2. Sensory transduction: This function is performed by non-motile cell. These type of Cilia recieves signals from outer environment and transduce that signal to the cell. These Cilia are present in olfactory route and visual route.
3. Embryonic development: This function is processed by non-motile Cilia and help in development of embryonic development by a hedgehog signaling. These Cilia help in tissue separation of developing embryo.
4. Movement of female eggs: This function is performed by motile cilia. These help in transportation of female egg from ovaries to uterus.
5. Cell movement: In single cell organism cilia helps in movement of cell.

Disorders of Cilia

Malfunction and abnormalities in Cilia formation causes disorders known as ciliopathies. Below are the some listed disorders related to Cilia.

1. Primary Ciliary Dyskinesia: Disorder of respiratory cilia.
2. Polycystic kidney disease: This leads to formation of fluid filled cyst in kidney.
3. Bardet Biedl syndrome: Mutation in Cilia sysnthesis gene reults in obesity, retinal disorders.
4. Joubert syndrome: This brain disorder result in malformation known as molar tooth sign.
5. Nephronophthisis: Development of fibrous tissue in kidney results in kidney failure.
6. Alstrom syndrome: This a rare genetic syndrome which consists of cardia problem, hearing issue and obesity.

Example of Cilia

Some example of different type of cilia are:

• Cilia in Lungs: Cilia are widely distributed throughout the lining of the trachea, or windpipe, and the bronchi, or lung airways, in the respiratory system. By keeping the airways free of debris and microbes as well as mucus, these respiratory cilia assist in preventing lung infections and preserving healthy airways.
• Fallopian tube Cilia: These help in transportation of eggs from fallopian tube to uterus.
• Brain ventricular system Cilia: These Cilia helps in proper circulation of CSF around brain.
• Kidney tubule Cilia: These Cilia aid proper filteration of fluid from the kidney.
• Olfactory Cilia: These Cilia helps in detection of smell or odour.
• Intestinal Cilia: These helps in movement of particles and absorption of nutrients.

Also Read: Small Intestine

Cilia Location

Small, hair-like structures called cilia are present in a wide range of species, including multicellular creatures like animals and plants and single-celled organisms like protozoa. They are found throughout the body in various places, and depending on where they are, they serve various purposes. The following are typical sites for cilia to be found:

• Respiratory Tract: Cilia are widely distributed throughout the lining of the trachea, or windpipe, and the bronchi, or lung airways, in the respiratory system.
• Reproductive System: The cilia of the female reproductive system are located in the oviducts, or fallopian tubes.
• Central Nervous System: Pendymal cilia are found on a subset of specialized brain cells. Cerebrospinal fluid, which nourishes and shields the brain and spinal cord, is moved about by these cilia.
• Digestive Tract: The lining of the intestines contains cilia, which facilitate the flow of food and the mingling of digestive secretions.
• Sensory Organs: Additionally, cilia are present in certain cells of sensory organs such as the retina and inner ear.

Importance of Cilia

Cilia are important for so many physiological processes and some of the importance in which Cilia helps are listed below.

• Locomotion: Cilia helps in motility of single cell organisms.
• Respiratory functions: Cilia facilitate the removal of mucus through trachea and bronchi
• Reproduction: Cilia aid in movement or transportation of eggs from fallopian tubes to uteurs.
• Sensory functions: Cilia aid in detection of external signal such as smell.
• Embryo development: In embryo development, hedgehog pathways signaling aided by Cilia.
• Transduction of signal: Receiving of signal from outside to pass it into the inside of cell.
• Environment sensing: Cilia responds to slightly change in the external environment such as mechanical stimuli.

Conclusion – Cilia

Several human body cells as well as those of several other cells have minute, hair-like projections called Cilia on their surface. In conclusion, cilia are complex and essential structures on several cell types’ surfaces. They have a variety of functions in biological processes, including fluid transport, sensory perception, and cell movement. From primitive single-celled organisms to large multicellular animals like humans, cilia play a critical role in the correct functioning of organisms through their coordinated beating movements or sensory reception. Understanding the structure, role, and importance of cilia contributes to our understanding of cellular biology and provides new perspectives on a range of illnesses and possible treatments.

Also Read:

• Flagella – Structure, Types and Functions
• Cell the Unit of Life Class 11 Notes CBSE Biology Chapter 8
• Cell Organelles
• Cilia And Flagella – Definition, Structure, Functions and FAQs
• Diagram of Respiratory System – Parts and Functions

FAQs on Cilia

What is Cilia and its Function?

Cilia are small, hair-like structures protruding from cell surfaces, aiding in movement and sensing in various organisms.

Where are Cilia in Humans?

In humans, cilia are found in the respiratory tract, lining the airways from the nose to the lungs, and also in the fallopian tubes and the brain’s ventricles.

What are Cilia and Why are they Important?

Cilia serve critical roles in cell motility, sensory perception, and fluid movement in various organs, contributing to overall health and functionality.

What is the role of Cilia in Lungs?

In the lungs, cilia play a crucial role in clearing mucus and foreign particles from the airways, helping to maintain respiratory health and prevent infections.

Is Flagella a Cilia?

Flagella and cilia are structurally similar but serve different functions; flagella typically function in cell propulsion, while cilia primarily aid in moving fluid or particles along cell surfaces.

What is the Difference Between Cilia and Flagella?

Cilia are typically shorter and more numerous, serving to move fluid or particles along cell surfaces, while flagella are longer and fewer in number, primarily used for cell propulsion.

Does Cilia Help in Locomotion?

Yes, cilia can play a role in locomotion.

Where is Cilia Present?

Cilia are present on the surface of cells lining various organs and structures in the body, including the respiratory tract, fallopian tubes, brain ventricles, and kidneys.