Cilia are microscopic, hair-like structures, which are protruding from the cell membrane of the eukaryotic cell. There are various functions in which Cilia help, including locomotion, signal transmission, sensing of signals and propelling of food particles. Cilia assist in several tasks, including signal transmission, food particle propulsion, motility, and signal perception.
Cilia are composed of the basal body, ciliary rootlet, transition zone and axoneme.
Basal Body
- The first part of the cilium structure is the basal body, which is considered to be the foundation of Cilia.
- Mammalian cell Cilia consist of subdistal appendages, nine triplet microtubules and distal appendages.
- Distal appendages attach the basal body of Cilia to the base of Cilia.
Ciliary Rootlet
- It is a cytoskeletal type of structure and has a diameter of 80 - 100nm, which protrudes from the basal body of Cilia at the proximal end.
- Ciliary rootlet is made up of protein known as rootlet protein, which is encoded by the CROCC gene.
Transition Zone
- The transition zone is also known as the ciliary gate, which is responsible for the entry and exit of proteins from Cilia.
- The ciliary gate consists of Y Y-shaped structure that connects it to the axoneme. The ciliary gate only allows selected protein entry.
Axoneme
- The core of Cilia is made up of microtubules known as axonemes.
- 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 five types of Cilia, which include
1) 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.
2) 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.
3) 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
4) 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.
5) Nodel Cilia
- These are also known as monocilium due to having only single cilium.
- They are present on early development stage of embryo.
Examples of Cilia
Some examples of different types 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.
- Fallopian tube Cilia: These help in the transportation of eggs from the fallopian tube to the uterus.
- Brain ventricular system Cilia: These Cilia help in the proper circulation of CSF around the brain.
- Kidney tubule Cilia: These Cilia aid proper filtration of fluid from the kidney.
- Olfactory Cilia: These Cilia help in the detection of smell or odour.
- Intestinal Cilia: These help in the movement of particles and the absorption of nutrients.
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 specialised 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.
Function of Cilia
There are so many functions of Cilia that depend upon the type of Cilia. Some of the functions are listed below.
- Movement of body fluids: This function is performed by motile Cilia. Motile Cilia are present in various cells. For instance, respiratory cells, Cilia help in sweeping and the movement of mucus.
- Sensory transduction: This function is performed by a non-motile cell. These types of Cilia receive signals from the outer environment and transduce that signal to the cell. These Cilia are present in the olfactory route and the visual route.
- Embryonic development: This function is processed by non-motile Cilia and helps in development of embryonic development through hedgehog signalling. These Cilia help in tissue separation of the developing embryo.
- Movement of female eggs: This function is performed by motile cilia. These help in the transportation of the female egg from the ovaries to the uterus.
- Cell movement: In a single-cell organism, cilia help in the movement of the cell.
Importance of Cilia
Cilia are important for so many physiological processes, and some of the important in that Cilia help with are listed below.
- Locomotion: Cilia help in the motility of single-cell organisms.
- Respiratory functions: Cilia facilitate the removal of mucus through the trachea and bronchi.
- Reproduction: Cilia aid in the movement or transportation of eggs from the fallopian tubes to the uterus.
- Sensory functions: Cilia aid in the detection of external signals such as smell.
- Embryo development: In embryo development, hedgehog pathways signalling is aided by Cilia.
- Transduction of signal: Receiving of signal from outside to pass it into the inside of the cell.
- Environment sensing: Cilia respond to slight changes in the external environment, such as mechanical stimuli.
Disorders of Cilia
Malfunction and abnormalities in Cilia formation cause disorders known as ciliopathies. Below are some listed disorders related to Cilia.
- Primary Ciliary Dyskinesia: Disorder of respiratory cilia.
- Polycystic kidney disease: This leads to the formation of fluid-filled cysts in the kidney.
- Bardet-Biedl syndrome: Mutation in the Cilia synthesis gene results in obesity, retinal disorders.
- Joubert syndrome: This brain disorder results in a malformation known as the molar tooth sign.
- Nephronophthisis: Development of fibrous tissue in the kidney results in kidney failure.
- Alstrom syndrome: This is a rare genetic syndrome that consists of cardiac problems, hearing issues and obesity.