Cilia - Definition, Structure, Types and Function

Aug 03, 2023, 16:45 IST

Introduction

Cilia are minute hair-like protrusions of the cell membrane on the outer surface of the cell. Cilia are found in eukaryotic cells, whereas prokaryotic cells bear similar structures called pili. They are used for locomotion and mechanoreception in primitive cells.

Types of Cilia

Two types of cilia are found in animal cells:

1. Motile Cilia- The motile cilia, also known as secondary cilia, seen in mammals beat in syncopated metachronal waves and are typically found in high numbers on the surface of cells. The respiratory tract is lined with multi-ciliated cells, which remove mucus by sweeping mucus-containing debris out of the lungs. The epithelial layer in the respiratory tract has about 200 motile cilia on each cell.

Smooth muscle contractions in the reproductive canal assist the cilia's beating as they transport the ovum from the ovary into the uterus.

The cerebrospinal fluid is circulated by ciliated ependymal cells in the brain's ventricles.

2.Non-motile Cilia- Animals have non-motile primary cilia on almost all types of cells, with blood cells being a notable exception. When compared to cells with motile cilia, most cells only have one cilium; however, olfactory sensory neurons, in which the olfactory receptors are located, each have roughly ten cilia. Some cell types, including retinal photoreceptor cells, have developed extremely specialized cilia. Kinocilia are a type of developed nonmotile cells that are present in the inner ear of humans. They have outer dynein arms but inner dynein arms are absent. They are stimulated by sound reaching the ears.

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Structure

• A cilium can be examined under the electron microscope to reveal that they are covered in a plasma membrane.
• Numerous microtubules run parallel to the longitudinal axis in their core, which is known as the axoneme.
• Nine doublets of radially organized peripheral microtubules in the axoneme are typically present, along with one pair in the center.
• This configuration of microtubules in the axoneme is known as the "9+2 array."
• One of the tubules of every peripheral doublet is connected to the central tubule of each doublet by a radial spoke, and the inner tubules are also encased by a central sheath.
• There are nine radiating spokes as a result. Additionally, linkers are used to connect the peripheral doublets.
• The basal bodies are a centriole-like structure that gives rise to cilia.

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Functions

• Protozoans belonging to the Phylum Ciliates have cilia as their main means of mobility. Both to respond to environmental changes and to transport extracellular fluid over the organism's surface, the movement happens.
• Some species use the cilia's rhythmic motion to sweep away pathogen-preventative mucus, foreign objects, and other bacteria.
• The motile cilia seen on various epithelial layers in the human body either help the ovum transfer vital substances through the oviduct or respiratory system to help in clearing out dust particles.
• Organ development and the control of the cell cycle are crucial functions of many cilia.
• Several proteins and receptors that function as chemoreceptors are found on the surface of primary or nonmotile cilia. The primary cilia of the kidney and retina, respectively, are important in the perception of urinary movement and light.
• Cilia are significant virulence components of pathogenic organisms that facilitate the colonization of numerous surfaces.
• Cilia have been linked to the release of vesicular ectosomes in various species.
• The routes for signal transduction that control the intracellular calcium ion concentration and polarity of planar cells depend heavily on cilia.

Ailments

• Ciliopathy- Ciliopathies are genetic syndromes of the ciliary linings in the body. The abnormal functioning of cilia causes diseases like PCD, Bardet-Biedll syndrome, Alstrom syndrome, etc.
• PCD or Primary Ciliary Dyskinesia is an autosomal recessive ailment in which the cilia lining the respiratory tracts. It causes chronic respiratory infections. Wet sputum is produced occasionally, and persistent cough and nasal congestion are symptoms of this rare disease.
• Bardet-Biedl syndrome- It is a pleiotropic abnormality in which rod and cone cell dystrophy causes loss in visibility.
• Alstrom-Hallgren syndrome- It is also an autosomal genetic condition characterized by obesity and rod-cone dystrophy.

Cilia: FAQs

Q1. What are cilia?

Ans. Cilia are slender finger-like protrusions of the cell membrane that aid in mechanoreception and movement.

Q2. What are the types of cilia?

Ans. Cilia may be of motile or nonmotile type.

Q3. What is the difference between cilia and flagella?

Ans. Cilia are numerous finger-like projections on the whole surface of the cell, whereas flagella are a few whiplash or tinsel-like projections on the cell body.

Q4. Where are cilia found in the human body?

Ans. Cilia are found in the lining of tracts of the internal organs like the respiratory tract, the fallopian tubes, etc.

Q5. What are the functions of cilia?

Ans. Cilia help in the mechanical reception and movement of the cell. They also line the tracts of the internal organs of the body to help in the flow of substances.

Q6. What are the ailments related to cilia?

Ans. Ciliopathy is the term given to genetic disorders related to cilia. Some ciliopathic disorders include Primary Ciliary Dyskinesia, Bardet-Biedl syndrome, Alstrom-Hallgren syndrome, etc.