Cilia- Definition, Structure, Ultrastructure, Functions

What are Cilia?

  • The word cilia is derived from the Latin word cili which means eyelash.
  • In the Latin word, Flagella means little whip.
  • They are microscopic which can’t be seen with naked eyes.
  • They have the contractile property and are filamentous processes of the cytoplasm.
  • Food currents are created by the cilia.
  • They act as sensory organs.
  • They perform different mechanical functions.
  • Cilia and flagella though it looks similar, they vary in the various aspects i.e number, size, functions.
Cilia- Definition, Structure, Ultrastructure, Functions
Cilia- Definition, Structure, Ultrastructure, Functions. Created with BioRender.com

Distinguishing features of both the cilia and the flagella are as follows:

a. Number

  • The number of cilia is 3000 t0 14000 or more
  • Flagella are few which maybe 1 or 2

b. Location

  • Cilia are present all over the surface of the cell.
  • Flagella are found at one end of the cell.

c. Length

  • Cilia are shorter. They are 5 to 10 μm in length. Cilia are the appendages of the cytoplasm.
  • Flagella are longer. They are the longer processes which can be up to 150 μm long.

d. Cilia possess the sweeping or pendular stroke motion but flagella have undulatory motion.

e. Cilia beat in the coordinated rhythm but the flagella beat independently.

Sterocilia and Kinocilia

  • The cytoplasmic extensions from the cell are called sterocilia. They are immobile too.
  • Kinocilia are the true cilia.
  • The sterocilia is present in the epithelial cells of the epididymis, macula, and crista of the internal ear.
  • In the inner ear, sterocilia is present in hair cells. It helps in the transduction of sound.
  • They are devoid of microtubules.
  • There is the presence of about 3000 microfilaments. 
  • These filaments are placed longitudinally and have definite polarity, helical symmetry.
  • Around the filaments, cross-bridges are present.
  • Flagella are present in:
    • in the protozoans of the class Flagellata
    • choanocyte cells of the sponges
    • spermatozoa of the Metazoa
    •  in the algae 
    •  gamete cells
  • The cilia are found in the:
    • protozoans of the class Ciliata 
    • members of other classes 
    • ciliated epithelium of the Metazoa. 
  • Cilia help in the locomotion as they are present in the external surface of the body. Example: Platyhelminthes, Nemertines, Echinodermata, Mollusca, and Annelida.
  • The internal cavity is lined by the cilia.
  • Cilia is also present in the air passages of the respiratory system and the reproductive tracts.
  • Cilia is absent in the nematode worms and arthropods.
  • Cilia in the sperm helps in locomotion but it does not contribute to locomotion in the other mammalian system. 

Structure of the Cilia

Cilia are composed of the following components. They are:

  • the shaft or cilium:  From the free surface of the cell, a projection arises which is slender and cylindrical. It is called the shaft or the cilium
  • the basal body or granule: It is the cellular organelle that is centriole-like. From it, the cilium originates.
  • Ciliary rootlets: By the ciliary or the basal plate, the basal body remains separated from the cilium. Its functions are: 
    • Termination of the C Tubule of each triplet of basal body
    • Beginning of two central microtubules.
  • Both the cilia and the flagella are the extension of the cytoplasm.  They are thread-like. 
  • They are delicate structures.

Ultrastructure of the Cilia

The cilia or the flagellum of the eukaryotic cell is made up of three parts:

  • central axoneme or shaft
  • plasma membrane 
  • cytoplasmic matrix 

A. Ciliary Membrane

  • The thickness of the ciliary membrane is 9.5nm.
  • It is present physically continuous with the plasma membrane.
  • It is poor in protein.
  • Some specific protein is present in the ciliary membrane.
  • These proteins act as the barrier and prevent the loss of ATP.
  • It also prevents the loss of some essential ions. These ions are required to be present in a particular concentration so that they will provide energy. This energy further helps in the ciliary movement.

Ciliary necklace

  • The presence of the ciliary necklace is an unusual feature that is found in the membranes of all somatic cilia.
  • The ciliary necklace is present at the base of this organelle.
  • It can be observed in the electron micrographs of the freeze-fractured cilium.
  • In the cilium, the microtubules and the  basal bodies make contact with the membrane
  • In that region, there is the presence of the ciliary necklace.
  • The ciliary necklace helps in the differentiation of the ciliary membrane.

B. Matrix

  • Watery space is present in the bounded space of the cilium which is called a matrix.
  • In the ciliary matrix, eleven microtubules of the axoneme are embedded in it. 
  • Similarly, the other interconnecting proteins are also embedded in the matrix. 

C. Axoneme

  • The axoneme is the axial basic microtubular structure of the cilia and flagella.
  • It helps in motility.
  • Its diameter is 0.2 to 10µm.
  • Along the length, the cilia gradually get thinner whereas it is thicker at the base.

Major protein structures of the axoneme of the cilia and flagella:

The Axoneme components are:

  • Tubulin: Tubulin is the principal component of the microtubule.
  • Dynein:  It produces bending. Dynein projects from microtubule doublets. It then interacts with the adjacent doublets.
  • Nexin link: It helps to hold the adjacent microtubule doublets together.
  • Radial spokes: From the nine outer doublets, extension occurs inward to the central pair.
  • Sheath projections: From the central pair of the microtubules, the projection occurs as a series of sidearms. It also helps in the regulation of the ciliary beat.

Functions of Cilia

  • It helps in locomotion.
  • In the lower aquatic animals, cilia create the food currents.
  • Ciliary movements in the respiratory tract play a role to eliminate the solid particles which are present in it.
  • The ciliary movement also protects from the disease and it can clear the different micro-organisms and the dust.
  • Mucociliary clearance is the primary innate defense mechanism that provides protection from airborne pathogens.
  • From the oviduct, the eggs are driven out with the help of vibratile cilia.
  • They are also responsible for cellular communication and molecular trafficking.
  • Non-motile cilia present in the kidney helps to sense the urine flow.
  • Non-motile cilia help to detect the signals where it functions as the sensory apparatus of the cell.

References

  1. Verma, P. S., & Agrawal, V. K. (2006). Cell Biology, Genetics, Molecular Biology, Evolution & Ecology (1 ed.). S.Chand and Company Ltd.
  2. Mall, M. A. (2008). Role of cilia, mucus, and airway surface liquid in mucociliary dysfunction: lessons from mouse models. In Journal of aerosol medicine and pulmonary drug delivery (Vol. 21, Issue 1, pp. 13–24). https://doi.org/10.1089/jamp.2007.0659 
  3. https://www.biologydiscussion.com/cytoplasm/4-main-components-of-the-cytoplasm-with-diagram/36769.

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