Notes on Motile Cilia

Motile cilia are very small, appearing like a whip. The beating of the cilia generates a directional fluid flow. 

Nowadays, the flow generated by the beating of the cilia is a topic of great interest. Defective motility in cilia can cause a severe human disease known as motile ciliopathies. Cilia are found in abundance in organisms; motile cilia are found in diverse organs including the nervous system. 

But the role of motile cilia in the organism has yet to be understood by scientists despite their diversity. 

Motile cilia are known as evolutionarily conserved organelles. 

Multiple motile cilia or single pair flagella, which are similar in structure to that of cilia, are present even in unicellular organisms. 

Some examples of organisms with motile cilia Paramecium caudatum, Tetrahymena, etc.

Cilia 

Cilia are short and eyelash-shaped filaments that are present in large numbers in the cells of tissues and some unicellular organisms.

In jellyfish-like organisms, cilia trigger the discharge of a tingling device, which gives rise to the light sensitivity rods of the mammalian retina. 

They also trigger smell detecting units that are present in the olfactory neurons of the mammalian body.

Cilia, which look like flagella or have similar structures to flagella, have a central core known as the axoneme. 

The axoneme has two central microtubules that are surrounded by an outer ring with nine pairs of microtubules.

The outer rings that are present in microtubules are surrounded by a membrane, which is continuous with the cell membrane of the organism.

The growth of cilia is controlled by the basal body, which lies inside the cell surface at the base of a cilium.

Types of cilia 

There are mainly three types of cilia are present:

1. Non-motile cilia 

Non-motile cilia are also known as primary cilia; they are solitary and non-motile and are found in most mammalian cells.

 Non-motile cells are mainly present in mitochondria, endoplasmic reticulum and Golgi apparatus. Non-motile cilia are involved in sensory functions.

2. Motile cilia

Motile cilia are involved in the movement of organisms. 

They are mainly present in the specialised lining present in the oviduct, airways, and ventricular system of the brain.

3. Nodal cilia

Nodal cilia are mainly similar to the non-motile cilia in structure except they have dynein arms, which are responsible for movement and spinning.

Nodal cilia move in clockwise directions which results in the movement of extraembryonic fluids through the nodal surface.

Motile cilia 

Motile cilia are very important in a variety of physiologic processes. 

Generally, there are three divisions of motile cilia.

 Cells that are present at embryonic nodes have a single pair of cilia. On the other hand, the cilia present in the respiratory tract are multi-ciliated.

Sperm cells contain flagella; these are specialised and responsible for the movement of sperm in the reproductive tract of females.

Most of the motile cilia have a 9+2 arrangement, but some also have a 9+0 arrangement structure.

 All cilia are covered with a plasma membrane. 

Their core, which is generally known as an axoneme, possesses several microtubules running parallel to the long axis.

The cilia formed in a lab can push debris along the surface of a bubble.

Functions of motile cilia 

• Motile cilia help in the movement or locomotion of organisms from one place to another.
• Motile cilia help in food-capturing in some organisms.
• Motile cilia help humans and some animals in cleaning atmospheric air for their respiration in the nasal passages.
• Sometimes motile cilia also act as sense detection.
• Cilia play a very important role in the cell cycle as well as in the development of animals.
• Motile cilia also help some proteins carry out their functions properly.
• Motile cilia play an important role in the communication that takes place in cells and molecular trafficking.

Conclusion 

Motile cilia are microscopic and cannot be seen by regular eyes.

Cilia are remarkable cellular devices that power cell motility and transduce extracellular signals.

Motile cilia arise from the basal bodies; cilia and flagella both are similar in chemical composition, have identical ultrastructure, and serve the same purpose – that is their movements either propel the organism or move the medium past a fixed cell. However, the two are distinguishable by their number, size, and mode of beating.