Cytoskeleton

 Cytoskeleton Structure and Function

The cytoskeleton is the skeleton of the cell. Just like the skeleton of our body which provides us the specific shape, strength, movement, and support, our cells also have a skeleton that gives support and protection to the cell in case of any compressive forces.

What is a Cytoskeleton?

The cytoskeleton is a network of filaments and tubules which are composed of specific proteins.

Components of the Cytoskeleton

There are three main components of the cytoskeleton which make the framework of the cell. These are;

• Microfilaments
• Intermediate filaments
• Microtubules

Composition of cytoskeletal components

Microfilaments 

Microfilaments are the smallest of all the cytoskeletal components. These are 7nm wide. These are also known as actin filaments because they are composed of actin proteins. Their flexibility is very high which is beneficial for cells.

Microfilaments are composed of G-actin or globular actin monomers which fuse to form polymers called filamentous actin. These polymers form double helical structures and make up the structure of microfilaments. 

Intermediate Filaments

Intermediate filaments are bigger than microfilaments and smaller than microtubules. These are 10 nm wide. These are the most tough and resilient of all the cytoskeletal components. The rod-shaped monomers of intermediate filaments meet each other and make a coil of dimer. Almost 8 dimers join together and an intermediate filament is formed.

Based on their location in different tissues, intermediate filaments are divided in the following main types;

1. Lamins: These are found in almost every cell of the body.
2. Keratins: These are present in epithelial cells.
3. Vimentin: These are coated within the fibroblast cells.
4. Desmin: These filaments are found in muscle cells.
5. Neurofilaments: These filaments are found in nerve cells.

Microtubules

Microtubules are the largest of all the cytoskeletal components. These are 25 nm  =in width. These are a little bit flexible.

Microtubules have two types of monomers, alpha tubulin and beta tubulin. Both join together to form tubulin dimers which are then joined by GTP and form a chain called proto tubulin. Thirteen proto-tubulins make a hollow tube called a microtubule. The cavity of microtubules is called lumen. 

Each microtubule has a negative end which points towards the nucleus and a positive end which points toward the periphery of the cell. 

Cytoskeleton Functions

The key function of the cytoskeleton is to maintain the shape of the cell and provide support. But it also allows the cell to be able to make particular movements, to adapt to compressive forces, and helps cellular organelles to perform their functions properly. 

Besides all these functions, the cytoskeletal components perform different functions individually, which are discussed below; 

Microfilaments

Cell Migration

Microfilaments cause the cells to be able to squeeze out and change their morphology to fit through tiny spaces due to their flexibility factor. For example, in diapedesis, when a pathogen enters the body, white blood cells squeeze out through capillary cell spaces due to the flexibility factor of actin molecules. The white blood cells then, detect and kill the pathogen.

Cytokinesis

One of the particular functions of microfilaments is in cytokinesis after the division of the nucleus. The actin molecules start pulling the cells from both sides and create a constriction ring which makes the cells apart.

Cellular Extensions

On the apical portion of the cell membrane, the actin molecules form small finger-like projections that allow some particular movements and perform other specific functions.

For example, microfilaments form microvilli in the gastrointestinal tract which are the smallest cellular extensions on the apical surface of cells. The microvilli increase the surface area of the cells and play an important in the digestion of food and absorption of nutrients.

Another example of cellular extensions is the presence of stereocilia which are the apical projections on the cell surfaces of the inner ear. These are a bit larger than microvilli. They have an important function in balance and hearing.

Cell Junctions

Microfilaments help in cell junctions and keep the cells through and closer to each other to prevent cell separation. 

Transport of m=Materials

Actin filaments help to push cellular contents out of the cell by binding these contents to the cell membrane and sending them outside of the cells by exocytosis.

Similarly, actin proteins help to create invaginations in the cell and suck the actual content into the cell by endocytosis.

Muscle Contraction

Microfilaments are associated with myofilaments in muscle cells and cause muscle contraction. When myosin binds to actin, it shortens the microfilaments due to which muscle contraction occurs.

Intermediate Filaments

Cell Shape Maintenance

Intermediate filaments help the cell to maintain its shape despite many compressive forces.

Cell junctions

In connective tissues, intermediate filaments cause desmosome junctions and give more protection to cells, especially in cardiac cells. Cardiac cells continuously stretch and there is always a risk of cell separation so intermediate filaments do not allow cells to get apart and make them connected. Intermediate filaments also connect the epidermis with the dermis through junctions called hemidesmosomes junctions.

Microtubules

Intracellular Transport

Microtubules control the intracellular transport of organelles and vesicles. For example, in neurons, microtubules act as a train track for motor proteins which carry neurotransmitters containing vesicles to the cell membrane which are then secreted out of the cell.

Spindle Fiber Formation

Microtubules join together to form spindle fiber formation. The spindle fibers pull the chromosomes due to which chromosomes move towards opposite poles and nucleus division takes place.     

Cilia and Flagella Synthesis        

The base of cilia and flagella is composed of microtubules. The flagella are present in sperm cells, bacteria amoeba, etc.   

The cilia are present in cells of the respiratory tract. These cilia continuously beat and move up to the trachea and remove dust or other harmful particles outside the body through coughing.