Everything You Need To Know About The Cytoskeleton

The cytoskeleton is an important part of cell biology. It provides support to the cell, and is characterized as being part of the cytoplasm, and enables the cell to function as it should.

There are three primary components of the cytoskeleton – the microfilaments, microtubules, and intermediate fibers/filaments.

Each of these components play a crucial role in the functioning of a cell and cytoskeleton, while also having unique roles.

To learn more about the cytoskeleton, including what it is, the structure and location, as well as the various functions, keep reading. Here, we will go through everything you need to know.

About The Cytoskeleton

The cytoskeleton has been a hot topic of research for some time. Recently, it has been found that the cytoplasm, which is part of the cytoskeleton, is actually involved in the energy transfer and information processing with neurons.

There has also been a recent discovery of three distinct nucleotides around the “barbed” end of the actin filaments. This has explained why the actin filaments typically grow far faster at one end rather than equally. 

The final profound finding is that the cytoskeleton can have an effect on the immune system. If there are found to be defects within the cytoskeleton, the immune system can be impaired, which will have a negative effect on the organism. 

However, there will always be more to learn as our science and technology advances. While the cytoskeleton aids in providing structural support for the cells, the protein filament network within the cell can be adjusted.

This can happen when it receives a number of cues from internal or external stimuli to alter itself.  

What Is It?

The cytoskeleton is a fibrillar-structured system within a cell that permeates the cytoplasm. It is described as being a part of the cytoplasm, and it is known to provide internal structural support for the cell.

However, it is also known to be involved in a number of movements. This movement is based on the cytoskeleton anchoring different cellular structures, such as flagellum. However, it is also involved in moving cellular substances. 

There are three primary components that make up the cytoskeleton, which we will go through in the section below. 

Structure And Location

The cytoskeleton is made up of microfilaments, microtubules, and intermediate fibers or filaments. We will go through each of these components in the sections below.

Microfilaments

Microfilaments are filamentous structures that are found in the cytoskeleton. They are made of actin monomers (f-action) and g-factors.

These g-factors are also known as g-actin, and polymerize to create filaments of actin polymers (f-actin). Two helical coiled f-actin make up one strand of filament, also known as microfilament. 

These microfilament strands are known to possess both positive and negative ends, which contribute to the filament’s regulation at either end.

Regarding microfilament development, some interesting findings have been discovered recently. Monomers are added to the positive side of the filament at a much faster rate when compared to the negative end.

At these positive ends, there are also ATP caps, which help stabilize the filament in times of rapid growth. 

Microfilaments are the most narrow structures in the cytoskeleton. Only messing between 3 and 5 nm in diameter, these tiny structures are made of actin.

This means that microfilaments can quickly and easily be assembled and help the cell function as it should when they are required. 

These structures are typically found in the cell periphery, where they are attached to the microvilli to the plasma membrane.

This means that they can be found in places like the pericanalicular zone and make up the pericanalicular meshwork. So, while microfilaments are incredibly tiny, they are very versatile and helpful structures.

Microtubules

Microtubules are the biggest components in the cytoskeleton. They have a diameter of anywhere from 15 and 10 nm, and are made of tubulin, a type of globular protein.

This protein is composed of alpha and beta tubulin and kd polypeptides. As such, they are vastly different from microfilaments, which are the smallest component in the cytoskeleton, and are made of different material.

Every microtubule is made of around 13 linear protofilaments, which are created around a core that is hollow. These components are made of polar structures, like microfilaments.

Because of this, they have two distinct ends, and those ends have a negative and positive charged end. Like microfilaments, the positive end of the structure will grow faster compared to the negative end. 

When the conditions are favorable in the cell, the heterodimers of tubulin will assemble to create protofilaments that are linear. These filaments will then assemble to create the microtubules, which are like hollow straws.

These microtubules are organized into cell bundles, much like microfilaments, However, these components have been known to be incredibly unstable, and some microtubules go through the cycles of growth quickly, reducing their population.

The polymerization and depolymerization of the beta Bulin monomers is what causes the instability in microtubules.

When there are shrinking phases, heterodimer subunits get removed from particular ends of the tubes and added during a phase of growth.

This instability has been attributed to the high variance of the microtubule bundle sizes and also the internal organization. 

Intermediate Fibers/Filaments (IF)

Intermediate fibers are unique components of the cytoskeleton. Unlike the other components, these fibers are made up from a large family of polypeptides.

Because of this, there is a wide array of various intermediate filaments in different cells.

Research has shown that there are more than 50 types of intermediate filaments, which can be classified into six primary groups. These groups are as follows:

• Type I & II – this group has around 15 different proteins that are found in the majority of epithelial cells.
• Type III – this group contains proteins such as desmin and vimentin which can be found in white cells, smooth muscles, and glial cells, among many more. 
• Type IV – this group includes proteins like ainternexin, which is found in nerve cells, and neurofilament proteins.
• Type V – this group has a number of proteins, such as lamins.
• Type VI – this group includes things like nestin, which can be found in neurons. 

Keratin is one of the most common proteins involved in the intermediate filament formation. You may know keratin from learning about one’s nails, skin, and hair.

The central rod domains of two polypeptide chains are coiled together during the assembly of these filaments. When this happens, they form a coiled structure known as a dimer.

These dimers then come together and form tetramers, which assemble onto either ends to finally form protofilaments. Eventually, these protofilaments come together to create the intermediate filaments. 

Around eight protofilaments make up one intermediate filament. These filaments are typically apolar as opposed to microfilaments and microtubules, which have polar ends.

This is primarily because of the fact that these intermediate fibers are made up of antiparallel tetramers.  

In terms of size, these intermediate fibers are typically between 8 and 10 nm in diameter. This gives meaning to their name “intermediate”, as they are not the smallest like microfilaments, but are not as large as microtubules.

These components are seen as being more stable compared to the other components, making them more permanent as a result. 

White these intermediate fibers are not components that experience dynamic instability like microtubules, the proteins in these filaments frequently get modified.

This typically happens through phosphorylation, and it plays a key role in their assembly inside a cell. 

Depending on the type of cell, intermediate filaments can vary greatly. However, they will extend from the cell membrane all the way to the nucleus surface.

The distinct and elaborate networks they create in the cytoplasm allow the fibers to associate with various other components in the cytoskeleton. This allows them to contribute to the function of the cell as a whole. 

Functions Of The Cytoskeleton

The cytoskeleton is known for helping the cell’s structural integrity remain strong. You may think of it as the scaffolding, or similar product, that keeps the cell in shape.

Without a cytoskeleton, the cell would have little to no structural integrity, and it would suffer. 

However, the cytoskeleton does more than keep the shape of a cell. It has a number of other functions that are important in the functioning of a cell.

So, in order to have a good understanding of the cytoskeleton, it is imperative that we take a look at these other important functions. Each component of the cytoskeleton plays a key role, which we will explore in the sections below. 

Microfilament Functions

The motile structure of the cells is usually what distributes the microfilaments, As such, they are able to be found in a number of different structures within a cell, such as the cilia and flagellum.

In these places, they play a key role in contributing to cell movement in a number of organisms. These actin filaments have also been found to have a role in the formation of other structures, however.

One of these structures is the lamellipodium, which allows cells to travel across substrates. 

However, they also have a purpose beyond cell motility. Microfilaments play a role in the movement of different organelles, which can be seen during cell division, where the actin ring is involved.

Not only that, but microfilaments, along with myosin, pay a key part in the pinching of cells in the middle. This eventually leads to the cell dividing.

When ATP energy is present, these two components also have a part in moving vesicles and organelles within a cell.

Actin filaments and myosin are also responsible for contraction in muscle cells.

This happens via the sliding activity of the actin filaments. This means that they play key parts in many processes and activities outside of being a part of a cell’s structural integrity.

Without microfilament, there would be no way of transporting cell components and materials, since the actin acts as a track of movement. 

Microtubules Function

These structures are some of the most stiff in cells, though this is particularly the case in animal cells.

With a high resilience, microtubules are great at protecting the cell components from a number of forces that could be extremely harmful.

These components are important in the way of protecting the cell from potentially damaging forces, but they also have roles in other areas, too. 

Microtubules are a key part of contributing to the cell’s internal environment via the architectural framework. They have been proven to organize various cell organelles in order to help establish cell polarity. 

These components are also important in chromosomal segregation. The microtubules make up part of the spindle apparatus.

This apparatus is responsible for separating chromosomes when cell division occurs. Because of this, they have a clear role in the cell division that frequently occurs.

Transportation is another key area where microtubules play a part. Here, microtubules contribute to the transport network inside cells much like microfilaments.

This component acts much as the tracks that materials can be transported on to get from one part of the cell to another. There are two groups of microtubules that make this possible, namely the dyneins and kinesins. 

Finally, microtubules also play a role in motility within the cell. The proteins that are associated with microtubules aid in generating movement and force in structures like the flagella. This helps increase motility within the cell.

Intermediate Filament Functions 

The intermediate filament primarily serves as structural support within the cell. When cells experience a lot of physical stress, these intermediate cells help keep it together and provide the support that the cell needs.

Since this component is more permanent compared to microfilaments and microtubules, they are often seen as the most important factor for maintaining cell structure. 

Without the intermediate filament, cells would likely be unable to retain structure, even with the help of the other components in the cytoskeleton. As such, these filaments are incredibly important in this regard. 

With that being said, intermediate filaments do play a part in other processes within the cell. Some of these are as follows:

• They aid in the stretching of the epithelial cells
• They aid in muscle contraction by forming the bridges between Z discs
• They help strengthen the nuclear membrane and protect the contents of the nucleus
• They support axons when they get larger

To Summarize 

The cytoskeleton is a crucial part of a cell. It helps the cell retain its structure with the help of its three primary components.

While the microfilaments, microtubules, and intermediate fibers aid in cell structure, they also have other roles to play within the cell. 

For microfilaments, their other main role is in cell motility. Without these actin filaments, the cell would be unable to function as it should. Not only that, but this component is also pivotal in pinching cells – a process in cell division. 

Microtubules, the largest component in the cytoskeleton, are responsible for motility, transportation, and also chromosomal segregation on top of cell structure.

These also help protect the cell against damage from external forces. As such, without microtubules, cells would be likely to sustain damage that would negatively impact them.

Intermediate filament is likely the most important component for structural support in the cytoskeleton. This component aids the cell in times of high stress and allows it to retain its structure without being damaged.

However, even intermediate filaments have other roles within the cytoskeleton and cell overall.

These include aiding muscle contraction, supporting increasing axon sizes, stretching epithelial cells, and strengthening the nuclear membrane. 

Each component of the cytoskeleton is crucial for a healthy and functioning cell. While all components are tiny, the help they provide allows the cell to do its job, and even be protected from outside forces.  

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