It is the framework that shapes a cell.
The cytoskeleton is a network of fibers that form the “infrastructure” of eukaryotic cells, prokaryotic cells, and archaea.
Just like in a human being, the skeleton also helps keep all the organs in place.
In eukaryotic cells , these fibers consist of a complex mesh of motor protein filaments that aid in cell movement and stabilize the cell.
The cytoskeleton extends throughout the cell’s cytoplasm and directs several important functions.
- It helps the cell maintain its shape and supports the cell.
- A variety of cellular organelles are held in place by the cytoskeleton.
- Helps the formation of vacuoles.
- The cytoskeleton is not a static structure, but it can disassemble and reassemble its parts to allow internal and general mobility of the cell.
- The types of intracellular movement supported by the cytoskeleton include transport of vesicles into and out of a cell, manipulation of chromosomes during mitosis and meiosis, and migration of organelles.
- The cytoskeleton makes cell migration possible since cell motility is necessary for the construction and repair of tissues, cytokinesis (the division of the cytoplasm) in the formation of daughter cells and in the responses of immune cells to germs.
- The cytoskeleton helps in the transport of communication signals between cells.
- It forms bulges similar to cell appendages, such as cilia and flagella, in some cells.
Components of the cytoskeleton
The eukaryotic cytoskeleton is a network of three long filament systems, created from the repetitive assembly and disassembly of dynamic protein components.
The primary filament systems that comprise the cytoskeleton are microtubules, actin filaments, and intermediate filaments.
It creates an internal architecture to give the cell its shape through elaborate bonds to itself, the plasma membrane, and internal organelles.
Through a series of intercellular proteins, the cytoskeleton gives the cell its shape, provides support, and facilitates movement through three main components: microfilaments, intermediate filaments, and microtubules.
The cytoskeleton helps the cell move in its environment and controls the movement of all internal functions of the cell.
Microtubules have different functions that contribute to the work of the cytoskeleton.
Microtubules are hollow rods that function primarily to help support and shape the cell, form the centrioles of a cell, are the basis for flagella and cilia in a cell, and function as pathways or the pathway by which they displace the transport vesicles (organelles).
Microtubules are typically found in all eukaryotic cells. They vary in length and are approximately 25 nm (nanometers) in diameter.
A microtubule is made up of tubulin proteins arranged to form a hollow straw-like tube, and each tubulin protein consists of two subunits, α-tubulin and β-tubulin.
Microfilaments are the smallest of the three parts of the cytoskeleton, being only about seven nanometers in diameter.
These helically shaped filaments are made up of G-actin proteins.
Actin microfilaments or filaments are thin, solid rods that are active in muscle contraction.
Microfilaments are particularly prevalent in muscle cells.
Similar to microtubules, they are typically found in all eukaryotic cells.
Microfilaments are primarily made up of the contractile protein actin. They also participate in the movement of organelles.
The middle filaments are slightly larger, eight to twelve nanometers, and these keratin-based filaments roll together to form a cord shape.
Intermediate filaments can be abundant in many cells and provide support for microfilaments and microtubules by holding them in place.
These filaments are found in epithelial cells and neurofilaments in neurons.
A number of motor proteins are found in the cytoskeleton.
As their name suggests, these proteins actively move the fibers of the cytoskeleton.
As a result, the molecules and organelles are transported around the cell.
Motor proteins are powered by ATP, which is generated through cellular respiration.
There are three types of motor proteins involved in cell movement.
These move by microtubules carry cellular components along the way and are normally used to move organelles towards the cell membrane.
Dyneins are similar to kinesins and are used to push cellular components into the nucleus.
The dyneins also work to slide microtubules past each other, as seen in the movement of cilia and flagella.
Myosins interact with actin to make muscle contractions. They are also involved in cytokinesis, endocytosis, and exocytosis.
The cytoskeleton helps make cytoplasmic transmission possible.
Also known as cyclosis, this process involves the movement of the cytoplasm to circulate nutrients, organelles, and other substances within a cell.
Cyclosis also aids in endocytosis and exocytosis, or the transport of substance in and out of a cell.
As the microfilaments of the cytoskeleton contract, they help direct the flow of cytoplasmic particles.
When the microfilaments attached to the organelles contract, the organelles crawl and the cytoplasm flows in the same direction.
Cytoplasmic transmission occurs in both prokaryotic and eukaryotic cells.
In protists, such as amoebae, this process produces extensions of the cytoplasm known as pseudopods.
These structures are used to capture food and for locomotion.
The structure of the cytoskeleton is modified by adhesion to neighboring cells or to the extracellular matrix.
The strength and type of these adhesions are essential to regulate the assembly or disassembly of the components of the cytoskeleton.
This dynamic property allows cell movement, which is governed by forces (internal and external).
This information is detected by mechanosensors and disseminated through the cytoskeleton, leading to chemical signaling and response.
Although the subunits of the three filament systems are present throughout the cell, the differences in the structures of the subunits and the attractive forces between them impart to each system variable stability and different mechanical properties.
These characteristics explain its distribution in particular structures and regions of the cell.
Numerous proteins associated with the cytoskeleton also help regulate the spatial and temporal distribution of the cytoskeleton.
The organization and assembly of one filament system are influenced by the others in a coordinated way for most cellular functions.
Accessory proteins organize filaments into higher-order structures.
Crosslinking of the filaments by specific motors or multivalent binding proteins (accessory proteins) increases stability and forms higher order structures.
This organization facilitates the generation of long-term contractile forces and, at times, admits compressive forces while being dynamic.
These structures are connected across cells through junctions and thus facilitate mechanotransduction and cumulative response at the tissue or organ level.
Accessory proteins are a critical part of the signaling network that integrates extra and intracellular signals (for example, force, ions, among others) with the assembly modules of the cytoskeleton.
These can be specific to certain types of filaments. For example, fimbrin binds only to actin filaments, while others such as plectin are not specific.
Accessory factors can also help regulate stability, mechanical properties, and force production for individual filaments within the larger structure.
For example, fascina crosslinks actin filaments into rigid bundles that have mechanical strength to generate outstanding force, while filamin crosslinks actin filaments into gel-like networks that are flexible and produce less force.
Tropomyosin stabilizes actin filaments and regulates myosin association to control the time of contraction.
The microtubule organization center creates a global organization of the microtubule network to establish the polarity and positioning of the cell organelles.
The nuclear lamina is composed of intermediate filaments and the mitotic spindle (made of microtubules). The laminae are mechanically tensioned with the continuous network of chromosomes and nuclear matrix.
The intermediate filaments also form flexible cables from the cell surface to the center to form a “cage” around the nucleus.
These accessory protein-equipped structures have additional strength relative to individual filaments.
For example, filaggrin groups keratin filaments in the top layer of skin cells, providing resistance to physical stress and water loss.