Cytoskeleton- Definition, Structure, Functions and Diagram

The cytoskeleton is a network of protein fibers that extends throughout the cytoplasm of eukaryotic cells. The word "cytoskeleton" comes from the Greek words "kytos", meaning "cell", and "skeletos", meaning "skeleton". The cytoskeleton provides structural support, shape, and movement to the cell and its organelles. It also plays important roles in cell division, signaling, and transport.

The cytoskeleton is composed of three main types of filaments: microtubules, microfilaments, and intermediate filaments. Each type of filament has a different structure, function, and distribution in the cell. The cytoskeleton is not a static structure, but rather a dynamic and adaptable one that can change in response to the needs of the cell. The cytoskeleton can assemble and disassemble its filaments, rearrange them into different patterns, and interact with other proteins and molecules.

The cytoskeleton is essential for the survival and function of eukaryotic cells. It enables cells to maintain their shape and integrity, resist mechanical stress, and adapt to different environments. It also allows cells to perform complex movements such as crawling, swimming, and contracting. Furthermore, it facilitates the transport of materials and signals within and between cells. The cytoskeleton is involved in many cellular processes such as cell growth, differentiation, communication, and death.

In this article, we will explore the structure, functions, and diagram of the cytoskeleton in more detail. We will also introduce a fourth component of the cytoskeleton called the microtrabecular lattice. We will start by describing the microtubules and their functions in the next section.

The cytoskeleton is composed of three main types of fibers: microtubules, microfilaments and intermediate filaments. These fibers differ in their diameter, structure, composition and functions. The diagram below shows the general organization and distribution of these fibers in a typical animal cell.

As you can see from the diagram, microtubules are the thickest and most rigid fibers, forming a network that radiates from the centrosome near the nucleus. Microtubules are involved in cell division, intracellular transport and movement of cilia and flagella.

Microfilaments are the thinnest and most flexible fibers, forming a meshwork under the plasma membrane and extending into cellular projections such as microvilli and pseudopodia. Microfilaments are involved in cell shape, muscle contraction and cell motility.

Intermediate filaments are intermediate in size and stability, forming a scaffold that supports the nuclear envelope and other organelles. Intermediate filaments are involved in cell strength, tissue integrity and cell adhesion.

In addition to these three types of fibers, some cells also contain a microtrabecular lattice, which is a three-dimensional network of interlinked filaments that fills the cytoplasm and anchors various cellular components. The microtrabecular lattice is involved in cell shape changes and cytoplasmic streaming.

The cytoskeleton is not a static structure, but rather a dynamic one that can rearrange itself according to the needs of the cell. The cytoskeleton is regulated by various proteins that bind to, modify or sever the fibers. The cytoskeleton also interacts with other cellular structures such as membranes, organelles and extracellular matrix.

The cytoskeleton is essential for many cellular processes and functions. In the next points, we will explore each type of fiber in more detail and learn about their specific roles in the cell.

Microtubules are the thickest components of the cytoskeleton, a network of protein filaments that gives shape and support to cells . They are hollow cylinders with a diameter of about 24 nanometers and a wall thickness of about 6 nanometers . They are composed of alpha and beta tubulin, two globular proteins that form dimers and then polymerize into protofilaments . A single microtubule contains 10 to 15 protofilaments, usually 13 in mammalian cells, that wind together to form the tube . Microtubules can grow or shrink by adding or removing tubulin molecules .

Microtubules have polarity, meaning they have two distinct ends: a plus (+) end and a minus (-) end . The plus end has beta-tubulin exposed and grows faster than the minus end, which has alpha-tubulin exposed . In animal cells, microtubules radiate outwards from an organelle called a centrosome, which is a microtubule organizing center (MTOC) . In plant and fungal cells, the nuclear envelope serves as an MTOC.

Microtubules have various roles in cell movement, division, and transport of materials . Some of their functions are:

• Cell Movement: Microtubules give structure and motility to cilia and flagella, which are hair-like or whip-like extensions of some cells that help them move or move fluids around them . Cilia are found in cells lining the respiratory tract and the fallopian tubes, where they prevent foreign particles from entering the lungs or help transport the egg to the uterus, respectively. Flagella are found in some bacteria and human sperm cells, where they enable swimming. Microtubules also allow whole cells to crawl or migrate by contracting at one end of the cell and expanding at another.
• Cell Division: Microtubules play a key role in forming the mitotic spindle, a structure that is formed during mitosis (cell division) in eukaryotic cells . The mitotic spindle consists of microtubules that originate from the centrosomes and attach to the chromosomes at their centromeres (the regions where sister chromatids are joined) . The microtubules then pull the sister chromatids apart and move them to opposite poles of the cell, ensuring equal distribution of genetic material between the daughter cells .
• Transport of Materials: Microtubules serve as tracks for the movement of organelles and vesicles within the cell . This movement is powered by motor proteins that bind to microtubules and use ATP to walk along them. Some examples of motor proteins are kinesin and dynein, which move towards the plus and minus ends of microtubules, respectively. By transporting organelles and vesicles, microtubules facilitate processes such as secretion, endocytosis, phagocytosis, and intracellular signaling.

Microfilaments are the thinnest cytoskeletal fibers, with a diameter of about 7 nm. They are composed of two intertwined strands of a globular protein called actin. For this reason, microfilaments are also known as actin filaments.

Actin is powered by ATP to assemble its filamentous form, which serves as a track for the movement of a motor protein called myosin. This enables actin to engage in cellular events requiring motion, such as cell division in animal cells and cytoplasmic streaming in plant cells.

Some of the functions of microfilaments are:

• They maintain the shape of the cell by forming a network under the plasma membrane called the cortex. The cortex gives the cell mechanical strength and flexibility. It also allows the cell to change shape and move by extending and retracting projections called pseudopodia (false feet).
• They form contractile components of cells, mainly of the muscle cells. In muscle cells, actin and myosin filaments are arranged in parallel bundles called sarcomeres. When these filaments slide past each other, they cause muscle contraction.
• They enable white blood cells to move to the site of an infection and engulf the pathogen by a process called phagocytosis. Microfilaments help to form the phagocytic cup that surrounds and engulfs the foreign particle.
• They participate in cytokinesis, which is the division of the cytoplasm during cell division. In animal cells, a ring of actin and myosin filaments forms at the equator of the cell and constricts to form a cleavage furrow that separates the two daughter cells. In plant cells, microfilaments help to guide vesicles containing cell wall materials to the middle of the cell, where they fuse to form a cell plate that divides the two daughter cells.

Microfilaments are dynamic structures that can rapidly assemble and disassemble depending on the needs of the cell. They are regulated by various proteins that bind to actin and affect its polymerization, stability, and interactions with other molecules.

Intermediate filaments (IFs) are the fibers of the middle-order in the cytoskeleton, having a diameter of about 10 nm. They are composed of a family of related proteins that share common structural and sequence features. Unlike microtubules and microfilaments, IFs are not powered by nucleotides and do not have polarity or dynamic instability. They are more stable and less prone to depolymerization than the other cytoskeletal elements.

There are six major types of IFs in eukaryotic cells, each composed of different proteins and expressed in different cell types. They are:

• Type I and II: Keratin filaments, found in epithelial cells that line the surfaces and cavities of the body. They provide mechanical strength and resistance to abrasion and stress. They also form the main component of hair, nails, feathers, horns, and scales.
• Type III: Vimentin filaments, found in mesenchymal cells such as fibroblasts, endothelial cells, and immune cells. They help maintain cell shape and integrity, as well as anchoring organelles and stabilizing cell junctions.
• Type IV: Neurofilaments, found in neurons. They support the long axons of nerve cells and regulate their diameter, which affects the speed of nerve impulses.
• Type V: Nuclear lamins, found in the inner membrane of the nucleus. They form a meshwork that supports the nuclear envelope and chromatin, as well as playing a role in DNA replication, transcription, and cell cycle regulation.
• Type VI: Nestin filaments, found in neural stem cells and some muscle cells. They are involved in cell division and differentiation.

The functions of IFs can be summarized as follows:

• Intermediate filaments contribute to cellular structural elements and are often crucial in holding together tissues like skin.
• Intermediate filaments help cells withstand mechanical stress and deformation by forming a network that distributes tension across the cell.
• Intermediate filaments anchor organelles such as the nucleus, mitochondria, and lysosomes to specific locations within the cell.
• Intermediate filaments interact with other cytoskeletal elements such as microtubules and microfilaments to coordinate cell movement and shape changes.
• Intermediate filaments participate in signal transduction pathways that regulate cell growth, survival, and differentiation.

Besides the three main types of cytoskeletal fibers, there is another component of the cytoskeleton that has been discovered more recently. This is the microtrabecular lattice, a three-dimensional network of interlinked filaments that fills the cytoplasm and connects to the other cytoskeletal elements.

The microtrabecular lattice is composed of actin-binding proteins that cross-link actin filaments and other cytoskeletal fibers into a mesh-like structure. The lattice is dynamic and can change its shape and organization in response to cellular signals and mechanical forces.

The microtrabecular lattice has several functions in the cell, such as:

• Anchoring organelles. The lattice provides a scaffold for the attachment and positioning of various cellular organelles, such as ribosomes, lysosomes, mitochondria, and endoplasmic reticulum .
• Regulating cytoplasmic viscosity. The lattice modulates the fluidity and movement of the cytoplasm, which affects the transport and diffusion of molecules and vesicles.
• Facilitating cell shape changes and motility. The lattice can deform and reorganize to allow the cell to change its shape and move in different directions. For example, the lattice is involved in the formation of pseudopods, lamellipodia, and filopodia, which are protrusions of the plasma membrane that enable cell crawling.
• Transmitting mechanical signals. The lattice can sense and transmit forces from the extracellular matrix or neighboring cells to the nucleus and other organelles, which can trigger changes in gene expression and cellular behavior.

The microtrabecular lattice is an important component of the cytoskeleton that contributes to the structural and functional organization of the cell. It is still an active area of research, as many aspects of its composition, regulation, and interactions are not fully understood.

The cytoskeleton is a network of protein fibers that provides structural support, shape, and movement to the cells. It consists of three main types of filaments: microtubules, microfilaments, and intermediate filaments. Each type of filament has a distinct structure, composition, and function in the cell.

Microtubules are hollow tubes made of tubulin subunits that can assemble and disassemble at their ends. They are involved in various cellular processes such as:

• Transporting vesicles, organelles, and molecules along their tracks using motor proteins like kinesin and dynein.
• Forming the spindle apparatus that separates chromosomes during cell division.
• Organizing the centrosome, which is the main microtubule-organizing center in animal cells.
• Building the cilia and flagella, which are hair-like extensions that enable cells to swim or move fluids.

Microfilaments are thin strands made of actin subunits that can twist into helical structures. They are involved in various cellular processes such as:

• Maintaining the cell shape and resisting tension by forming a cortex under the plasma membrane.
• Forming contractile bundles that interact with myosin motor proteins to generate force and movement in muscle cells and other cell types.
• Forming pseudopods that extend and retract to allow cells to crawl or engulf particles.
• Forming the cleavage furrow that pinches the cell into two during cytokinesis.

Intermediate filaments are rope-like structures made of various proteins that have a similar structure but different amino acid sequences. They are involved in various cellular processes such as:

• Providing mechanical strength and stability to the cells and tissues by forming a network that connects the nucleus, organelles, and plasma membrane.
• Supporting the nuclear envelope and anchoring the nuclear lamina, which is a meshwork of proteins that lines the inner surface of the nucleus.
• Forming specialized structures such as keratin filaments in epithelial cells, desmin filaments in muscle cells, neurofilaments in nerve cells, and vimentin filaments in mesenchymal cells.

In addition to these three types of filaments, some cells also have a microtrabecular lattice, which is a three-dimensional network of interlinked cytoskeletal fibers. This lattice may serve as an anchor for various cellular components such as ribosomes and lysosomes.

The cytoskeleton is not a static structure but a dynamic one that constantly changes its organization and function in response to the needs of the cell. It is regulated by various factors such as ATP, calcium ions, phosphorylation, and interactions with other proteins. The cytoskeleton plays a vital role in many cellular functions such as cell division, cell motility, intracellular transport, cell signaling, and cell differentiation.

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