Have you ever heard of the cytoskeleton? It's like a scaffold inside a cell that helps all the organelles and molecules stay in place and move around. Basically, it's like a network of tiny filaments that reach all over the cell. Scientists used to think that everything inside a cell just floated around randomly, but they were wrong! They noticed that there was an order to things and that the cytoskeleton was responsible for it. Even though it's called a "skeleton," it's actually really flexible and does more than just hold things up.

Cytoskeleton definition

The cytoskeleton does a lot of important jobs for the cell! It helps to keep the cell in shape and flexible, and it also moves things around inside the cell. There are three types of protein fibers that make up the cytoskeleton in eukaryotic cells: microfilaments, intermediate filaments, and microtubules. Each type has a specific job and is made up of different materials. Even prokaryotic cells have a cytoskeleton, but it's not as complex as the one found in eukaryotic cells. Still, it helps them move around using their flagella. So, the cytoskeleton is like the cell's personal superhero, always there to save the day!

Cytoskeleton structure and function

The cytoskeleton is like the cell's skeleton, providing support and helping it to move around. It's made up of different parts, each with its own job. These components help the cell keep its shape, move things around, and function properly. In the next section, we'll talk more about these parts, what they're made of, and what they do. So get ready to learn about the cool components that make up the cytoskeleton!

Microfilaments:

Microfilaments are the thinnest of the cytoskeletal fibers, composed of only two intertwined protein threads. The threads are made up of chains of actin monomers, thus, microfilaments are commonly called actin filaments. Microfilaments and microtubules can be quickly disassembled and reassembled in different parts of the cell. Their primary function is to maintain or change the cell shape and to aid in intracellular transport (figure 1).

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Actin filaments form a dynamic mesh in the portions of the cytoplasm that are adjacent to the plasma membrane. This microfilament mesh is connected to the plasma membrane and, with the bordering cytosol, forms a gel-like layer all around the internal side of the membrane (note how in figure 1, left, the actin filaments are more abundant at the edge of the cytoplasm). This layer, called the cortex, contrasts with the more fluid cytoplasm in the interior. In cells with outward extensions of the cytoplasm (like microvilli in nutrient-absorbing intestinal cells), this microfilament network forms bundles that enlarge into the extensions and reinforce them  (figure 2).

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In muscle cells, actin filaments interact with thicker filaments of myosin to generate mechanical forces for physiological functions. The myosin filaments have “arms” that attach to two continuous actin filaments, and the myosin “arms” move along the microfilaments dragging them closer to each other, causing the muscle cell to contract. This type of cell movement is known as muscle contraction.

In addition to muscle contraction, actin and myosin proteins also allow for other types of cell movement. For example, cells can crawl by extending the leading edge primarily through remodeling of the actin cytoskeleton, forming new adhesive contacts at that leading edge while releasing adhesions to the rear, and bulk internal movement forward to “catch up” with the leading edge. Myosin proteins can also select bundled actin for motility, which is a measure of a motor's ability to move on bundles in preference to single actin filaments, given equal amounts of actin and motor.

Overall, actin filaments and myosin proteins provide structural support and cell motility. They allow for muscle contraction, cell crawling, and the selection of bundled actin for motility.

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Actin filaments play a crucial role in various types of cell movement, including ameboid movement, phagocytosis, cytoplasmic streaming, and cytokinesis.

In unicellular protists such as Amoeba, actin filaments facilitate the formation of pseudopodia, which are cytoplasmic extensions that help the cell to move along a surface. Similarly, animal cells like white blood cells use ameboid movement to crawl inside our body and engulf food particles or pathogens. This process is called phagocytosis.

Actin filaments are also involved in cytoplasmic streaming, which occurs in all eukaryotic cells but is particularly useful in large plant cells. The localized contractions of actin filaments and the cortex produce a circular flow of the cytoplasm inside the cell, which accelerates the distribution of materials through the cell. During cell division in animal cells, a contractile ring of actin-myosin aggregates forms the segmentation groove and keeps tightening until the cell’s cytoplasm divides into two daughter cells. This process is called cytokinesis, which is the part of cell division (meiosis or mitosis) where the cytoplasm of a single cell splits into the two daughter cells. In summary, actin filaments are versatile components of the cytoskeleton that play important roles in various cellular processes, including cell movement, phagocytosis, cytoplasmic streaming, and cytokinesis.

Intermediate filaments

Intermediate filaments have an intermediate diameter size between microfilaments and microtubules and vary in composition. Each type of filament is made up of a different protein, all belonging to the same family that includes keratin (the main component of hair and nails). Multiple strings of fibrous protein (like keratin) intertwine to form one intermediate filament.

Due to their sturdiness, their main functions are structural, such as reinforcing the shape of the cell and securing the position of some organelles (for example, the nucleus). They also coat the interior side of the nuclear envelope, forming the nuclear lamina. The intermediate filaments represent a more permanent support frame for the cell. Intermediate filaments are not disassembled as commonly as actin filaments and microtubules.

Microtubules

Microtubules are hollow structures composed of tubulin molecules, which are arranged to form a tube. They are the thickest of the cytoskeletal components and serve as tracks that guide organelles and other cellular components' movement. Each tubulin is a dimer made of two slightly different polypeptides, alpha-tubulin and beta-tubulin. Microtubules can be disassembled and reassembled in different parts of the cell, and their origin, growth, and anchorage are concentrated in regions of the cytoplasm called microtubule-organizing centers (MTOCs).

Microtubules play a crucial role in cell division, as they guide the movement of chromosomes during mitosis and meiosis. They also serve as tracks that guide vesicles from the endoplasmic reticulum to the Golgi apparatus and from the Golgi apparatus to the plasma membrane. Motor proteins like dynein can move along a microtubule, transporting attached vesicles and organelles inside the cell.

Cilia and flagella are extensions of the plasma membrane that serve in cell movement. Both structures have the same "9 + 2" pattern, which is composed of nine pairs of microtubules arranged in a ring and two microtubules in the center. The basal body anchors the microtubule assembly to the rest of the cell and has a "9 + 0" pattern, with nine groups of microtubules arranged in triplets and no microtubules in the center.

In summary, microtubules are the thickest of the cytoskeletal components and play a critical role in cell division, vesicle transport, and cell movement through cilia and flagella.

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The basal body is structurally very similar to a centriole with a “9 + 0” pattern of microtubules triplets. Indeed, in humans and many other animals, when a sperm enters the egg, the basal body of the sperm flagellum becomes a centriole.

How do cilia and flagella move?

Dyneins are motor proteins that are attached along the most external microtubule of each of the nine pairs that form a flagellum or cilium. They cause the bending of the microtubule by pulling on the outer microtubule of the adjacent pair and releasing it. Dyneins synchronize to be active only at one side of the flagellum or cilium at a time, alternating the direction of bending and producing a beating movement. A flagellum usually undulates, while a cilium moves in a back-and-forth motion.

Microfilaments, intermediate filaments, and microtubules are all components of the cytoskeleton. Microfilaments are composed of actin proteins and maintain or change the cell shape, aid in cell movement, and aid in intracellular transport. Intermediate filaments are composed of several intertwined fibrous filaments of proteins and provide structural support and secure the position of some organelles. Microtubules are hollow tubes composed of tubulin proteins and function in intracellular transport, chromosome movement during cell division, and are the structural component of cilia and flagella. Motor proteins are proteins that associate with cytoskeletal components to produce movement of the entire cell or components of the cell.

Cytoskeleton in animal cells

Animal cells have a unique cytoskeletal feature, the centrosome, which is a region found near the nucleus and functions as a microtubule-organizing center. The centrosome contains a pair of centrioles, which are composed of nine triplets of microtubules in a "9 + 0" arrangement. Centrosomes play a crucial role in cell division, as they help pull the duplicated chromosomes to opposite sides during cell division. However, the function of centrioles in other eukaryotic cells without centrioles is not entirely clear.

In animal cells, the cytoskeleton plays a more critical role in structural support and maintenance of cell shape than in plant cells. This is because plant cells have cell walls that are mainly responsible for support. The centrosome is mainly involved in cell division, and the centrioles in it are essential for the organization of microtubules.

Cytoskeleton - Key takeaways

The cytoskeleton is crucial for providing both structural support and flexibility to the cell, and it is composed of three types of protein fibers: microfilaments, intermediate filaments, and microtubules. Microfilaments or actin filaments provide mechanical support to maintain or change cell shape, generate cytoplasmic streaming, and participate in cytokinesis. Intermediate filaments are sturdier and provide a more permanent support frame for the cell and some organelles. Microtubules are hollow tubes composed of tubulin and serve as tracks that guide intracellular transport, pull chromosomes during cell division, and are the structural components of cilia and flagella.

Flagella and cilia are motile appendages formed by a ring of nine pairs of microtubules with two more in its center. They are used to move the entire cell (flagella and cilia) or substances along the surface of a tissue (cilia). A centrosome is a microtubule-organizing center found in animal cells that contains a pair of centrioles and is more active during cell division, where it plays a crucial role in pulling duplicated chromosomes to opposite sides.

Cytoskeleton

What is cytoskeleton?

Cytoskeleton is a dynamic internal frame made of proteins involved in structural support of the cell, maintenance and change of cell shape, intracellular organization and transport, cell division, and cell movement. 

What happens in the cytoskeleton?

Structural support, intracellular organization and transport, maintenance or changes in cell shape, and cell movement happen with the involvement of cytoskeletal elements and motor proteins.

What are the 3 functions of the cytoskeleton?

Three functions of the cytoskeleton are: structural support to the cell, guide the movement of organelles and other components within the cell, and movement of the entire cell.

Do plant cells have cytoskeleton?

Yes, plant cells have a cytoskeleton. However, unlike animal cells, they do not have a centrosome with centrioles.

What is the cytoskeleton made of?

The cytoskeleton is made of different proteins. Microfilaments are made of actin monomers, microtubules are made of tubulin dimers, and different types of intermediate filaments are made of one of several different proteins (for example, keratin).

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