The cytoskeleton is an extensive network of filamentous or tubular proteins found in the cytoplasm of cells, with a shape and composition that can vary according to the cell's needs (Laguna & Serrano, 2021). The cytoskeleton is characterized by its three-dimensional shape, which provides structure and volume to both the cytoplasm and cellular organelles, allowing them to carry out their functions (Rothschuh Osorio, 2023). It is a geodesic structure, where the balance of opposing forces maintains the stability of the whole. Additionally, it possesses flexibility and firmness due to the specific properties of the proteins that comprise it. In correspondence with Rothschuh Osorio (2023), the cytoskeleton is composed of four types of structures: microtubules, microfilaments, intermediate filaments, and cilia or flagella, each with a specific function.
Functions
The function of the cytoskeleton is multiple and fundamental for the cell (Rothschuh Osorio, 2023). On one hand, it organizes the cellular space by serving as the matrix on which organelles are located, ensuring that each of them fulfills its function in the appropriate place. Previously, organelles were thought to be dispersed in the cytosol, the liquid substance of the cytoplasm, but it was later revealed that the cytoplasm also contains a fiber matrix called the cytoskeleton (Rothschuh Osorio, 2023). On the other hand, the cytoskeleton supports the cell by maintaining its shape and rigidity, allowing it to adopt irregular shapes according to its needs (Laguna & Serrano, 2021). According to Rothschuh Osorio (2023), this is especially useful for animal cells, which lack a cell wall like plant cells.
Similarly, the cytoskeleton allows ordered movement within the cell, being flexible and facilitating the displacement of small movements that occur within the cell, such as cytoplasmic streaming in plant cells (Rothschuh Osorio, 2023). These movements are called cellular motility. Finally, in correspondence with Rothschuh Osorio (2023), the cytoskeleton regulates biochemical processes within the cell by allowing the flow of components manufactured within organelles, which can then be transported within the cell as part of its vital functions.
Structure
Cytoskeleton in Eukaryotic Cells
Microtubules are the largest cytoskeletal fibers of the three that exist, with a diameter of 25 nm (Khan Academy, n.d.). These fibers are formed by a hollow tube of tubulin proteins, which come in two forms: alpha and beta (Rothschuh Osorio, 2023; Khan Academy, n.d.). Like actin filaments, microtubules are dynamic structures that can grow and disassemble rapidly by adding or removing tubulin proteins, and they have directionality, meaning their ends are different from each other (Khan Academy, n.d.). Microtubules play an important structural role in the cell, as they enable it to resist compressive forces. Additionally, they have other more specialized functions, such as forming tracks for motor proteins kinesins and dyneins, which transport vesicles and other cargo within the cell. According to Khan Academy (n.d.), they also organize into a structure called the spindle during cell division, which is responsible for separating chromosomes.
The cytoskeleton is composed of three types of fibers, with microfilaments being the thinnest of them (Khan Academy, n.d.). These fibers have a diameter of 7 nm and are formed by the joining of many monomers of a protein called actin, which are organized in a structure similar to a double helix (Khan Academy, n.d.). For this reason, microfilaments are also called actin filaments (Khan Academy, n.d.). According to Khan Academy (n.d.), actin filaments exhibit directionality, meaning their ends have a distinct structure.
Actin filaments perform various functions in the cell, such as serving as tracks for the movement of a motor protein called myosin, which also forms filaments, and participating in many cellular functions that require movement, due to their relationship with myosin (Khan Academy, n.d.). For example, in animal cell division, an actin and myosin ring divides the cell into two daughter cells. Additionally, in correspondence with Khan Academy (n.d.), actin filaments are abundant in muscle cells, where they constitute overlapping filament structures called sarcomeres, which allow muscle contraction by sliding actin and myosin filaments past each other.
Actin filaments also function as tracks within the cell for the transport of cargoes, such as vesicles with proteins or even organelles, which are carried by individual myosin motors that "walk" along the actin filament bundles (Khan Academy, n.d.). Actin filaments have the ability to assemble and disassemble rapidly, allowing them to play an important role in cellular motility, such as the movement of white blood cells in the immune system. Finally, according to Khan Academy (n.d.), actin filaments have essential structural functions in the cell, as they form a network in the outermost region of the cytoplasm, which is connected to the plasma membrane by special proteins, giving shape and structure to the cell.
Intermediate filaments are fibers composed of different fibrous proteins, the type of which varies depending on the cellular tissue where they are found (Rothschuh Osorio, 2023; Khan Academy, n.d.). For example, keratin is a protein that forms intermediate filaments in hair, nails, and skin (Khan Academy, n.d.). These fibers have a size intermediate between microfilaments and microtubules, with a diameter between 8 and 10 nm (Khan Academy, n.d.). These fibers are only found in animal cells and are the strongest fibers of the cytoskeleton (Rothschuh Osorio, 2023). Unlike microfilaments, intermediate filaments are more stable and have an important structural function in the cell (Khan Academy, n.d.). According to Khan Academy (n.d.), their specialty is to resist tension, and among their functions, they stand out in maintaining cell shape and anchoring the nucleus and other organelles in position.
In eukaryotic cells, microtubules are part of three more specialized structures: flagella, cilia, and centrosomes (Khan Academy, n.d.). Flagella are cellular extensions resembling hairs that allow the movement of the entire cell, as in sperm cells. Flagella are scarce, and if a cell possesses them, it usually has one or a few. Cilia are similar to flagella but shorter and more abundant. When motile cilia cover the cells of a tissue, their synchronized movement facilitates the transport of materials over the tissue surface. For example, in correspondence with Khan Academy (n.d.), cilia in the cells of the upper respiratory system help to remove dust and particles towards the nasal passages.
Despite their differences in number and size, motile flagella and cilia have a common internal structure. This consists of 9 pairs of microtubules arranged in a circle, with an extra pair of microtubules in the center of the ring. Motile flagella and cilia move thanks to motor proteins called dyneins, which travel along the microtubules generating force. The coordinated movement of dyneins and the structural connections between the pairs of microtubules result in a regular beating pattern. Another characteristic is that the cilium or flagellum has a basal body at its base. The basal body is composed of microtubules and plays a fundamental role in the assembly of the cilium or flagellum. Additionally, the basal body regulates the entry and exit of proteins. According to Khan Academy (n.d.), the basal body is actually a modified centriole.
Centrioles are better known for their function in centrosomes, which are structures that act as microtubule-organizing centers in animal cells (Khan Academy, n.d.). A centrosome consists of two centrioles arranged at right angles to each other and surrounded by a mass of "pericentriolar material," which provides anchoring sites for microtubules. The centrosome duplicates before cell division, and the pair of centrosomes appears to have an important function in organizing the microtubules that separate the chromosomes during cell division. However, the exact function of centrioles in this process is still unclear. According to Khan Academy (n.d.), some cells can divide without centrosomes, such as plant cells, which do not have them, or animal cells from which the centrosome has been removed.
Cytoskeleton in Prokaryotic Cells
The cytoskeleton of prokaryotic cells was unknown until recently, as it was thought that only eukaryotic cells possessed these structures (Rothschuh Osorio, 2023). However, it has been discovered that prokaryotic cells also have a cytoskeleton, which performs similar functions to those of eukaryotic cells but is formed by different proteins. According to Rothschuh Osorio (2023), among these proteins are: MreB and ParM, which resemble actin; proteins of the WACA family, which participate in the biogenesis and assembly of cilia and flagella in unicellular organisms; crescentin, which is equivalent to intermediate filaments; and FtsZ, which is analogous to tubulin.
References
Khan Academy. (s.f.). El Citoesqueleto. Khan Academy. Recuperado 13 de febrero de 2024, de https://es.khanacademy.org/science/biology/structure-of-a-cell/tour-of-organelles/a/the-cytoskeleton
Laguna, M., & Serrano, C. (2021, septiembre 21). Citoesqueleto. Ken Hub. https://www.kenhub.com/es/library/anatomia-es/citoesqueleto
Rothschuh Osorio, U. (2023, noviembre 30). Citoesqueleto: Qué es, Características, Función y Estructura. Ecología Verde. https://www.ecologiaverde.com/citoesqueleto-que-es-caracteristicas-funcion-y-estructura-4675.html
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