The cytoskeleton is a dynamic and intricate network of protein filaments located within the cytoplasm of all eukaryotic cells. It serves as the cell’s internal scaffolding, providing structural support and enabling various cellular processes. This network constantly reorganizes, allowing cells to adapt and respond to changes in their internal and external environments. The cytoskeleton’s flexibility and adaptability are central to its role in maintaining cellular integrity and facilitating diverse functions.
Maintaining Cell Shape and Stability
The cytoskeleton provides the necessary structural support for cells to maintain their distinct shapes and resist external mechanical forces. Intermediate filaments, which are rope-like fibers with a diameter of about 10 nanometers, provide tensile strength, allowing cells to stretch and resist tearing. These filaments form a supportive mesh around the nucleus and throughout the cytoplasm, anchoring organelles and contributing to the integrity of tissues, such as skin, hair, and nails.
Microtubules, which are rigid, hollow rods approximately 25 nanometers in diameter, also play a significant role in maintaining cell shape, especially in elongated cells, by providing internal rigidity. They are dynamic structures that undergo continuous assembly and disassembly, a process called dynamic instability, which allows for rapid changes in cell morphology. Their dynamic behavior, often stabilized by microtubule-associated proteins (MAPs), helps determine cell shape and polarity.
Microfilaments, also known as actin filaments, are the thinnest components of the cytoskeleton, measuring about 7 nanometers in width. Composed of actin, they form dense networks beneath the plasma membrane, known as the cell cortex. This cortical network supports the plasma membrane and enables changes in the cell’s surface, contributing to cell shape and enabling processes like microvilli or lamellipodia formation. The cell cortex generates tension under the cell membrane, allowing the cell to change shape during migration or division.
Powering Cell Movement
The cytoskeleton is the driving force behind the movement of entire cells, enabling various forms of cellular movement. Amoeboid movement, characteristic of cells like white blood cells, involves the dynamic reorganization of actin filaments. This process is driven by the polymerization of actin at the leading edge, pushing the membrane forward, and often involves the contraction of myosin at the rear. While myosin contributes to contraction, actin polymerization alone can induce cell polarity and retrograde flow, enabling movement in various environments.
The beating of cilia and flagella, found on some cell surfaces, is another example of cytoskeleton-powered movement. These whip-like appendages are composed of microtubules arranged in a 9+2 axoneme structure, with nine doublet microtubules surrounding a central pair. Motor proteins, particularly dynein, cause these microtubules to slide, leading to bending movements that enable cells like sperm to swim or clear airways.
General cell migration, observed during embryonic development, wound healing, and immune responses, also relies heavily on the cytoskeleton. Cells polarize and extend protrusions, such as lamellipodia and filopodia, at their leading edge. These extensions are formed by actin reorganization and are regulated by actin-binding proteins and Rho GTPases. Subsequent contractions, often involving actin and myosin, propel the cell forward as adhesive attachments at the trailing edge are released.
Orchestrating Internal Traffic and Cell Duplication
The cytoskeleton orchestrates complex internal cellular activities, including intracellular transport and cell duplication. Microtubules act as “railroad tracks” for long-distance transport within the cell. Motor proteins, kinesins and dyneins, bind to cellular cargo, such as organelles, vesicles, and macromolecules, and “walk” along these microtubule tracks. Kinesins move cargo towards the “plus end” of the microtubule, often away from the cell center, while dyneins transport cargo towards the “minus end,” usually towards the nucleus or cell interior.
Microfilaments are also involved in intracellular transport, for shorter distances and in processes like cytoplasmic streaming. Cytoplasmic streaming is the large-scale flow of cytoplasm within a cell, often observed in large plant cells, amoebae, and fungi. This movement is driven by myosin motor proteins attached to organelles, which move along fixed actin filament bundles, moving the surrounding cytoplasm. This process helps to speed up the transport of molecules and organelles throughout the cell, especially in cells where diffusion alone would be too slow.
During cell duplication, or cell division, microtubules and microfilaments play distinct yet coordinated roles to ensure accurate chromosome segregation and cell partitioning. Microtubules form the mitotic spindle, responsible for segregating chromosomes. Kinetochore microtubules attach to specialized regions on condensed chromosomes called kinetochores, pulling sister chromatids to opposite poles of the dividing cell. This dynamic process ensures each daughter cell receives a complete set of genetic material.
Following nuclear division, microfilaments, specifically actin filaments, form a contractile ring that divides the cytoplasm in a process called cytokinesis. This ring, composed of filamentous actin and the motor protein myosin-2, forms just inside the plasma membrane at the cell’s equator. As the actin ring contracts, it creates a cleavage furrow that deepens, eventually pinching the cell into two separate daughter cells. Its formation and contraction are regulated to ensure equal distribution of cellular components.