The cytoskeleton is a sophisticated, three-dimensional network of protein filaments that extends throughout a cell’s cytoplasm. This internal scaffolding acts as the cell’s skeleton, providing mechanical support and determining its overall shape. Far from being a static framework, this network is highly dynamic, constantly assembling and disassembling in response to the cell’s immediate needs. This dynamic internal architecture facilitates internal organization, enables cellular movement, and is directly involved in critical processes like cell division.
The Three Primary Filament Types
The cytoskeleton is constructed from three distinct types of protein filaments, differentiated primarily by their composition and diameter.
Actin Filaments (Microfilaments)
Actin filaments, also known as microfilaments, are thin, flexible fibers measuring about seven nanometers in diameter. Actin filaments are polymers formed from globular actin subunits and are typically concentrated just beneath the cell membrane, where they form a dense network that influences the cell’s surface shape and flexibility.
Intermediate Filaments
Intermediate filaments represent the middle tier, with a diameter ranging from eight to twelve nanometers. Unlike the other two types, intermediate filaments are composed of a wide variety of proteins, such as keratin in epithelial cells or neurofilaments in nerve cells. These filaments are relatively stable and non-polar, providing tensile strength and mechanical resistance against physical stress.
Microtubules
The largest components are the microtubules, which are rigid, hollow cylinders approximately 25 nanometers wide. They are built from the polymerization of alpha and beta tubulin protein dimers. Microtubules often originate from a central organizing center and radiate outwards, functioning as structural elements that resist compressive forces and establish long-range organizational pathways within the cell.
Providing Structural Support
The cytoskeleton’s static role involves maintaining the specific, often complex, morphology required for different cell types. For instance, the long, slender shape of a neuron’s axon or the flattened, sheet-like structure of an epithelial cell is stabilized by the underlying cytoskeletal framework.
Intermediate filaments are particularly specialized for this supportive role, acting like strong internal cables that withstand pulling forces and mechanical strain. This high tensile strength is why these filaments are abundant in tissues that experience significant wear and tear, such as the skin. These filaments also help anchor the nucleus and other organelles in fixed positions.
Microtubules contribute by acting as stiff girders that resist compression, helping to establish and maintain cell polarity, which is the structural and functional difference between one end of a cell and the other. Actin filaments, concentrated near the plasma membrane in a layer called the cortex, provide localized mechanical support. This cortical layer helps define the cell’s surface features and contributes to its ability to resist tension near its edges.
Enabling Cell Dynamics and Mobility
Beyond its structural duties, the cytoskeleton is a dynamic machine that powers movement both inside the cell and for the cell itself.
Intracellular Transport
Intracellular transport relies heavily on microtubules, which serve as “railroad tracks” for molecular motor proteins. Specialized proteins like kinesin and dynein bind to cargo, such as vesicles or organelles, and use the energy from ATP hydrolysis to “walk” along the microtubule surface. Kinesin motors generally move cargo toward the periphery of the cell, while dynein motors transport materials toward the cell’s center, ensuring precise delivery.
Cell Motility
Actin filaments govern cell motility, enabling cells like white blood cells to crawl or change shape rapidly. This is achieved through the controlled polymerization and depolymerization of actin monomers, which push the cell membrane forward to create extensions like lamellipodia.
Cell Division
The cytoskeleton is also fundamental to cell division, a process requiring highly coordinated movement. Microtubules form the mitotic spindle apparatus, which captures and separates duplicated chromosomes into the two daughter cells. Following chromosome separation, a contractile ring composed of actin filaments and myosin motor proteins constricts, pinching the cell membrane in two to complete cytokinesis.