Cytoskeleton is the dynamic internal structural network that helps cells maintain shape, organise their contents, move materials, change morphology and, in many cases, generate mechanical force. It is composed principally of three major filament systems in eukaryotic cells: actin filaments, microtubules and intermediate filaments. These systems differ in molecular composition, diameter, mechanical properties, organisation and associated proteins. The cytoskeleton is not comparable to a rigid scaffold because its filaments are continuously assembled, disassembled, rearranged and transported. Motor proteins and regulatory proteins interact with cytoskeletal filaments to produce movement and organise intracellular structures. Cytoskeletal organisation is therefore central to cell architecture, intracellular transport, cell division, migration and responses to mechanical forces.
Actin filaments are polymers of actin and are particularly important near the cell cortex, where they help determine cell shape and interact with the plasma membrane. Actin networks and bundles participate in cell migration, endocytosis, cytokinesis and muscle contraction. Myosin motor proteins move along actin filaments and can generate contractile forces or transport selected cargo. Actin polymerisation can itself generate force as filaments grow against membranes, contributing to processes such as protrusion during cell migration. Actin organisation is controlled by numerous binding proteins that regulate nucleation, elongation, branching, bundling, severing and turnover. Consequently, cells can rapidly remodel actin networks in response to signals and mechanical requirements.
Microtubules are cylindrical polymers composed of alpha and beta tubulin. They have a larger diameter than actin filaments and possess structural polarity, with plus and minus ends that differ in growth and associated regulatory behaviour. Microtubules exhibit dynamic instability, alternating between phases of growth and shortening. Their organisation is particularly important for long range intracellular transport and chromosome segregation during cell division. Kinesin and dynein motor proteins transport vesicles, organelles and protein complexes along microtubules. In many animal cells, microtubules are organised from centrosomes, although other nucleation sites and arrangements occur in different cell types and organisms.
Intermediate filaments are a diverse family of fibrous proteins that provide mechanical strength and resistance to deformation. Unlike actin and microtubules, intermediate filaments are not generally polar structures and do not serve as conventional tracks for ATP dependent motor proteins. Different cell types express different intermediate filament proteins, including keratins in epithelial cells, vimentin in many mesenchymal cells and neurofilament proteins in neurons. Intermediate filaments can connect to cell junctions and the nuclear envelope, helping distribute mechanical forces through tissues. Their properties make them especially important where cells experience stretching, compression or other mechanical stresses.
The cytoskeleton is essential for intracellular transport. Molecular motors move cargo along defined filament tracks, allowing vesicles, organelles and molecular complexes to reach specific cellular locations. Kinesins generally transport many types of cargo toward microtubule plus ends, while dyneins generally move cargo toward minus ends, although specific motor behaviour depends on the molecular context. Myosins transport cargo along actin networks and can generate contractile forces. Cytoskeletal transport works with membrane trafficking systems to position mitochondria, endosomes, lysosomes, secretory vesicles and other structures. This organised transport is particularly important in large cells such as neurons, where distances between the cell body and distal processes can be substantial.
The cytoskeleton is central to cell division. During mitosis, microtubules form the mitotic spindle, which attaches to chromosomes and helps segregate duplicated chromosome sets. Actin and myosin contribute to cytokinesis in many animal cells by forming a contractile ring that constricts the cell cortex between daughter cells. Cytoskeletal rearrangements also occur during meiosis and during developmental changes in cell shape. Accurate regulation of filament assembly and motor activity is essential because errors can cause chromosome missegregation, abnormal cell division or structural defects. Cytoskeletal organisation therefore links physical cell architecture with the accurate inheritance of cellular and genetic components.
Cytoskeletal systems also contribute to cell movement and tissue organisation. Migrating cells coordinate actin polymerisation, adhesion, contractility and membrane trafficking to extend protrusions and move their bodies. Microtubules help organise polarity and intracellular transport during migration, while intermediate filaments provide mechanical resilience. In tissues, cytoskeletal networks connect cells to one another and to extracellular structures through specialised adhesion complexes. These connections allow mechanical forces to be transmitted across groups of cells. Cytoskeletal responses can also influence signalling because mechanical forces and filament associated proteins interact with biochemical pathways controlling growth, differentiation and gene expression.
Cytoskeletal dysfunction is associated with numerous diseases because filament systems are required throughout the body. Mutations in keratin genes can produce epithelial fragility, while defects in neuronal cytoskeletal proteins can interfere with axonal structure and transport. Microtubules and their associated proteins are important targets of several therapeutic drugs because disrupting microtubule dynamics can interfere with rapidly dividing cells. Cytoskeletal components are also involved in infection because some pathogens exploit host cytoskeletal systems for entry, movement or intracellular trafficking. A common misconception is that the cytoskeleton is simply an internal skeleton that provides shape. It is instead a highly dynamic regulatory and mechanical system that coordinates structure, movement, transport, division, signalling and adaptation throughout the cell.
