The cytoskeleton is a dynamic network of protein polymers (microfilaments, intermediate filaments, microtubules) extending throughout the cytoplasm. It maintains cell shape, organizes organelles, provides tracks for transport, generates cell movement, and is essential for division. Microfilaments (actin) are thin and involved in contraction; microtubules (tubulin dimers) are thick, hollow, and form spindles; intermediate filaments provide mechanical strength.
Use electron microscopy to visualize each filament type and describe its structure. Use live-cell imaging to observe dynamic reorganization during mitosis and migration.
The cytoskeleton is static—it constantly polymerizes and depolymerizes. Microtubules exist only for division—they are permanent tracks. All filament types function independently—they work cooperatively.
From your study of eukaryotic cells, you know that these cells are large and internally complex compared to prokaryotes. But how does a cell maintain its shape, move organelles to the right locations, crawl toward a wound, or physically divide in two? The answer is the cytoskeleton — a dynamic, interconnected network of protein filaments that serves as the cell's structural framework, transportation system, and mechanical engine all at once.
The cytoskeleton comprises three main filament types, each built from different protein subunits and specialized for different roles. Microfilaments (also called actin filaments) are the thinnest, at about 7 nm in diameter. They are polymers of the protein actin, which assembles into two intertwined helical strands. Actin filaments are concentrated beneath the plasma membrane, where they form a meshwork called the cell cortex that determines cell shape and drives movements like crawling, phagocytosis, and cytokinesis (the physical pinching apart of daughter cells). When the motor protein myosin walks along actin filaments, it generates contractile force — the same mechanism that powers muscle contraction.
Microtubules are the largest cytoskeletal filaments, at 25 nm in diameter. They are hollow tubes assembled from dimers of α-tubulin and β-tubulin, arranged in a ring of 13 protofilaments. Microtubules radiate outward from the centrosome near the nucleus and serve as tracks for long-distance intracellular transport: the motor protein kinesin carries cargo toward the plus end (cell periphery), while dynein carries cargo toward the minus end (cell center). During cell division, microtubules form the mitotic spindle that segregates chromosomes. Microtubules are highly dynamic — they undergo rapid cycles of growth and shrinkage called dynamic instability, allowing the cell to quickly reorganize its transport network in response to signals.
Intermediate filaments are the middle-sized filaments (about 10 nm) and the most mechanically robust. Unlike actin and tubulin, intermediate filaments are built from a diverse family of proteins — keratins in epithelial cells, vimentin in mesenchymal cells, neurofilaments in neurons, and lamins lining the nuclear envelope. They do not have motor proteins associated with them and are not involved in motility. Instead, their role is purely structural: they resist tensile (stretching) forces and distribute mechanical stress across the cell and between cells via connections to desmosomes and hemidesmosomes. While microfilaments and microtubules are dynamic and constantly remodeling, intermediate filaments are more stable and provide the cell's baseline mechanical integrity. Together, the three systems cooperate — actin provides contractile force, microtubules provide organization and transport, and intermediate filaments provide mechanical resilience — creating a versatile architecture that adapts to the cell's changing needs.