Cytoskeleton

Microfilaments (actin filaments, ≈7 nm) are two-stranded helical polymers of globular G-actin forming polarised F-actin with a fast-growing barbed (+) and slow pointed (−) end. They form a dense cortex beneath the plasma membrane, core bundles of microvilli, stress fibres and the adhesion belt of adherens junctions. With myosin motors they generate contraction (muscle sarcomeres, the contractile ring of cytokinesis) and drive cell migration (lamellipodia, filopodia), phagocytosis and endocytosis.

Intermediate filaments (≈10 nm) are rope-like, non-polar polymers of fibrous proteins with high tensile strength; they have no motors and are the most stable system. They are cell-type specific: keratins (cytokeratins) in epithelia, anchored at desmosomes and hemidesmosomes; vimentin in mesenchymal cells (fibroblasts, endothelium, leukocytes); desmin in muscle; glial fibrillary acidic protein (GFAP) in astrocytes; neurofilaments in neurons; and lamins in the nuclear lamina of all cells. This specificity is used in tumour immunohistochemistry.

Microtubules (≈25 nm) are hollow tubes of 13 protofilaments of α/β-tubulin heterodimers, nucleated with their minus ends at the centrosome (γ-tubulin ring complexes) and plus ends growing toward the periphery. They show dynamic instability — alternating growth and rapid shrinkage depending on whether a GTP-tubulin "cap" is present. Microtubules form the mitotic spindle, the axoneme of cilia and flagella, and the tracks for intracellular transport: kinesins move cargo toward the plus end (anterograde), cytoplasmic dynein toward the minus end (retrograde). Microtubule-associated proteins such as tau stabilise them.