Cytoskeleton

The cytoskeleton is a dynamic network of protein filaments that extends throughout the cytoplasm of eukaryotic cells. It provides mechanical support, defines cell shape, and organizes the cell’s interior. The cytoskeleton comprises three principal filament systems: actin filaments, intermediate filaments, and microtubules, each with distinct properties and roles.

Actin filaments, or microfilaments, are about 7 nanometers in diameter and polymerize from actin monomers. They form the cell cortex, support the plasma membrane, and drive movements such as cell crawling, vesicle trafficking, and muscle contraction in conjunction with myosin. Actin filaments undergo rapid turnover through polymerization and treadmilling.

Intermediate filaments are a diverse group of approximately 10-nanometer filaments that provide mechanical strength and resilience. They resist shear and help organize the nuclear envelope and cytoplasmic networks. Different tissues express distinct keratins, vimentin, desmins, neurofilaments, and lamins.

Microtubules are hollow tubes about 25 nanometers in diameter, composed of α- and β-tubulin dimers. They radiate from microtubule organizing centers and serve as tracks for motor proteins that direct intracellular transport of vesicles and organelles. They form the mitotic spindle during cell division and assemble cilia and flagella.

The cytoskeleton is highly dynamic, with assembly and disassembly regulated by nucleotide hydrolysis (ATP for actin, GTP for tubulin) and by various binding proteins. Signaling pathways, including Rho-family GTPases, control remodeling during migration, division, and morphogenesis. The cytoskeleton interacts with the plasma membrane and organelles to shape cells and coordinate cellular processes, and its disruption is linked to developmental disorders and disease.