Cytoskeleton
 
 The cytoskeleton consists primarily of three types of filament: microtubules (about 25 nm in diameter), actin-containing microfilaments (initially thought to be 5 to 7 nm in diameter but now known to be about 10 nm in diameter), and intermediate filaments (about 10 nm in diameter). The latter were so named because they were intermediate in size between the long-established and well-characterized microtubules and microfilaments. Microtubules are filamentous structures often many microns in length that occur in the cytoplasm of eukaryotic cells. The actins are a highly conserved family of eukaryotic proteins, and actin filaments are perhaps best known for the highly specific interactions they make with myosin thick filaments that results in muscle contraction. Many actin-binding proteins play important roles in establishing the cytoskeleton in vivo and in allowing it to function appropriately. For example, fimbrin and a-actinin aggregate and cross-link microfilaments into bundles, severin severs actin filaments, b-actinin acts as an actin filament-capping protein, and ponticulin (a transmembrane protein) binds actin filaments and spectrin binds and links the microfilaments to the cell membrane. See Actin, Microfilament, Microtubules, Intermediate Filaments, Cytokeratins, and Keratins for further information on various components of the cytoskeleton.