microtubule

microtubule A microscopic tubular structure, with an external diameter of 24 nm and of variable length, found in a wide range of eukaryotic cells. Microtubules are composed of numerous subunits of the globular protein tubulin and occur singly or in pairs, triplets, or bundles. Microtubules help cells to maintain their shape (see cytoskeleton); they also occur in cilia and eukaryotic flagella (see undulipodium) and the centrioles and form the spindle during nuclear division. A further role is in the intracellular transport of materials and movement of organelles. Formation of microtubules is initiated at microtubule-organizing centres (MTOCs). Compare microfilament; intermediate filament.