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 can occur singly or in pairs, triplets, or bundles. Each tubulin protein is a dimer consisting of two slightly different polypeptides—α‎-tubulin and β‎-tubulin—which gives each microtubule distinct ‘plus’ and ‘minus’ ends. Each tubule can add or lose tubulin dimers from its ‘plus’ end, depending on levels of free GTP-bound dimers. If these are abundant, the microtubule binds more dimers and gets longer; if they are in short supply, dimers dissociate from the microtubule, which gets shorter. This state is called dynamic instability.

Microtubules help cells to maintain their shape by resisting compressive forces (see cytoskeleton); they also occur in cilia and eukaryotic flagella (sense 2) 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.