Microtubules are protein structures found within cells, one of the components of the cytoskeleton. They have diameter of ~ 24 nm and varying length from several micrometers to possible millimeters in axons of nerve cells.

    Microtubules are polymers of α- and β-tubulin dimers. They are hollow cylinders, which can be thought of as 13 protofilaments of tubulin arranged into a cylinder, or as a single spiral with 13 subunits in one helical turn of the spiral. Microtubules have a polarity: Each microtubule has a (+) and a (-) end, with the α subunit being exposed at the (-) end and the β subunit exposed at the (+). Gain or loss of tubulin subunits from the (+) end can change the length of a microtubule.

    Microtubules are nucleated and organised by the microtubule organizing centers (MTOCs), such as centrosomes and basal bodies. They are part of a structural network (the cytoskeleton) within the cell's cytoplasm, but, in addition to structural support, microtubules are used in many other processes, as well. They are capable of growing and shrinking in order to generate force, and there are also motor proteins that move along the microtubule. A notable structure involving microtubules is the mitotic spindle used by eukaryotic cells to segregate their chromosomes correctly during cell division. Microtubules are also part of the cilia and flagella of eukaryotic cells (prokaryote flagella are entirely different).