Microtubules are a key constituent of the cell's structural framework and are responsible for a diverse range of functions within the cell. They are cylindrical polymers, 25 nm in diameter and can grow to be several hundred micrometers in length. Tubulin, the protein which is the main component of microtubules, self-assembles to form the walls of the cylinder in a highly-ordered, helical lattice arrangement. Functionally, microtubules fill a wide variety of roles within the cell. The function often considered most important is the role played in cell division.