Microtubules: These can be observed only with the help of electron microscope. These were first observed in 1953 by Robertis and Franchi in the axons of medulated nerve fibres. These are cylindrical tubules, 20 - 25 nm in diameter. They are composed of subunits of the protein tubulin. These subunits are termed ? - tubulin and � - tubulin. Functions 1. Microtubules act as a scaffold to determine cell shape. 2. They provide a set of ?tracks? for cell organelles and vesicles to move on. 3. Microtubules also form the spindle fibres for separating chromosomes during mitosis. 4. When arranged in geometric patterns inside flagella and cilia, they are used for locomotion. 5. Microtubules causes movement of pigment granules in the chromatophores. 6. Microtubules are components of centrioles and basal bodies.