Microtubules. Transmission electron micrograph of purified microtubules from bovine brain tissue. Microtubules are the protein filaments that form the cytoskeletons in cells. They are thought to guide the movement of organelles within the cell and can bring about cell elongation by their own extension. They are also found in cilia and flagella, where they play a role in the beating movements of these structures, and in centrioles and cell-division spindles. Each microtubule consists of a hollow cylinder, about 25 nanometres in diameter, made of 13 filaments of the protein tubulin. Magnification x21,300 at 10x8 inch size. Negatively stained with phosphotungstic acid.