Cell microtubules. Fluorescent light micrograph of a culture cell showing its nucleus (blue) and microtubules (green). Microtubules form part of the cytoskeleton, the internal structural units of a cell that assist in transporting material around the cell, and maintaining its shape. Microtubules are composed of polymers of the proteins alpha and beta tubulin. They are involved in transporting organelles around the cell, notably in mitosis (cell division), and they form the scaffold of cilia and flagella. The other main components of the cytoskeleton are actin and intermediate filaments (neither seen). Magnification: x600 when printed 10cm high.