False-colour tubulin immunofluorescence micrograph of tissue culture cells showing the microtubule component of the cytoskeleton, the three- dimensional array of protein molecules that permeates the cytoplasm of a cell. Here, the cells have been fixed and stained with an antibody which binds to the microtubules; the antibody is labelled with the dye fluorescein, which fluoresces green in ultraviolet light. The apparently empty space in the centre of each cell represents the nucleus. Microtubules are thought to act as direction-markers within the cell - a sort of cytoplasmic railway network. Magnification: x275 at 35mm size. Dark green tint.