Fish skin cells. Epifluorescence light micrograph of two keratocytes (fish epidermal cells). The nuclei, which contain the cells' genetic information in the form of deoxyribonucleic acid (DNA), are orange. Microtubules are yellow and actin is blue. Microtubules and actin are protein filaments, they form the cell cytoskeleton, which gives the cell its shape and the capacity for directed movement. When scales are plucked and placed on a glass slide, individual cells become extremely motile due to a wound healing response. As they also have a simple structure, these cells are used as a model system to study cell locomotion, metastasis (spread) of cancer cells, and wound healing.