Just as the bones of vertebrate skeletons provide internal support for the body, the protein fibers of the cytoskeleton provide internal support for cells (Fig. 10.3). All eukaryotic cells have at least two cytoskeletal elements, microtubules and microfilaments. Animal cells have a third element, intermediate filaments. All three of these cytoskeletal elements are long chains, or polymers, made up of protein subunits. In addition to providing structural support, microtubules and microfilaments enable the movement of substances within cells as well as changes in cell shape.