The shape of eukaryotic cells in maintained by a structure known as the cytoskeleton, comprising three different type of cytoplasmic filaments: microtubules, microfilaments (actin-containing filaments) and intermediate filaments. The cytoskeleton in therefore expected to be a traget for intracellular second messengers that promote changes in cell shape. These include elevation of the cytoplasmic concentration of calcium ions. Most of the known cytoskeletal receptors for calcium are associated with the actin filament system and these may be important in regulating many types of cell motility, including locomotion, phagocytosis and secretion.