CYTOSKELETON

a system of tiny filaments or fibres that is present in the cytoplasm of eukaryotic cells (cells containing a nucleus). The cytoskeleton organizes other constituents of the cell, maintains the cell's shape, and is responsible for the locomotion of the cell itself and the movement of the various organelles within it. The filaments that comprise the cytoskeleton are so small that their existence was only discovered because of the greater resolving power of the electron microscope. Three major types of filaments make up the cytoskeleton: actin filaments, microtubules, and intermediate filaments. Actin filaments occur in a cell in the form of bundles of parallel fibres; they help determine the shape of the cell and also help it adhere to the substrate. The constantly changing arrays of actin filaments help move the cell and mediate specific activities within it, such as cell cleavage during mitosis. Microtubules are longer filaments that are constantly assembling and disassembling; they play a crucial role in moving the split chromosomes to the newly forming daughter cells during mitosis, and bundles of microtubules form the cilia and flagellae found in protozoans and in the organs of some multicellular animals. Intermediate filaments, in contrast to actin filaments and microtubules, are very stable structures that form the true skeleton of the cell. They anchor the nucleus and position it within the cell, and they give the cell its elastic properties and its ability to withstand tension.