Between the nucleus and plasma membrane of all eukaryotic cells is a system of interconnected protein filaments collectively called the cytoskeleton. The cytoskeleton is a network of fibers extending throughout the cytoplasm. Elements of the cytoskeleton reinforce, organize, and move cell structures, anchoring many organelles.
Microtubules are long, hollow cylinders that consist of subunits of the protein tubulin. They form a dynamic scaffolding for many cellular processes, rapidly assembling when they are needed and then disassembling when they are not. For example, before a eukaryotic cell divides, microtubules assemble, separate the cell’s duplicated chromosomes, then disassemble. As another example, microtubules that form in the growing end of a young nerve cell support and guide its lengthening in a particular direction.
Microfilaments are fibers that consist primarily of subunits of the globular protein actin. They strengthen or change the shape of eukaryotic cells. Crosslinked, bundled, or gel-like arrays of them make up the cell cortex, which is a reinforcing mesh under the plasma membrane. Actin microfilaments that form at the edge of a cell drag or extend it in a certain direction. Myosin and Actin microfilaments interact to bring about contraction of muscle cells.
Intermediate filaments that support cells and tissues are the most stable elements of the cytoskeleton. These filaments form a framework that lends structure and resilience to cells and tissues. Some kinds underlie and reinforce membranes. The nuclear envelope, for example, is supported by an inner layer of intermediate filaments called lamins. Other kinds connect to structures that lock cell membranes together in tissues.
Microtubules control the beating of cilia and flagella, locomotor appendages of some cells. Cilia and flagella differ in their beating patterns Cilia – Cilia (singular, cilium) are short, hairlike structures that project from the surface of some cells. Cilia are usually more profuse than flagella. The coordinated waving of many cilia propels cells through fluid, and stirs fluid around stationary cells. Flagella – Eukaryotic flagella are structures that whip back and forth to propel cells such as sperm through fluid. They have a different internal structure and type of motion than flagella of bacteria.