<P> A large - scale example of an action performed by the cytoskeleton is muscle contraction . During contraction of a muscle, within each muscle cell, myosin molecular motors collectively exert forces on parallel actin filaments . This action contracts the muscle cell, and through the synchronous process in many muscle cells, the entire muscle . </P> <P> In 1903, Nikolai K. Koltsov proposed that the shape of cells was determined by a network of tubules that he termed the cytoskeleton . The concept of a protein mosaic that dynamically coordinated cytoplasmic biochemistry was proposed by Rudolph Peters in 1929 while the term (cytosquelette, in French) was first introduced by French embryologist Paul Wintrebert in 1931 . When the cytoskeleton was first introduced, it was thought to be a uninteresting gel - like substance that help organelles stay in place . </P> <P> Eukaryotic cells contain three main kinds of cytoskeletal filaments: microfilaments, microtubules, and intermediate filaments . Each cytoskeletal filament type is formed by polymerization of a distinct type of protein subunit and has its own characteristic shape and intracellular distribution . Microfilaments are polymers of the protein actin and are 7 nm in diameter . Microtubules are composed of tubulin and are 25 nm in diameter . Intermediate filaments are composed of various proteins, depending on the type of cell in which they are found; they are normally 8 - 12 nm in diameter . The cytoskeleton provides the cell with structure and shape, and by excluding macromolecules from some of the cytosol, it adds to the level of macromolecular crowding in this compartment . Cytoskeletal elements interact extensively and intimately with cellular membranes . A number of small - molecule cytoskeletal drugs have been discovered that interact with actin and microtubules . These compounds have proven useful in studying the cytoskeleton and several have clinical applications . </P> <P> All filaments interact with accessory proteins that regulate and link the filaments to other cell compounds and each other . The accessory proteins are essential for controlled assembly of cytoskeletal filaments in particular locations, and they include motor proteins . </P>

The smallest of the cytoskeleton elements is the