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A system of filaments found in the cytoplasm of cells and responsible for the maintenance of and changes in cell shape, cell locomotion, movement of various elements in the cytoplasm, integration of the major cytoplasmic organelles, cell division, chromosome organization and movement, and the adhesion of a cell to a surface or to other cells.

Three major classes of filaments have been resolved on the basis of their diameter and cytoplasmic distribution: actin filaments (or microfilaments) each with an average diameter of 6 nanometers, microtubules with an average diameter of 25 nm, and intermediate filaments whose diameter of 10 nm is intermediate to that of the other two classes. The presence of this system of filaments in all cells, as well as their diversity in structure and cytoplasmic distribution, has been recognized only in the modern period of biology.

A technique that has greatly facilitated the visualization of these filaments, as well as the analysis of their chemical composition, is immunofluorescence applied to cells grown in tissue culture. See Immunofluorescence

Actin is the main structural component of actin filaments in all cell types, both muscle and nonmuscle. Actin filaments assume a variety of configurations depending on the type of cell and the state it is in. They extend a considerable distance through the cytoplasm in the form of bundles, also known as stress fibers since they are important in determining the elongated shape of the cell and in enabling the cell to adhere to the substrate and spread out on it. Actin filaments can exist in forms other than straight bundles. In rounded cells that do not adhere strongly to the substrate (such as dividing cells and cancer cells), the filaments form an amorphous meshwork that is quite distinct from the highly organized bundles. The two filamentous states, actin filament bundles and actin filament meshworks, are interconvertible polymeric states of the same molecule. Bundles give the cell its tensile strength, adhesive capability, and structural support, while meshworks provide elastic support and force for cell locomotion.

Microtubules are slender cylindrical structures that exhibit a cytoplasmic distribution distinct from actin filaments. Microtubules originate in structures that are closely associated with the outside surface of the nucleus known as centrioles. The major structural protein of these filaments is known as tubulin. Unlike the other two classes of filaments, microtubules are highly unstable structures and appear to be in a constant state of polymerization-depolymerization. See Centriole

Intermediate filaments function as the true cytoskeleton. Unlike microtubules and actin filaments, intermediate filaments are very stable structures. They have a cytoplasmic distribution independent of actin filaments and microtubules. In the intact cell, they anchor the nucleus, positioning it within the cytoplasmic space. During mitosis, they form a filamentous cage around the mitotic spindle which holds the spindle in a fixed place during chromosome movement.


(cell and molecular biology)
Protein fibers composing the structural framework of a cell.
References in periodicals archive ?
Among these proteins, smooth muscle gamma-actin, transgelin, cofilin-1, and myosin are cytoskeleton proteins.
If the cytoskeleton is destroyed but the actin is not allowed to enter the nucleus, the little bits of actin just sit in the cytoplasm, and the stem cells do not become bone cells.
Thus, the mature differentiated lymphocytes from healthy donors in the regulation of the volume by hypotonic stress are influenced by plasmalemma surface pool reserves, the use of which depends on the elastic properties of cytoskeleton components, and the concentration of intracellular signaling molecules involved in signal transduction from receptors to cell nucleus.
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They cover cytoskeletal and motility proteins, cytoskeleton in cell and tissue architecture, intracellular transport, cellular motility, and cell division.
For instance, recently showed that chromosome movements during mid-prophase of meiosis in budding yeast result from a trans-acting force generated at the level of the global cytoskeleton network, suggesting that extranuclear mechanical trans-acting signals could also regulate chromosomal metabolism in other ways.
The aim of this study was to investigate the roles of several protein components (MreB, MreC, and MreD) of the prokaryotic cytoskeleton for the faithful transmission of the chromosomal terminus into daughter cells.
These twisty fibers (below, in yellow; cell's nucleus in magenta) are part of a cell's internal scaffolding known as the cytoskeleton.