Post # 163 from Silica Thread
Cytoskeleton
Cytoskeleton - Wikipedia, the free encyclopedia
....
The eukaryotic cytoskeleton 
Actin cytoskeleton of
mouse embryo fibroblasts, stained with
phalloidin Eukaryotic cells contain three main kinds of cytoskeletal filaments, which are microfilaments, intermediate filaments, and microtubules. 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.
[2] Cytoskeletal elements interact extensively and intimately with cellular membranes.
[3] Actin filaments / Microfilaments
Main article:
Microfilament
Around 6 nm in diameter, this filament type is composed of two intertwined actin chains.
Microfilaments are most concentrated just beneath the
cell membrane, and are responsible for resisting tension and maintaining cellular shape, forming cytoplasmatic protuberances (like
pseudopodia and
microvilli- although these by different mechanisms), and participation in some cell-to-cell or cell-to-matrix junctions.
In association with these latter roles, microfilaments are essential to
transduction.
They are also important for cytokinesis (specifically, formation of the cleavage furrow) and, along with myosin, muscular contraction.
Actin/Myosin interactions also help produce cytoplasmic streaming in most cells.
Further information: Actin Intermediate filaments
Main article:
intermediate filament These filaments, around 10 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents of the cytoskeleton.
Although little work has been done on intermediate filaments in plants, there is some evidence that cytosolic intermediate filaments might be present, and plant nuclear filaments have been detected.
Like actin filaments, they function in the maintenance of cell-shape by bearing tension (
microtubules, by contrast, resist compression. It may be useful to think of micro- and intermediate filaments as cables, and of microtubules as cellular support beams).
Intermediate filaments organize the internal tridimensional structure of the cell, anchoring organelles and serving as structural components of the
nuclear lamina and
sarcomeres. They also participate in some cell-cell and cell-matrix junctions.
Different intermediate filaments are:
Microtubules 
Microtubules in a gel fixated cell.
Main article:
microtubule
Microtubules are hollow cylinders about 23 nm in diameter (lumen = approximately 15nm in diameter), most commonly comprised of 13 protofilaments which, in turn, are polymers of alpha and beta
tubulin.
They have a very dynamic behaviour, binding
GTP for polymerization. They are commonly organized by the
centrosome.
In nine triplet sets (star-shaped), they form the centrioles, and in nine doublets oriented about two additional microtubules (wheel-shaped) they form cilia and flagella. The latter formation is commonly referred to as a "9+2" arrangement, wherein each doublet is connected to another by the protein dynein. As both flagella and cilia are structural components of the cell, and are maintained by microtubules, they can be considered part of the cytoskeleton.
They play key roles in:
Kat
August 7th, 2009, 03:24 PM
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