Cytoskeleton


The eukaryotic cytoskeleton. Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.

The cytoskeleton is a cellular "scaffolding" or "skeleton" contained, as all other organelles, within the cytoplasm. It is contained in all cells, including plant and animal cells as well as prokaryotic and eukaryotic cells. It is a dynamic structure that maintains cell shape, enables some cell motion (using structures such as flagella and cilia), and plays important roles in both intra-cellular transport (the movement of vesicles and organelles, for example) and cellular division.

The eukaryotic cytoskeleton

Eukaryotic cells contain three kinds of cytoskeletal filaments.

Actin filaments

Around 7 nm in diameter, this filament is composed of two actin chains oriented in an helicoidal shape. They are mostly concentrated just beneath the plasma membrane, as they keep cellular shape, form cytoplasmatic protuberances (like pseudopodia and microvilli), and participate in some cell-to-cell or cell-to-matrix junctions and in the transduction of signals. They are also important for cytokinesis and, along with myosin, muscular contraction.

Intermediate filaments

These filaments, 8 to 11 nanometers in diameter, are the more stable (strongly bound) and heterogeneous constitutents of the cytoskeleton. They organize the internal tridimensional structure of the cell (they are structural components of the nuclear envelope or the sarcomeres for example). They also participate in some cell-cell and cell-matrix junctions.

Different intermediate filaments are:
• made of vimentins, being the common structural support of many cells.
• made of keratin, found in skin cells, hair and nails.
• neurofilaments of neural cells.
• made of lamin, giving structural support to the nuclear envelope.

Microtubules

They are hollow cylinders of about 25 nm, formed by 13 protofilaments which, in turn, are polymers of alpha and beta tubulin. They have a very dynamic behaviour, binding GTP for polymerization. They are organized by the centrosome.

They play key roles in:
• intracellular transport (associated with dyneins and kinesins they transport organelles like mitochondria or vesicles).
• the axoneme of cilia and flagella.
• the mitotic spindle.
• synthesis of the cell wall in plants.

A fourth eukaryotic cytoskeletal element, microtrabeculae, were proposed by Keith Porter in the 1960s. Porter's lab observed short, filamentous structures of unknown molecular composition in electron micrographs of whole cells. Due to their filamentous appearance and association with known cytoplasmic structures, microtrabeculae were speculated to represent a novel filamentous network distinct from microtubules, filamentous actin, or intermediate filaments. However, they were later shown to be an artifact of certain types of fixation treatments by Hans Ris and others.

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