Cytoskeleton
The cytoskeleton is a cellular "scaffolding" or "skeleton" contained, as all other organelles, within the cytoplasm. 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.
In Eukarya
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 protuberancies (like pseudopods and microvillus), paticipate in some cell-to-cell or cell-to-matrix junctions and the transduction of signals. They are also important for cytokinesis and, along with myosin, muscular contraction.
See also: actin, microfilaments
Intermediate Filaments
These 8 to 11 nanometers in diameter filaments are the more rigid (non-dynamical) and heterogenous 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-to-cell and cell-to-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.
See also: intermediate filaments
Microtubules
They are hollow cylinders of about 25 nm., formed by 13 protofilaments which, in turn, are polymers of alfa 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 (asociated 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.
See also: microtubules
In bacteria
The bacterial cytoskeleton controls cell length and width and the site of cell division.