Despite the irregularity of some cell types, all varieties of the eukaryotic cell have defined shapes and movements that are visible under the microscope. In each case, a cell’s shape and its functional polarity are provided by a three-dimensional filamentous protein network called the Cytoskeleton. The cytoskeleton extends throughout the cell and is attached to the plasma membrane and internal organelles, providing a framework for organization and structure in the cell. Surprisingly, the cytoskeleton is not a static structure at all, but is highly dynamic, constantly changing over the length of the cell (microns) and over subcellular lengths (10’s or 100’s of nanometers).
(Actin filaments are shown in red, microtubules in green, and the nuclei are in blue.)
The cytoskeleton is composed of three major filament types, each of which has a distinct set of organizations and functions. Each filament system is composed of a polymer of assembled subunits, which undergo regulated assembly and disassembly, giving the cell the flexibility to construct or remove specialized structures as needed. The three filament types, actin/microfilaments, microtubules, and intermediate filaments, are described below.
Actin filaments / Microfilaments
Around 7 nm in diameter, this filament is composed of two actin chains oriented in a circular shape. They are mostly concentrated just beneath the cell 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. Actin/Myosin interactions also help reduce cytoplasmic streaming in most cells.
Intermediate filaments
These filaments, 8 to 11 nanometers in diameter, are more stable (strongly bound) than actin filaments, and heterogeneous constituents 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 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 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.
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