Cytoskeleton

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Overview of the cell and its contents.

The cytoskeleton is a collective term that refers to an extensive network of filamentous or tubular intracellular proteins of varying morphology and composition scattered within the cytoplasm of a cell. It consists of three structurally and functionally distinct components: microfilaments, intermediate filaments and microtubules.

Microfilaments, which are the smallest components of the cytoskeleton, are made up of fine protein strands of actin. Each filament consists of two  strands (protofilaments) coiled together to form a helix. Microfilaments form a robust interlacing meshwork on the cytoplasmic side of the plasma membrane called the cell cortex. By this, they not only protect the cell against deformation, but also support changes in cell morphology when rearranged.

Intermediate filaments are slightly larger in diameter and more rigid compared to microfilaments. The type of protein that makes up these filaments depends on the type of body cells in which they are found. For example, keratin intermediate filament family is found mainly in epithelial cells whereas desmin is in muscle cells. The principal function of intermediate filaments is to provide structural support for cells and stabilize the plasma membrane at intercellular junctions.

Microtubules are the largest type of cytoskeleton. They consist of hollow tubules composes of long chains of the protein tubulin. Microtubules do not have a fixed structure and can be assembled and disassembled at the centrosome, depending on the specific function being carried out. The centrosome is a spherical structure which lies in close proximity to the nucleus of the cell. Within the centrosome is a centriole which serves as a microtubule organization center that supports growth and elongation of microtubules. Microtubules play a role in maintaining cell shape, location and movement of organelles, as well as chromosomes and cell motility.  

The cytoskeleton performs several important functions in the cell. These include:

• Maintenance of cell shape and rigidity by providing mechanical support, allowing cells to assume irregular shapes.
• Provision of support to specialized cell surface projections such as microvilli, cilia and filopodia
• Aids in forming structural polarity and different functional domains within a cell
• Restriction of specific organelles to fixed locations within the cell such as  the Golgi apparatus in close proximity to the nucleus and endoplasmic reticulum
• Provision of specific structures (tracks) for intracellular cargo transport of vesicles, macromolecules, chromosomes during cell division (mitosis and meiosis) and movement of the entire cell such as during chemotactic extravascular migration of leukocytes
• Performing highly specialized functions such as the contraction of the sarcomere in skeletal muscle cells and the movement of flagella