The Cytoskeleton: A Network of Cellular Fibers

The cytoskeleton, derived from the Greek words "Kytos" meaning cell and "skeleton" referring to a dried body, is a complex network of interconnected filaments and tubules that span from the nucleus to the plasma membrane within eukaryotic cells. Although not visible under an ordinary microscope, the cytoskeleton plays a crucial role in maintaining cell shape, providing mechanical support, and facilitating cellular movements. It is composed of three different types of fibers: microfilaments, microtubules, and intermediate filaments, each with unique structural and functional characteristics.

                                    Cytoskeleton

Microfilaments: Actin Filaments for Cellular Structure

Microfilaments, also known as actin filaments, are long protein fibers with a diameter of approximately 7 nm. They can occur in bundles or form a mesh-like network within the cell. Actin filaments are made up of globular actin monomers twisted together in a helical manner. They play a key role in maintaining cellular structure and are found in various cellular structures, such as microvilli in intestinal cells. Additionally, actin filaments act as tracks along which organelles, such as chloroplasts, can move in a specific direction.

 

Microtubules: Hollow Cylinders for Cellular Organization

Microtubules, named after the Greek words "micros" meaning small or little, and "tubus" meaning pipe, are small, hollow cylinders with a diameter of approximately 25 nm and lengths ranging from 0.2 to 25 μm. They are composed of protein subunits called tubulin, which come in two forms - alpha and beta tubulin - and combine to form tubulin dimers. Microtubules have a distinctive structure with 13 rows of tubulin dimers surrounding an apparent empty central core as observed in electron micrographs. They radiate from a structure called the centrosome and are involved in maintaining cell shape and providing tracks along which organelles can move within the cell.

 

Intermediate Filaments: Fibrous Proteins for Cellular Support

Intermediate filaments are protein fibers that wrap around each other and have a diameter of 8 to 10 nm, hence the name "intermediate" as they are larger than actin filaments but smaller than microtubules. The basic protein subunit of intermediate filaments is called vimentin, although some cells use other fibrous proteins instead. Recent research has revealed that intermediate filaments are highly dynamic, meaning they can assemble and disassemble. They play a role in supporting the nuclear envelope and plasma membrane in some cells. For example, in the skin, intermediate filaments made of the protein keratin provide mechanical support to skin cells.