intermediate filament
/ in-ter-MEE-dee-it FIL-uh-ment /
Imagine the strong, twisted ropes used to moor a ship. They are not stiff like a pole and not for moving anything; their job is simply to take a strong pull without snapping. Intermediate filaments are the cytoskeleton's version of such ropes. They are tough, rope-like fibers whose main job is to give cells mechanical strength so they can withstand stretching and pulling.
Their name comes from their thickness: at about 10 nanometers across, they sit between the thinner microfilaments (about 7 nm) and the thicker microtubules (about 25 nm). Unlike the other two fiber types, intermediate filaments are not built from round, ball-shaped subunits but from long, rod-shaped proteins that wind together into a stable cable. They also have no plus and minus ends and do not serve as tracks for motor proteins, so they are more about endurance than about movement.
Intermediate filaments come in many tissue-specific kinds. Keratins make your skin, hair, and nails tough; vimentin strengthens connective tissue cells; neurofilaments help nerve cells keep their long thin shape; and lamins line the inside of the nucleus and support its envelope. Because they bear load rather than turning over quickly, faulty intermediate filaments cause diseases of fragile tissue, such as skin that blisters from gentle rubbing.
The keratin filaments in the cells of your epidermis link to neighboring cells through desmosomes, spreading mechanical stress across the whole sheet so the skin does not tear.
Intermediate filaments work best as a tissue-wide network, not cell by cell.
Unlike microfilaments and microtubules, intermediate filaments are absent from many simpler organisms; they are a specialty of animal cells that face strong mechanical stress.