Intermediate filaments have great strength, and their main function is to enable cells to withstand the mechanical stress that occurs when cells are stretched. The filaments are called “intermediate” because, in the smooth muscle cells where they were first discovered, their diameter (about 10 nm) is between that of the thinner actin filaments and the thicker myosin filaments.

Intermediate filaments are the toughest and most durable of the cytoskeletal filaments: when cells are treated with concentrated salt solutions and nonionic detergents, the intermediate filaments survive, while most of the rest of the cytoskeleton is destroyed.

Intermediate filaments are found in the cytoplasm of most animal cells. They typically form a network throughout the cytoplasm, surrounding the nucleus and extending out to the cell periphery. There, they are often anchored to the plasma membrane at cell–cell junctions called desmosomes, where the plasma membrane is connected to that of another cell.

Intermediate filaments are also found within the nucleus of animal cells. There, they form a meshwork called the nuclear lamina, which underlies and strengthens the nuclear envelope. In this section, we see how the structure and assembly of intermediate filaments makes them particularly suited to strengthening cells and protecting them from tearing.