A network of fibers exists throughout the cytoplasm, supporting the cell and maintaining its shape--a function that is particularly important in animal cells, which have no cell wall. In addition to stabilizing the cell, the cytoskeleton provides anchorage for organelles even cytosolic enzyme molecules.

The cytoskeleton is also involved several types of cell motility, ranging from changes in cell location to movements of certain cell parts. This generally requires an interaction between the cytoskeleton and proteins called motor molecules, which power cilia and flagella, and also cause muscle cells to contract. Vesicles may travel on cytoskeletal "tracks". The cytoskeleton also manipulates the plasma membrane to form food vacuoles during phagocytosis. There is evidence that cytoskeleton may be able to transmit mechanical forces from the cell's surface to its interior, perhaps even into the nucleus.

There are three main types of fibers: microtubules (the thickest of the three), microfilaments (the thinnest), and intermediate filaments (middle range, as the name suggests).

MICROTUBULES

These fibers are found in the cytoplasm of all eukaryotic cells. They are straight, hollow rods measuring about 25 nm in diameter, and from 200 nm to 25 micrometers in length. The wall of the tube is made from a globular protein called tubulin. Microtubules elongate by adding tubulin molecules to the ends, and can be disassembled and their tubulin used to build other microtubules elsewhere.

They shape and support the cell, serving as tracks which organelles with motor molecules can move along.

MICROFILAMENTS

Unlike microtubules, microfilaments are solid robs, approximately 7 nm in diameter. They are a twisted double chain of subunits of the globular protein actin. They seem to be present in all eukaryotic cells, bearing tension. In combination with other proteins, they often form a three-dimensional network just inside the membrane, supporting the cell's shaping and giving the outer cytoplasmic layer a slightly more solid consistency than the inner cytoplasm.

INTERMEDIATE FILAMENTS

The diameter of these tension-bearing filaments ranges from 8-12 nm. This is a very diverse class of cytoskeletal elements, each type constructed from different families of proteins. Intermediate filaments are more permanent than the other fibers. Chemical treatments that would disassemble the others do not remove these filaments, which retain the original shape of the cell. A cage of intermediate filaments usually surrounds the nucleus.