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Cell Biology
The World of Cells
The Cytoskeleton:
Microtubules and Microfilaments
The cytoskeleton of a cell is a network of filaments and fibers found in the cytoplasm. It determines cell shape and is also involved in cell division, movement of organelles, movement of the cell and the adhesion of the cell to other cells. There are three classes of filament recognizable under the electron microscope based on size, distribution and function.
Microfilaments sometimes called actin filaments are about 4 to 6 nm in diameter. Microfilaments are the smallest filaments in the cytoskeleton. Most lie just beneath the surface of the cell where they form a mesh or web. Anchored in the cell membrane, a filament may be connected to one or more organelles. Lengthening or shortening of the filament moves the attached organelle around the interior of the cell.
Myosin filaments are composed of the protein myosin, the major macromolecule in these filaments. Myosin is also found in muscle cells where it plays a role in contraction.
Intermediate filaments are called by a variety of names. These 10 nm filaments are made of keratin and seen in different forms in many different types of cells. Their function is still not certain; possibly they may be structural, holding the cell in shape.

In addition to the filaments described above there are more complex fibers and structures, such as:

Microtubules Hollow and cylindrical, microtubules are 20 to 25 nm in diameter and composed of 13 smaller protofilaments. These fibers seem to have at least three functions. They form parts of the cilia and flagella, and are found in association with centrioles and basal bodies. Possibly they are also involved in moving other organelles around within the cytoplasm, in cell division and in cytoplasmic organization. A protein called tubulin is the major structural component in these fibers. Unlike the filaments listed previously, microtubules are very unstable and can be seen forming and disassembling as they are needed and used.
Composed of nine sets of microtubules, centrioles are tiny structures about 0.4 µm long by 0.15 µm wide, usually found in pairs at right angles to each other in animal cells. Being hollow, cylindrical organelles, they appear to act as organizers for other collections of specialized microtubules. Cilia and flagella have small centriolar structures at their bases, called the basal bodies.
Figure legend: Seen from inside the cell, a tangle of filaments intertwine to form a supporting and regulating network. The largest of these are the microtubules which are cylinders of tubulin protein subunits. Dynamic structures, they are built up and broken down as needed by the cell. Smaller filaments, such as microfilaments, are more stable and often lie just beneath the surface of the cell. They may be involved in shape changes.

© 2001, Professor John Blamire