THE BASICS
    The cytoskeleton is transparent and is often "invisible" under electron microscopes.  It usually isn't included in diagrams of the cell but is an extremely important and complex structure.  The cell's cytoskeleton maintains it's shape, keeps organelles in their place, and moves parts of the cell to where in needs to go.  There are three main types of proteins in the cytoplasm, microfilaments (actin filaments), intermediate filaments, and microtubules.

MICROFILAMENTS
    Actin filaments are formed by the monomers of the protein actin polymerizing to form long, thin fibers.  These fibers are about 8 nm in diameter.  Microfilaments form a band below the plasma membrane to provide mechanical strength to the cell.  They also link cell surface receptors to cytoplasmic proteins.  Microfilaments anchor the centrosomes at the opposite ends of the cell during mitosis .  They also pinch divide animal cells apart during cytokinesis.  Microfilaments can generate cytoplasmic streaming in some cells.  Actin filaments are approximately a third fo the diameter of a microtubule.

INTERMEDIATE FILAMENTS
    Intermediate filaments are cytoplasmic fiber that average 10 nm in diameter.  These fibers are classified into four major types, each made of one or more proteins.  Even though they are made of different proteins intermediate filaments have a similar job in the cell of providing a supporting framework within the cell.  Intermediate filaments have other functions also, these include keeping the thick and thin filaments of muscle cells in a fixed position and providing mechanical strength to the long axons found in some neurons.

MICROTUBULES
    Microtubules are straight, hollow cylinders that have a diameter of about 25 nm.  They can be different sizes but can grow up to 1000 times as long as they are thick.  There are specialized microtubules  called spindle fibers that aid in the movement of chromosomes during cell division.