Cytoskeleton
All eukaryotic cells have an internal scaffolding called a cytoskeleton. The cytoskeleton gives the cells its shape, participates in whole cell movement and the internal movement of organelles and structures. The cytoskeleton is made up of various elements including microtubules, intermediate filaments and micro filaments.
Microtubules are large (on a microscopic scale) molecules composed of thousands of units of the protein tubulin. Microtubules play an important role in cell division. They make up the spindle apparatus which attaches to chromosomes and moves the chromosomes and sister chromatids around during cell division. Microtubules are also responsible for transport of vesicles within cells. Neurotransmitters are produced by the Nissl bodies located in the soma of neurons but released from the axons of the cell. The cell body and the axon can be pretty far apart. Microtubules extend from the soma to the tips of the axon. Vesicles containing neurotransmitters are shuttled down the microtubules from the cell body to the axon.
Intermediate filaments are fibrous protein molecules that are intermediate in size between the microfilament and the microtubules. Intermediate filaments also play a role in cell shape. They are found beneath the nuclear envelop and determine the shape of the nucleus. They serve as anchors for organelles holding them in place. You may notice in some epithelial tissues that the nucleus is always in the same place, center for simple squamous for example.... the intermediate filaments are holding the nuclei in place.
Microfilaments are the smallest diameter cytoskeletal element. Actin, a contractile globular protein, is the primary protein associated with microfilaments. Microfilaments play a role during cell division also. At the end of nuclear division the cytoplasm must also divide. Microfilaments in the cytoskeleton form the cleavage furrow which allows the cells to 'pinch off' and separate. Microfilaments are also involved in amoeboid movement and the formation of pseudopods. This type of movement is seen in some white blood cells and allows these cells to leave the peripheral blood stream and enter the tissue.
