The inside of a cell is much more than a sack of watery cytoplasm and organelles. Within the cytoplasm is a complex scaffold of proteins that comprise the cytoskeleton. The cytoskeleton is made up of three major components: microfilaments, intermediate filaments, and microtubules.
The smallest cytoskeletal fibers are microfilaments , made of polymerized subunits of the globular protein actin , which are strung together like beads on a string. Microfilaments play a critical role in cell motility, where they facilitate cellular migration or, as in the case of muscle cells, contraction. The movement of white blood cells illustrates the type of movement created by microfilaments. This type of movement is called "amoeboid movement" and it is the result of the cytoplasm flowing and extending within the cell membrane.
As the name implies, intermediate filaments are somewhat larger than microfilaments, yet smaller than microtubules. Intermediate filaments are made of fibrous proteins wrapped around one another to form a thick, cable-like structure. Just as cables are specialized to withstand large forces, intermediate filaments play an important role in supporting cell structures and anchoring organelles in the correct position within the cell. Unlike microfilaments and microtubules, intermediate filaments are relatively static within the cell.
Microtubules are the largest cytoskeletal element that we will discuss. Microtubules are hollow tubes consisting of rows of paired tubulin molecules. Microtubules are important to major cellular events such as mitosis and meiosis and they also have a major structural role within the cell. Cilia and flagella are hair-like projections on certain cells that play a role in cell motility. The beating motion of cilia and flagella is the result of the action of microtubules.
Cancer cells are characterized by their rapid cell divisions. Given the importance of the cytoskeleton in cell division, it is the target of a number of cancer therapeutic drugs. For example, Taxol is a drug that is used to treat a number of cancers including breast and lung cancer. The drug works by targeting tubulin and disrupting the mitotic spindle. This causes mistakes in chromosome segregation and cell division and eventually kills the cell. However, one of the major problems with targeting the cytoskeleton with drug therapies is that there are many adverse side effects associated with these drugs. This is because the cytoskeleton plays important roles throughout the cell cycle of all cells including health cells. Several drug companies are actively searching for drugs that target the cytoskeletal elements specifically involved in cancer.