The cell cytoskeleton is a network of fibrillar proteins organized into filaments or tubules. These originate as precursor protein molecules synthesized by ribosomes in the cytoplasm. The precursor molecules then polymerize to form filaments. As an example, large numbers of actin filaments frequently occur in the outer zone of the cytoplasm, called the ectoplasm, to form an elastic support for the cell membrane. Also, in muscle cells, actin and myosin filaments are organized into a special contractile machine that is the basis for muscle contraction.

A special type of stiff filament composed of polymerized tubulin molecules is used in all cells to construct strong tubular structures, the microtubules. Figure 1 shows typical microtubules from the flagellum of a sperm.

Fig1.  Microtubules teased from the flagellum of a sperm.  (From Wolstenholme GEW, O’Connor M, and the publisher, JA Churchill, 1967. Figure 4, page 314. Copyright the Novartis Foundation, formerly the Ciba Foundation.)

Another example of microtubules is the tubular skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the cilium. This structure is discussed later in the chapter and is illustrated in Figure 2. Also, both the centrioles and the mitotic spindle of the mitosing cell are composed of stiff microtubules.

Fig2. Structure and function of the cilium. (Modified from Satir P: Cilia. Sci Am 204:108, 1961. Copyright Donald Garber: Executor of the estate of Bunji Tagawa.)

Thus, a primary function of microtubules is to act as a cytoskeleton, providing rigid physical structures for certain parts of cells. The cytoskeleton of the cell not only deter mines cell shape but also participates in cell division, allows cells to move, and provides a track-like system that directs the movement of organelles within the cells.