The membrane-enclosed organelles discussed in the preceding chapters constitute one level of the organizational substructure of eukaryotic cells. A further level of organization is provided by the cytoskeleton, which consists of a network of protein filaments extending throughout the cytoplasm. The cytoskeleton provides a structural framework for the cell, serving as a scaffold that determines cell shape and the positions of organelles. In addition to this structural role, the cytoskeleton is responsible for cell movement and the transport of organelles and other structures (such as mitotic chromosomes) through the cytoplasm. Importantly, the cytoskeleton is much less rigid and permanent than its name implies. Rather, it is a dynamic structure that is continually reorganized as cells move and change shape-for example, during mitosis and cell division.

The cytoskeleton is composed of three principal types of protein filaments: actin filaments, microtubules, and intermediate filaments (Figure 14.1), which are held together and linked to subcellular organelles and the plasma membrane by a variety of accessory proteins. This chapter discusses the structure and organization of each of these three major components of the cytoskeleton as well as the roles of actin filaments and microtubules in cell motility, organelle transport, and cell division.