Figure 1. The three major classes of cytoskeletal filaments: (a) microfilaments (composed of actin monomers); (b) microtubules (composed of - and -tubulin); (c) intermediate filaments (various different types of monomer).

Figure 2. Polymerization of actin. (a) The rate of polymerization or depolymerization depends on the actin monomer concentration. Note that each end behaves differently. The critical concentration for each end is the concentration of actin monomers at which there is no net polymerization or depolymerization. (b) Kinetics of polymerization of an actin solution above the critical concentration for barbed ends. The long delay is caused by the slow formation of three-subunit nuclei.

Figure 3. Contrasting actin-rich structures. (a) Microvilli. In these long finger-like structures on the surfaces of absorptive epithelia, the actin filaments are bundled in the same direction (isotropically). (b) Lamellipodium. The actin-rich edges at the leading edges of motile cells contain actin filaments crosslinked at an angle to one another.