Figure 16–3 A neutrophil in pursuit of bacteria. In this preparation of human blood, a clump of bacteria (white arrow) is about to be captured by a neutrophil. As the bacteria move, the neutrophil quickly reassembles the dense actin network at  its leading edge (highlighted in red) to push toward the location of the bacteria (Movie 16.1). Rapid disassembly and reassembly of the actin cytoskeleton in this cell enables it to change its orientation and direction of movement within a few minutes. (From a video recorded by  David Rogers.) remain strikingly dynamic and are continuously remodeled and replaced every 48 hours, even within stable cell-surface structures that persist for decades.
Besides forming stable, specialized cell-surface protrusions, the cytoskeleton is also responsible for large-scale cellular polarity, enabling cells to tell the difference between top and bottom, or front and back. The large-scale polarity information conveyed by cytoskeletal organization is often maintained over the lifetime of the cell. Polarized epithelial cells use organized arrays of microtubules, actin filaments, and intermediate filaments to maintain the critical differences between the apical surface and the basolateral surface. They also must maintain strong adhesive contacts with one another to enable this single layer of cells to serve as an effective physical barrier (Figure 16–4).
Cytoskeletal filaments can reach from one end of the cell to the other, spanning tens or even hundreds of micrometers. Yet the individual protein molecules that form the filaments are only a few nanometers in size. The cell builds the filaments by assembling large numbers of the small subunits, like building a skyscraper out of bricks. Because these subunits are small, they can diffuse rapidly in the cytosol, whereas the assembled filaments cannot. In this way, cells can undergo rapid structural reorganizations, disassembling filaments at one site and reassembling them at another site far away.
Actin filaments and microtubules are built from subunits that are compact and globular—actin subunits for actin filaments and tubulin subunits for microtubules—whereas intermediate filaments are made up of smaller subunits that are themselves elongated and fibrous. All three major types of cytoskeletal filaments form as helical assemblies of subunits (see Figure 3–22) that self-associate, using a combination of end-to-end and side-to-side protein contacts. Differences in the structures of the subunits and the strengths of the attractive forces between microvillus terminal web of actin
Figure 16–4 Organization of the cytoskeleton in polarized epithelial cells.
All the components of the cytoskeleton cooperate to produce the characteristic shapes of specialized cells, including the epithelial cells that line the small intestine, diagrammed here. At the apical (upper) surface, facing the intestinal lumen, bundled actin filaments (red) form microvilli that increase the cell surface area available for absorbing nutrients from food. Below the microvilli, a circumferential band of actin filaments is connected to cell–cell adherens junctions that anchor the cells to each other. Intermediate filaments (blue) are anchored to other kinds of adhesive structures, including desmosomes and hemidesmosomes, that connect the epithelial cells into a sturdy sheet and attach them to the underlying extracellular matrix; these structures are discussed in Chapter 19. Microtubules (green) run vertically from the top of the cell to the bottom and provide a global coordinate system that enables the cell to direct newly synthesized components to their proper locations.
them produce important differences in the stability and mechanical properties of each type of filament. Whereas covalent linkages between their subunits hold together the backbones of many biological polymers—including DNA, RNA, and proteins—it is weak noncovalent interactions that hold together the three types of cytoskeletal polymers. Consequently, their assembly and disassembly can occur rapidly, without covalent bonds being formed or broken.