Regulation of the dynamic behavior and assembly of cytoskeletal filaments allows eukaryotic cells to build an enormous range of structures from the three basic filament systems. The micrographs in Panel 16–1 illustrate some of these structures. Actin filaments underlie the plasma membrane of animal cells, providing strength and shape to its thin lipid bilayer. They also form many types of cell-surface projections. Some of these are dynamic structures, such as the lamellipodia and filopodia that cells use to explore territory and move around. More stable arrays allow cells to brace themselves against an underlying substratum and enable muscle to contract. The regular bundles of stereocilia on the surface of hair cells in the inner ear contain stable bundles of actin filaments that tilt as rigid rods in response to sound, and similarly organized microvilli on the surface of intestinal epithelial cells vastly increase the apical cell-surface area to enhance nutrient absorption. In plants, actin filaments drive the rapid streaming of cytoplasm inside cells.
Microtubules, which are frequently found in a cytoplasmic array that extends to the cell periphery, can quickly rearrange themselves to form a bipolar mitotic spindle during cell division. They can also form cilia, which function as motile whips or sensory devices on the surface of the cell, or tightly aligned bundles that serve as tracks for the transport of materials down long neuronal axons. In plant cells, organized arrays of microtubules help to direct the pattern of cell wall synthesis, and in many protozoans they form the framework upon which the entire cell is built.
Intermediate filaments line the inner face of the nuclear envelope, forming a protective cage for the cell’s DNA; in the cytosol, they are twisted into strong cables that can hold epithelial cell sheets together or help nerve cells to extend long and robust axons, and they allow us to form tough appendages such as hair and fingernails.
An important and dramatic example of rapid reorganization of the cytoskeleton occurs during cell division, as shown in Figure 16–2 for a fibroblast growing in a tissue-culture dish. After the chromosomes have replicated, the interphase microtubule array that spreads throughout the cytoplasm is reconfigured into the bipolar mitotic spindle, which segregates the two copies of each chromosome into daughter nuclei. At the same time, the specialized actin structures that enable the fibroblast to crawl across the surface of the dish rearrange so that the cell stops moving and assumes a more spherical shape. Actin and its associated motor protein myosin then form a belt around the middle of the cell, the contractile ring, which constricts like a tiny muscle to pinch the cell in two. When division is complete, the cytoskeletons of the two daughter fibroblasts reassemble into their interphase structures to convert the two rounded-up daughter cells into smaller versions of the flattened, crawling mother cell.
Many cells require rapid cytoskeletal rearrangements for their normal functioning during interphase as well. For example, the neutrophil, a type of white
Actin flaments(also known as microflaments) are helical polymers of the protein actin. They are fexible structures with a diameter of 8 nm that organize into a variety of linear bundles, two-dimensional networks, and three-dimensional gels. Although actin flaments are dispersed throughout the cell, they are most highly concentrated in the cortex, just beneath the plasma membrane. (i) Single actin flament; (ii) microvilli; (iii) stress fbers (red) terminating in focal adhesions (green); (iv) striated muscle.
Micrographs courtesy of R. Craig (i and iv); P.T. Matsudaira and D.R. Burgess (ii); K. Burridge (iii).
Microtubulesare long, hollow cylinders made of the protein tubulin.