The cytoplasm of nucleated cells is supported by a cytoskeleton consisting of three types of fibers and a number of associated proteins. The functions of the fibers are to resist forces that would deform the cell, to allow the cell to change shape and move and some types of intracellular transport. The three types of fibers are microfilaments (MF), intermediate filaments (IF) and microtubules (MT). Although the lengths of the fibers are indeterminate as they are actively extended and shortened during cellular activities, their diameters are fairly uniform between cell types. Microfilaments and MTs have diameters of approximately 7 and 25 nm respectively. As their name implies, IFs have diameters between these values of 8–11 nm.
Microfilaments are made of the protein actin. They are relatively flexible filaments but, cross-linked into bundles, they can withstand compression. Microtubules are composed of tubulin proteins arranged into hollow rods that are rigid and can resist both compression and tension. Intermediate filaments are built up from a number of types of proteins that are tissue specific, keratins in epidermal cells, desmin in muscles, for example. They form flexible cables whose high tensile strength allows the cell to resist excessive stretching.
Microfilaments and MTs form defined tracks within the cell for the transport of macromolecules and membranous structures. The two most common methods for this involves the movements of motor proteins along the filaments that are driven by the hydrolysis of ATP. The motor proteins of the MTs are dyneins and kinesins; those of the MFs are the myosins. Actin–myosin complexes are probably best known as the contractile apparatus of skeletal muscle tissues. Skeletal muscle tissue shows a multinuclear organization or syncitium arranged into fibers, which are surrounded by a basal lamina of extracellular matrix proteins, which forms a supporting sheath. Each fiber contains sarcoplasm (cytoplasm) that houses the contractile fibers of actin and myosin and is surrounded by a sarcolemma (plasma membrane). A network of elongated protein molecules about 150 nm long of the protein dystrophin is found within the sarcoplasm The dystrophin links actin filaments to a transmembrane complex of proteins that, in turn, is linked to components of the basal lamina (Figure 16.16). This complex arrangement of proteins provides mechanical support to the sarcolemma during muscle contraction.
The blood contains about 5 q 1012 erythrocytes per dm3 . Their major function is to carry dioxygen from the lungs to the general body tissues. The unique biconcave shape of erythrocytes is maintained by a cytoskeleton composed of five major proteins that form a network lining the inner sur-face of their plasma membranes (Figure 16.17). The spectrin–actin complex is thought to act in a manner that resembles that of the dystrophin–actin complex of skeletal muscle and provides mechanical support to the plasma membrane preventing its lysis during circulation. The network of proteins also allows erythrocytes to deform and spring back into shape as they are pumped through the narrow capillaries of the vascular system. The numbers of erythrocytes are maintained by a constant production in the bone marrow and the destruction of worn out or misshapen erythrocytes by the spleen. This destruction releases bilirubin, which is converted to bile salts in the liver and released into the gastrointestinal tract in bile . The iron from the hemoglobin is largely retained and reused by the body.