Figure 22–18 The four classes of muscle cells of a mammal. (a) Schematic drawings (to scale). (b–e) Scanning electron micrographs. Skeletal muscle fibers (b, from a hamster) are giant cells with many nuclei and are formed by cell fusion. the other types of muscle cells are more conventional, generally having only a single nucleus. heart muscle cells (C, from a rat) resemble skeletal muscle fibers in that their actin and myosin filaments are aligned in very orderly arrays to form a series of contractile units called sarcomeres, so that the cells have a striated (striped) appearance. the arrows in (C) point to intercalated discs—end-toend junctions between the heart muscle cells; skeletal muscle cells in long muscles are joined end-to-end in a similar way. Smooth muscle cells (d, from the urinary bladder of a guinea-pig) are so named because they do not appear striated; they belong to the connective-tissue family and are closely related to fibroblasts. note that the smooth muscle is shown here at a lower magnification than the other muscle types. the functions of smooth muscle vary greatly, from propelling food along the digestive tract to erecting hairs in response to cold or fear. myoepithelial cells (e, from a secretory alveolus of a lactating rat mammary gland) also have no striations, but unlike all other muscle cells they lie in epithelia and are derived from the ectoderm. they form the dilator muscle of the eye’s iris and serve to expel saliva, sweat, and milk from the corresponding glands. (b, courtesy of Junzo desaki;  C, from t. Fujiwara, in Cardiac muscle in handbook of microscopic anatomy [e.d. Canal, ed.]. berlin: Springer-verlag, 1986; d, courtesy of Satoshi nakasiro; e, from t. nagato et al., Cell Tissue Res. 209:1–10, 1980. with permission from Springer-verlag.)
Some myoblasts persist as Quiescent Stem Cells in the adult
Even though humans do not normally generate new skeletal muscle fibers in adult life, they still have the capacity to do so, and existing muscle fibers can resume growth when the need arises. Cells capable of serving as myoblasts are retained as small, flattened, and inactive cells lying in close contact with the mature muscle cell and contained within its sheath of basal lamina (Figure 22–20). If the muscle is damaged or stimulated to grow, these satellite cells are activated to proliferate, and their progeny can fuse to repair the damaged muscle or to allow muscle growth. Satellite cells, or some subset of the satellite cells, are thus the stem cells of adult skeletal muscle, normally held in reserve in a quiescent state but available when needed as a self-renewing source of terminally differentiated cells.
The process of muscle repair by means of satellite cells is, however, limited in what it can achieve. In one form of muscular dystrophy, for example, a genetic defect in the cytoskeletal protein dystrophin damages differentiated skeletal muscle cells. As a result, satellite cells proliferate to repair the damaged muscle fibers. This regenerative response is, however, unable to keep pace with the damage, and connective tissue eventually replaces the muscle cells, blocking any further possibility of regeneration. A decline of capacity for repair likewise contributes to the weakening of muscle in the elderly.
satellite cell activated to divide
Figure 22–19 Myoblast fusion in culture.