New Skeletal Muscle Fibers Form by the Fusion of Myoblasts

The previous chapter described how certain cells, originating from the somites of a vertebrate embryo at a very early stage, become determined as myoblasts, the precursors of skeletal muscle fibers. As discussed in Chapter 7, the commitment to be a myoblast depends on gene regulatory proteins of at least two families—the MyoD family of basic helix-loop-helix proteins, and the MEF2 family of MADS box proteins. These act in combination to give the myoblast a memory of its committed state, and, eventually, to regulate the expression of other genes that give the mature muscle cell its specialized character (see Figure 7-72). After a period of proliferation, the myoblasts undergo a dramatic switch of phenotype that depends on the coordinated activation of a whole battery of muscle-specific genes, a process known as myoblast differentiation. As they differentiate, they fuse with one another to form multinucleate skeletal muscle fibers (Figure 22-41). Fusion involves specific cell-cell adhesion molecules that mediate recognition between newly differentiating myoblasts and fibers. Once differentiation has occurred, the cells do not divide and the nuclei never again replicate their DNA.

Figure 22-41

Myoblast fusion in culture. The culture is stained with a fluorescent antibody (green) against skeletal muscle myosin, which marks differentiated muscle cells, and with a DNA-specific dye (blue) to show cell nuclei. (A) A short time after a change to (more...)

Myoblasts that have been kept proliferating in culture for as long as two years still retain the ability to differentiate and can fuse to form muscle cells in response to a suitable change in culture conditions. Appropriate signal proteins such as fibroblast or hepatocyte growth factor (FGF or HGF) in the culture medium can maintain myoblasts in the proliferative, undifferentiated state: if these soluble factors are removed, the cells rapidly stop dividing, differentiate, and fuse. The system of controls is complex, however, and attachment to the extracellular matrix is also important for myoblast differentiation. Moreover, the process of differentiation is cooperative: differentiating myoblasts secrete factors that apparently encourage other myoblasts to differentiate. In the intact animal, the myoblasts and muscle fibers are held in the meshes of a connective-tissue framework formed by fibroblasts. This framework guides muscle development and controls the arrangement and orientation of the muscle cells.

Muscle Cells Can Vary Their Properties by Changing the Protein Isoforms They Contain

Once formed, a skeletal muscle fiber grows, matures, and modulates its character according to functional requirements. The genome contains multiple variant copies of the genes encoding many of the characteristic proteins of the skeletal muscle cell, and the RNA transcripts of many of these genes can be spliced in several ways. As a result, a wealth of protein variants (isoforms) can be produced for the components of the contractile apparatus. As the muscle fiber matures, different isoforms are produced, adapted to the changing demands for speed, strength, and endurance in the fetus, the newborn, and the adult. Within a single adult muscle, several distinct types of skeletal muscle fibers, each with different sets of protein isoforms and different functional properties, can be found side by side (Figure 22-42). Slow muscle fibers (for sustained contraction) and fast muscle fibers (for rapid twitch) are innervated by slow and fast motor neurons, respectively, and the innervation can regulate muscle-fiber gene expression and size through the different patterns of electrical stimulation that these neurons deliver.

Figure 22-42

Fast and slow muscle fibers. Two consecutive cross sections of the same piece of adult mouse leg muscle were stained with different antibodies, each specific for a different isoform of myosin heavy chain protein, and images of the two sections were overlaid (more...)

Skeletal Muscle Fibers Secrete Myostatin to Limit Their own Growth

A muscle can grow in three ways: its fibers can increase in number, in length, or in girth. Because skeletal muscle fibers are unable to divide, more of them can be made only by the fusion of myoblasts, and the adult number of multinucleated skeletal muscle fibers is in fact attained early—before birth, in humans. Once formed, a skeletal muscle fiber generally survives for the entire lifetime of the animal. However, individual muscle nuclei can be added or lost. Thus, the enormous postnatal increase in muscle bulk is achieved by cell enlargement. Growth in length depends on recruitment of more myoblasts into the existing multinucleated fibers, which increases the number of nuclei in each cell. Growth in girth, such as occurs in the muscles of weightlifters, involves both myoblast recruitment and an increase in the size and numbers of the contractile myofibrils that each muscle fiber nucleus supports.

What, then, are the mechanisms that control muscle cell numbers and muscle cell size? One part of the answer lies in an extracellular signal protein called myostatin. Mice with a loss-of-function mutation in the myostatin gene have enormous muscles—two to three times larger than normal (Figure 22-43). Both the numbers and the size of the muscle cells seem to be increased. Mutations in the same gene turn out to be present in so-called “double-muscled” breeds of cattle (see Figure 17-51): in selecting for big muscles, cattle breeders have unwittingly selected for myostatin deficiency. Myostatin belongs to the TGFβ superfamily of signal proteins, and it is normally made and secreted by skeletal muscle cells. Its function, evidently, is to provide negative feedback to limit muscle growth. Small amounts of the protein can be detected in the circulation of adult humans, and it has been reported that the amount is raised in AIDS patients who show muscle wasting. Thus, myostatin may act as a negative regulator of muscle growth in adult life as well as during development. The growth of some other organs is similarly controlled by a negative-feedback action of a factor that they themselves produce. We shall encounter another example in a later section.

Figure 22-43

Regulation of muscle size by myostatin. (A) A normal mouse compared with a mutant mouse deficient in myostatin. (B) Leg of a normal and (C) of a myostatin-deficient mouse, with skin removed to show the massive enlargement of the musculature in the mutant. (more...)

Some Myoblasts Persist as Quiescent Stem Cells in the Adult

Even though humans do not normally generate new skeletal muscle fibers in adult life, the capacity for doing so is not completely lost. 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 22-44). If the muscle is damaged, these satellite cells are activated to proliferate, and their progeny can fuse to repair the damaged muscle. 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. Athletes who specialize in muscular strength often damage their muscle fibers and are thought to depend on this mechanism for muscle repair, resulting in regenerated fibers that are often highly branched.

Figure 22-44

A satellite cell on a skeletal muscle fiber. The specimen is stained with an antibody (red) against a muscle cadherin, M-cadherin, which is present on both the satellite cell and the muscle fiber and is concentrated at the site where their membranes are (more...)

The process of muscle repair by means of satellite cells is, nevertheless, limited in what it can achieve. In one form of muscular dystrophy, for example, differentiated skeletal muscle cells are damaged because of a genetic defect in the cytoskeletal protein dystrophin. 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 the muscle cells are eventually replaced by connective tissue, blocking any further possibility of regeneration. A similar loss of capacity for repair seems to contribute to the weakening of muscle in the elderly.

In muscular dystrophy, where the satellite cells are constantly called upon to proliferate, their capacity to divide may become exhausted as a result of progressive shortening of their telomeres in the course of each cell cycle (discussed in Chapter 17). Stem cells of other tissues, such as blood, are limited in the same way: they normally divide only at a slow rate, and mutations or exceptional circumstances that cause them to divide more rapidly can lead to premature exhaustion of the stem-cell supply.

Summary

Skeletal muscle fibers are one of the four main categories of vertebrate cells specialized for contraction, and they are responsible for all voluntary movement. Each skeletal muscle fiber is a syncytium and develops by the fusion of many myoblasts. Myoblasts proliferate extensively, but once they have fused, they can no longer divide. Fusion generally follows the onset of myoblast differentiation, in which many genes encoding muscle-specific proteins are switched on coordinately. Some myoblasts persist in a quiescent state as satellite cells in adult muscle; when a muscle is damaged, these cells are reactivated to proliferate and to fuse to replace the muscle cells that have been lost. Muscle bulk is regulated homeostatically by a negative-feedback mechanism, in which existing muscle secretes myostatin, which inhibits further muscle growth.