Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
Maintenance of Normal Tissue Organization
Origin, Modification, and Regeneration of Skeletal Muscle Tissue

Cells whose primary function is contraction are generally referred to as "Muscle cells," although they may otherwise exhibit considerable structural diversity. As discussed in Chapter 11, the contractile apparatus—comprising Actin and Myosin—is a fundamental feature of animal cells in general, but it is exceptionally well-developed in muscle cells. Mammals possess four Major Types of cells specialized for contraction: Skeletal Muscle fibers, cardiac muscle cells, smooth muscle cells, and myoepithelial cells (Fig. 17-36). They differ in function, Structure, and developmental pathway.

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Fig. 17-36. The four types of mammalian muscle cells. A. Schematic representations (drawn to scale). B–D. Scanning electron micrographs: B - skeletal muscle from the neck of a hamster; C - rat cardiac muscle; D - smooth muscle from the Urinary Bladder of a guinea pig; E - myoepithelial cells in an alveolus of a Cytology/practical/135.html">Lactating mammary gland of a rat. The arrows in photo C indicate intercalated discs (see Section 11.1.14). Note that the smooth muscle is shown at a lower magnification than the others. [B - courtesy of Junzo Desaki; C - T. Fujiwara, Cardiac Muscle, in: Handbook of Microscopic Anatomy (E. D. Canal, ed.). Berlin: Springer Verlag, 1986; D - courtesy of Satoshi Nakasiro; E - T. Nagato et al., Cell and Tissue Res., 209, 1-10, 1980.]

Although all of them apparently utilize actin and myosin to generate mechanical force, these Proteins differ somewhat in Primary Structure across various cell types, are organized differently within the intracellular space, and associate with distinct sets of contraction-regulatory proteins.

Skeletal muscle cells, whose contractile apparatus is examined in detail in Chapter 11, are responsible for virtually all voluntary movements. These cells can attain enormous dimensions (up to half a meter in length and up to 100 µm in diameter in adult humans) and, owing to their elongated shape, are also commonly referred to as muscle fibers. Each such cell is a syncytium containing multiple nuclei within a common Cytoplasm. In contrast, the other Three types of muscle cells have a more conventional structure, possessing only a single nucleus. Cardiac muscle cells resemble skeletal muscle fibers in that their actin and myosin filaments form ordered arrays, giving The Cell a striated appearance. Smooth muscle cells derive their name from the fact that, by contrast, they lack striations. The Functions of smooth muscle are remarkably diverse, ranging from propelling food along the digestive tract to causing Hair to stand on end in response to cold or fear. Myoepithelial cells (which are also non-striated) differ from the other three cell types in that they reside within the epithelium and originate from the ectoderm. These cells form the musculature of the iris, which dilates the pupil, and are also responsible for expressing saliva, sweat, and milk from the respective glands (see Fig. 17-36, D).

The four principal types of muscle cells can be further subdivided into various subtypes, each possessing distinct characteristics. However, we will focus our attention on skeletal muscle cells due to their fascinating developmental mechanism, unusual mode of injury repair, and striking capacity to undergo phenotypic modulation in the differentiated state.

17.6.1. New Skeletal Muscle Cells Form by the Fusion of Myoblasts [2, 34]

The preceding chapter described how specific cells originating from somites at a very early stage of vertebrate development become determined as myoblasts (i.e., precursors of skeletal muscle cells) and migrate into the adjacent embryonic Connective Tissue, or mesenchyme (Section 16.6.5). As noted in Section 10.1.8, this determination of cell fate as a myoblast (rather than a fibroblast, for instance) apparently involves the Activation of a specific master regulatory Gene. Following a period of proliferation, myoblasts fuse with one another to form multinucleated skeletal muscle cells (Fig. 17-37). Upon fusion, they undergo a dramatic phenotypic shift resulting from the coordinated activation of an entire battery of other genes (Section 10.1.8). Once myoblasts have coalesced into a syncytium, the DNA in their nuclei never replicates again. This fusion is mediated by specific mutual recognition among myoblasts; they do not fuse with neighboring non-muscle cells. The Molecular Basis of this recognition process remains unknown.

Myoblasts cultured for as long as two years still retain their capacity for differentiation and, upon appropriate modification of culture conditions, will fuse to form muscle cells. Fibroblast growth factor (FGF) appears to be a key component of the medium that Supports proliferation while preventing differentiation: if FGF is withdrawn, the cells rapidly cease dividing, fuse, and differentiate. However, the regulatory system is complex. For differentiation to proceed, myoblasts must, for example, adhere to the Extracellular matrix. Furthermore, the fusion process is cooperative: fusing myoblasts apparently secrete unidentified factors that stimulate other myoblasts to fuse.

Fig. 17-37. Myoblasts in vitro proliferate, align in an orderly fashion, and then fuse to form multinucleated muscle cells. Phase-contrast micrographs of a live culture at successive stages. Photo C is taken at a higher magnification, revealing the cross-striations (indicated by the long arrow) that appear as soon as the contractile apparatus begins to develop. Multiple nuclei within a single cell are visible (short arrows). (Courtesy of Rosalind Zalin.)

17.6.2. Muscle Cells Can Modify Their Properties via the Substitution of Specific Protein Isoforms [35]

Once formed, a skeletal muscle cell typically persists for the remainder of the animal's life, growing, maturing, and altering its properties in accordance with functional demands. The Genome contains various gene variants encoding many of the characteristic proteins of skeletal muscle cells, and the RNA transcripts of certain genes can undergo Alternative Splicing. This generates a multitude of variants—known as isoforms—of the contractile apparatus protein components. During muscle cell maturation, different combinations of isoforms are selected to suit the changing requirements for contraction speed and fatigue resistance in the fetus, newborn, and adult.

Within a single muscle, one can find side-by-side muscle cells of different types, each with its own distinct set of protein isoforms. In adult animals, two main types can be readily distinguished even with the unaided eye. Red muscle fibers, such as those found in dark chicken meat, are rich in the oxygen-binding protein Myoglobin. White muscle fibers, such as those in white chicken meat, contain considerably less myoglobin. This variation in myoglobin—a protein related to Hemoglobin—reflects differing metabolic demands for oxygen: red fibers rely primarily on Oxidative Phosphorylation, whereas white fibers depend on anaerobic Glycolysis. These metabolic profiles, in turn, are correlated with Different types of contractile activity. In response to stimulation, red fibers contract slowly, are less susceptible to fatigue, and are more efficient for sustained exertion. White fibers provide a rapid response, fatigue more readily, and are better suited for brief, powerful bursts of movement. Muscles such as the biceps typically contain a mixture of several muscle cell types in proportions optimized for the specific function of that muscle (Fig. 17-38).

Fig. 17-38. Consecutive cross-sections of a region of adult chicken muscle stained with fluorescent Antibodies specific for two different myosin isoforms. A. White (fast-twitch) cells stained with antibodies against "fast" myosin. B. Red (slow-twitch) cells stained with antibodies against "slow" myosin. (G. Gauthier et al., J. Cell Biol., 92, 471-484, 1982.)

By surgically altering muscle innervation or artificially stimulating muscles with implanted electrodes, one can demonstrate that the frequency of electrical stimulation profoundly influences the pattern of Gene Expression IN a muscle cell. If a "slow" muscle cell is stimulated at a frequency more appropriate for a "fast" muscle, it partially transforms into a fast muscle, and vice versa; this shift occurs, in part, through the replacement of protein isoforms. As we have seen (Section 17.4.3), such changes are not unique to muscle cells: switches in gene expression leading to The production of variant mRNA isoforms frequently occur during the maturation of a differentiated cell or in response to environmental cues.

17.6.3. Some Myoblasts Persist in the Adult Organism as Quiescent Stem Cells [36]

Muscle can grow in three ways: through an increase in the length of differentiated muscle fibers, an increase in their thickness, and an increase in their number. Because skeletal muscle cells are incapable of division, new fibers can arise only through the fusion of myoblasts. The number of multinucleated cells in skeletal muscles essentially reaches its adult level at a very early stage of development—prior to birth in humans. Subsequent massive increases in muscle mass occur through the enlargement of individual cells. Longitudinal growth of a muscle depends on the incorporation of additional myoblasts into existing multinucleated cells, primarily at their ends, which increases the number of nuclei per cell. Conversely, radial thickening of a muscle, such as that seen in weightlifters, results from an increase in the size and number of contractile myofibrils within each individual cell (Section 11.1.1) rather than A change in the number of muscle cells or their nuclei.

Nevertheless, a small population of myoblasts persists in the adult organism. These are small, flattened, quiescent cells that lie in intimate contact with mature muscle fibers, enclosed within the fiber's basal lamina. Upon muscle injury or exposure to fibroblast growth factor, these so-called satellite cells are stimulated to resume proliferative activity (Fig. 17-39), and their progeny can fuse to form new muscle fibers. Thus, these resident stem cells in mature skeletal muscle serve as a reserve pool that can act as a self-renewing source of terminally differentiated cells when needed.

Fig. 17-39. Autoradiograph of a multinucleated muscle fiber with associated satellite cells. The fiber was isolated from an adult rat and placed in a culture medium containing ³H-thymidine and an extract from damaged muscle, which stimulates satellite Cell Division. The dividing satellite cells (indicated by arrows) have incorporated the radiolabel (silver grains appear as black dots). Muscle cell nuclei are incapable of division and remain unlabeled. (R. Bischoff, Dev. Biol., 115, 140-147, 1986.)

Summary

Vertebrate skeletal muscle cells (fibers) represent one of four specialized cell types dedicated to contraction and are responsible for voluntary movements. Each cell is a syncytium formed by the fusion of myoblasts. Myoblasts can be stimulated to proliferate by growth factors such as FGF, but once fused, they lose the capacity to divide. Myoblast fusion is typically coupled with the initiation of muscle Cell Differentiation, during which numerous distinct genes are activated in a coordinated manner. Subsequently, these cells can modify their differentiated phenotype by altering the repertoire of protein isoforms they synthesize. In adult muscles, a fraction of myoblasts persists in a quiescent state as satellite cells. In the event of muscle injury, these cells function as stem cells—initiating proliferation and fusion to repair the loss of muscle fibers.