Muscle Cell Growth and Development (2024)

Muscle Cell Structure and Composition

The differentiated muscle cell in postnatal muscle is the muscle fiber, a highly specialized, long, cylindrical cell that can range in diameter from 10 to 100 mm and in length from millimeters up to many centimeters. The primary differences in fibers of different species are fiber length and number of fibers per muscle. Each fiber is surrounded by a 7.5- to 10-nm-thick plasmalemma, called the sarcolemma. The sarcolemma is a lipid bilayer like the cell membranes of other cells and has a lipid composition of roughly 60 percent protein, 20 percent phospholipid, and 20 percent cholesterol. Surrounding the sarcolemma is the basal lamina, or basem*nt membrane. This somewhat amorphous structure, 50 to 70 nm thick, is composed of mucopolysaccharides and collagen (types III and V). The cell membrane of muscle has a specialized structure—the motor endplate—which accommodates interaction with an axon from a motoneuron. In addition, the membrane maintains an electrical potential that is propagated from the motor endplate, down the membrane, and finally into the cell by a complex set of invagin*tions that form the transverse tubular system.

Muscle fibers contain the major organelles present in most cells. The most striking difference between muscle cells and the majority of other cells is their multinucleated nature. Depending on its size, an individual fiber may contain hundreds of nuclei. They are found just beneath the sarcolemma and seem to be randomly distributed along the length of the fiber. Mitochondria are present between the contractile elements of muscle; their concentration varies with the metabolic activity of the particular fiber. Ribosomes are dispersed within the cytoplasm, but very few are associated with endoplasmic reticulum, primarily because muscle fibers synthesize few secreted proteins. The endoplasmic reticulum in muscle has formed a specialized set of membrane structures called the sarcoplasmic reticulum. The primary function of this structure is regulation of free calcium ion concentration. When free calcium ion concentration is maintained below approximately 0.1 mM, contraction does not occur. But when the membrane is depolarized, the action potential reaches the interior of the cell through the transverse tubular system, calcium is released from the sarcoplasmic reticulum, the concentration approaches 1 mM, and contraction is activated. Lysosomes are not readily seen in muscle fibers, although lysosomal enzymes are present. The lysosomes are most likely sequestered in the sarcoplasmic reticulum.

By far the most unique subcellular aspect of muscle fibers is the contractile machinery, the myofibril. This is an aggregation of 12 to 14 proteins into highly organized contractile threads that are insoluble at the ionic strength of the cytoplasm in muscle cells. It is noteworthy that this specialized set of proteins constitutes about 55 percent of the total protein in muscle. Consequently, many developmental studies of muscle have focused on myofibrillar protein gene expression and synthesis, which are discussed later in this paper.

Myofibrils are composed of two main classes of filaments: thick filaments and thin filaments. Thick filaments measure approximately 15 nm by 1,500 nm. The major protein in thick filaments is myosin, which has the active site that hydrolyzes adenosine triphosphate (ATP) and the site that binds to actin in the thin filament. The thin filament is roughly 6 nm by 1,000 nm and is composed of actin, which forms the beaded backbone of the filament, and tropomyosin and troponin, which perform regulatory functions. At one end, thin filaments insert into a protein lattice called the Z-line; at the other end, they overlay with thick filaments in a hexagonal array. Additional small-diameter filament systems are present within myofibrils to provide an elastic component. Also, an intermediate-diameter filament system, found outside the periphery of the myofibril, links adjacent myofibrils and maintains their contractile units in register. Specific details of the ultrastructure of myofibrils and the biochemical properties of this interdigitating array of filaments can be found in Goll et al. (1984).

These features of muscle cells are common to all skeletal muscle fibers, but specific fibers have differentiated somewhat depending on their purpose. Some populations of fibers are primarily responsible for rapid contractions on an intermittent basis, while others have slower contraction speed and sustain contractile activity over extended periods of time. Muscle fiber types have been described extensively in many species; and their biochemical, physiological, and morphological differences are significant to problems of muscle growth and meat quality. A generalized scheme for describing fiber types classifies them on the basis of their contraction speed and on the energy metabolism pathways primarily used to provide energy for contraction. Peter et al. (1972) provided one of the most descriptive classification systems by grouping fibers into three general categories. Fibers that were dependent on oxidative metabolism and had slower contraction speeds were classified as slow-twitch, oxidative fibers (so). Fibers with faster contraction times that were dependent on anaerobic, or glycolytic, energy metabolism pathways were termed fast-twitch, glycolytic fibers (FG). A third broad category contained fast-twitch fibers that had glycolytic metabolic capabilities but also a significant capacity for oxidative metabolism; these were termed fast-twitch, oxidative-glycolytic fibers (FOG).

Contraction speed is correlated with myosin adenosine triphosphatase (ATPase) activity and, therefore, with the particular myosin isozymes synthesized by the fiber. Other myofibrillar protein isoform variations may also be associated with contractile properties. The complexity and degree of development of the sarcoplasmic reticulum, t-tubule system, and neuromuscular junctions have all been associated with contraction speed and fiber class. As expected, mitochondrial content and glycolytic enzyme content vary, among fiber types, as do energy substrates such as glycogen and triglyceride. Aspects of fiber type variation that affect muscle growth include the notable differences in fiber size that generally correlate with muscle fiber type. SO fibers are smaller in diameter than FG fibers, and FOG fibers tend to be intermediate in size. Smaller fiber diameters may facilitate efficient gas exchange in oxidative fibers. In addition, SO fibers tend to have higher nuclei concentrations and, therefore, lower protein concentrations per nucleus. Satellite cell frequency, however, is reportedly higher for SO fibers (Kelly, 1978b). Because individual muscles vary in fiber type composition, factors that differentially affect the development or growth of specific fiber types can result in alterations in muscle mass (for example, the transition from FG to FOG that can accompany aerobic conditioning). Reductions in fiber diameter and, consequently, muscle mass would be expected. Alterations in gene expression and in quantitative aspects of protein metabolism that are responsible for such fiber type transitions are poorly understood.

Chemical composition of muscle tissue can be quite variable, and the primary source of variation is intramuscular adipose tissue. It is clear that most of the variation in major constituents is minimized when expressed on a fat-free basis. Some compositional variation can be found in association with aging, but, in general, it is attributable to changes in moisture content. Skeletal muscle from very young animals has a high moisture content that decreases with maturity. As a result, protein concentration increases with maturity. Subtle changes in other constituents, such as glycogen, can vary among muscles and species, but these differences may not have major nutritional significance when considering the composition of muscle as a food.

The primary lipid fraction contributing to muscle tissue variation is triglyceride, which is stored in adipocytes within the muscle. These depositions are commonly referred to as marbling, and within the range of marbling found in the longissimus muscle of beef, the ether-extractable lipid (primarily triglyceride) varies from 1.77 to 10.42 percent on a wet weight basis (Savell et al., 1986).

Cholesterol content, on the other hand, is less variable. This can best be understood in light of its role in muscle tissue. Cholesterol is an integral part of cell membranes, mainly the plasma membrane. On a tissue basis across maturity groups and marbling contents within maturity groups, cholesterol content of beef muscle does not vary (Stromer et al., 1966). In addition, the amount of cholesterol per gram of whole steak was not significantly different among the five yield grades examined by Rhee et al. (1982). Furthermore, neither breed type nor nutritional background affected cholesterol content of lean muscle tissue in beef cows (Eichhorn et al., 1986). It is possible to find variation in cholesterol content of meat, however, because adipose tissue tends to have a higher cholesterol concentration than do muscle fibers. Consequently, variations in the amount of subcutaneous or inter-muscular fat consumed with the lean portion can alter cholesterol intake. It has been calculated that 37 to 56 percent of the cholesterol in a cooked rib steak of beef originates from subcutaneous and inter-muscular adipose tissue (Rhee et al., 1982).

In looking only at muscle cells, however, significant variations in cholesterol content have not been seen, even among most of the species used for muscle foods (Reiser, 1975; Watt and Merrill, 1963). This is also true for the amino acid composition of muscle. The majority of muscle cell proteins are myofibrillar and are very highly conserved across species. In addressing topics such as alteration of tissue composition to enhance nutritional quality, it is important to keep in mind that the biology of the animal or tissue must come first. Our ability to manipulate cells in animals has both physiological limits and ramifications.

Muscle Fiber Development

Muscle Fiber Protein Metabolism

Muscle protein metabolism encompasses a broad range of cellular activities, many of which are integral parts of energy metabolism in the whole animal. Most notable among these biochemical processes are the deamination of amino acids and the utilization of the carbon skeletons for energy production; supplying amino acids to the liver for gluconeogenesis is another important function. These aspects of muscle protein metabolism are obviously critical to the physiology of the animal, but they are not necessarily directly related to muscle growth. Consequently, this discussion dwells on two broad growth-related processes in muscle: protein synthesis and protein degradation. The quantitative balance between these two activities determines the net accumulation of protein in muscle.

A fundamental concept that has been widely appreciated only within the past decade or so is the fact that muscle protein is in a constant state of flux. Protein is constantly being degraded. It would not be out of the ordinary, for example, to experience a 5 to 10 percent rate of degradation of protein per day. To maintain muscle mass, the muscle would have to synthesize an amount of protein equivalent to 5 to 10 percent of its protein content on a daily basis. The ramifications of this are enormous when one considers the energetic costs of synthesizing one peptide bond and the total number of peptide bonds that must be degraded and resynthesized per day. It is easy to understand why protein turnover represents a significant factor in the "maintenance" energy requirements of an animal. It is also easy to see how the efficiency of growth or production could be enhanced if protein turnover could be altered in a favorable way.

A number of studies have demonstrated the balance between protein synthesis and degradation in domestic animals, laboratory animals, and humans and have revealed a general trend: In growing animals, synthesis and degradation rates are elevated with synthesis rate exceeding degradation rate; as maturity is approached, both synthesis and degradation rates decrease and ultimately reach a low and equal rate. With only minor variations, these trends have been observed in cattle, chickens, and laboratory animals (Lewis et al., 1984; MacDonald and Swick, 1981; McCarthy et al., 1983; Millward and Waterlow, 1978; Millward et al., 1976).

Certain metabolic hormones influence protein turnover; glucocorticoids, for example, cause muscle atrophy by depressing synthesis and degradation (McGrath and Goldspink, 1982). Synthesis rate is apparently depressed to a greater extent than degradation rate. Insulin, on the other hand, causes net accretion of protein, primarily by affecting synthesis rate (Tischler, 1981), and generally antagonizes the glucocorticoid effect on synthesis and degradation (Tomas et al., 1984). Thyroid hormone, T3, can increase degradation rate, but this modulation tends to follow the rate of synthesis (Millward, 1985), so there is a minimal change in protein accretion. Metabolites such as branched-chain amino acids or the keto acids of these amino acids may also be involved in depressing degradation (Mitch and Clark, 1984; Tischler et al., 1982). The integrated response of muscle to the inter-play of metabolites and metabolic hormones is not completely understood but represents an important feature of muscle protein accretion regulation.

In addition to the homeostatic regulation of protein turnover, relative rates of synthesis and degradation are altered during growth. Thus far, the only growth-related hormones that have been implicated in regulating protein degradation are the insulin-like growth factors, the somatomedins. Most of the work in this area has been conducted in vitro, where potent inhibitory effects have been observed (Ballard et al., 1986; Janeczko and Etlinger, 1984). The involvement is somewhat perplexing, since rapid growth rates in young animals are accompanied by increased rates of degradation, not decreased degradation. This point of contention, however, may be related to the in vitro assay system; the key element in the observation may be the decrease in degradation rate relative to synthesis rate.

Several physiological conditions have been shown to affect the rates of synthesis and degradation in skeletal muscle. Included among these are physical influences such as muscle stretching, which leads to hypertrophy (Goldspink, 1978; Summers et al., 1985). In vitro muscle stretching decreases protein degradation (Baracos and Goldberg, 1985). Inflammation, fever, and burns also have a dramatic effect by accelerating protein turnover (Goldberg et al., 1984); the common denominator in these observations and in the stretch-induced alteration in turnover may be calcium metabolism. In vitro, an influx of calcium into cells increases protein degradation (Silver and Etlinger, 1985). Furthermore, the calcium-induced elevation in degradation is of nonlysosomal origin (Furuno and Goldberg, 1986), as evidenced by the failure of lysosomal protease inhibitors to inhibit this calcium-induced response.

At present, an inadequate mechanistic understanding of the biochemical details of protein synthesis and degradation—especially degradation—is blocking progress in research on the regulation of these processes. Nutritional/physiological experimentation has provided an important descriptive base, but future progress depends on cellular and molecular details. As mentioned previously, new information on the regulation of myofibrillar protein isoform transitions and the structure and regulation of genes encoding these proteins will have a dramatic impact on our view of muscle protein synthesis regulation. Molecular details of the interaction of key hormones or their second messengers with myofibrillar protein genes should be forthcoming within the next decade.

It is traditionally assumed that lysosomal enzymes are responsible for intracellular protein degradation. These proteases are contained in lysosomes and are active at acidic pH. Several different proteases are grouped in this class and called cathepsins. Not all cathepsins are able to cleave peptide bonds in myofibrillar proteins; only cathepsins B1, D, H, and L have been found in muscle and are active on myofibrillar protein substrates (see Goll et al., 1983). A problem with attributing myofibrillar protein degradation in skeletal muscle to catheptic proteases is the fact that myofibrils or myofilaments have not been observed in lysosomal structures in muscle. Nor have lysosome-like organelles been observed in association with myofibrils. In addition, treatment of cells with lysosomal enzyme inhibitors failed to suppress calcium-induced protein degradation (Furuno and Goldberg, 1986). Although it has been possible in some eases to show correlations between lysosomal enzyme activity and protein degradation, the cause-and-effect relationship has not been proved.

A more likely mechanism for explaining myofibrillar protein degradation begins with the action of nonlysosomal cytoplasmic pro-teases that selectively cleave certain myofibrillar proteins, resulting in the disassembly of filaments in the myofibril (Dayton et al., 1975). Individual myofibrillar proteins or fragments of these proteins can then be taken up by lysosomes and degraded to individual amino acids. If such a scheme is accurate, one of the rate-limiting steps in the process would be the initial degradation steps accomplished by nonlysosomal pro-teases. Recent evidence suggests that activation of calcium-induced and injury-induced protein degradation in muscle does not involve a lysosomal mechanism (Furuno and Goldberg, 1986).

A couple of strong candidates have been suggested for this degradation role, the first of which is the calcium-dependent neutral protease described by Dayton et al. (1976). This protease, with a molecular weight of 110,000 daltons, is located inside skeletal muscle cells, as well as many other cell types, and is active at neutral pH. In skeletal muscle cells, it is found in the sarcoplasm and not in lysosomal structures or other intracellular membrane-bound organelles. The specificity of this protease is somewhat limited in that it generally cleaves only one or a few peptide bonds in a protein. In the myofibril, the proteins affected are troponin-T, troponin-I, tropomyosin, C-protein, filamin, desmin, the Z-line structure, and possibly titin (Goll et al., 1983). Many of these proteins have regulatory and structural significance. Note, however, that the primary proteins in the myofibril—actin and myosin—are apparently not hydrolyzed by this protease.

Although the regulatory details of this protease have not been elucidated, it is clear that calcium ions and a free sulfhydryl group are required for activity. It is also accepted that two forms of the protease exist, one that requires millimolar concentrations of calcium and another that only requires micromolar concentrations for activity. These are distinctly different proteins that share a high degree of sequence hom*ology. The active sites of these proteins are similar to those of papain, and the calcium-binding regions are similar to those of calmodulin (Emori et al., 1986). To add to the complexity of the system, an inhibitor of these proteases is also found in skeletal muscle. The physiological regulation of these different forms of the enzyme and inhibitor is not clear, but it may be crucial to an understanding of protein degradation and turnover.

Other soluble proteases may also be important components of the myofibril degradation process. Several alkaline or neutral proteases have the ability to hydrolyze actin or myosin, but most of these are not found in muscle cells. Perhaps the degradative system understood in greatest detail is an ATP-dependent protease system (Hershko and Ciechanover, 1982), which is found in many cells but has been most extensively studied in reticulocytes. This system is composed of a small, heat-stable protein called ubiquitin (because of its presence in a highly conserved form in most cells) that interacts with an activating enzyme in an ATP-dependent process to ultimately form a covalent isopeptide bond between the carboxyl group of the C-terminal glycine residue of ubiquitin and an epsilon amino group of a lysine on the target protein. The covalent attachment of ubiquitin is thought to target the protein for protease attack. The pro-teases responsible are ill-defined, but the end products are peptides and a released ubiquitin that can recycle. This system could be responsible for identifying proteins that were damaged structurally or otherwise in-activated. Other protease systems requiring ATP may also be present in cells, but their characterization is far from complete. The primary problems with proposed roles for these ATP-dependent proteolytic systems in muscle protein degradation are the lack of detailed information about the specificity of these systems for muscle proteins and the presence and location of these systems in muscle cells.

During normal growth and in many metabolic states, rates of synthesis and degradation tend to move in tandem. Even during fasting, degradation is depressed and not increased, presumably to spare protein. These observations suggest that during normal growth, protein synthesis may represent the primary site of regulation, and degradation may follow (Millward, 1985). It is virtually impossible to tie together endocrine and nutritional influences on animal protein degradation and the subcellular events that mediate these effects because of the present gap that exists between our knowledge of the cellular and biochemical mechanisms involved in skeletal muscle protein degradation and the whole animal and tissue level descriptions of the process. This does not eliminate the possibility of targeting degradation as a site for muscle growth regulation, but it makes it difficult to devise strategies to manipulate protein degradation to enhance the efficiency of muscle growth in meat animals.

Strategies for Regulating Muscle Development and Growth in Meat-Producing Animals

Significant research areas that can be layered over the muscle-specific problem are the integration of metabolism during growth and the manner in which tissue growth is coordinated within the animal. These topics are more general and, at face value, more pertinent to altering efficiency of protein accretion and the composition of the product than are the studies of specific cellular and biochemical events in developing and growing muscle. But progress in these areas can only proceed as rapidly as progress toward a mechanistic understanding of muscle growth.

Establishing muscle cellularity, in its broadest sense, involves prenatal fiber development and nuclear accretion during postnatal growth. Fiber development is the result of myogenesis that takes place in the developing embryo or fetus. The final event in this cascade of proliferation and differentiation is the fusion of myoblasts into multinucleated myotubes that mature into fibers. Currently, hormones and growth factors that stimulate and inhibit the proliferative and differentiative events in myogenic cells are being identified, but the factors that regulate the number of fibers that are formed from a given cohort of myoblasts have not been considered experimentally. It may be that innervation plays a key role in establishing fiber number and organization in muscle, since innervation is required to sustain fibers. For many years, it has been accepted that major differences in muscle mass in mature animals can be attributed in large part to differences in fiber number. Consequently, alterations in fiber number during late prenatal life would likely result in differences in muscularity. At present, however, there is probably insufficient mechanistic detail to suggest specific approaches. A critical question in this regard is whether it is advisable to increase muscularity prenatally; in cattle, for example, increased management problems associated with dystocia could offset any advantages due to increased muscle growth potential. In swine or poultry, this problem may not be so acute.

Cellularity could conceivably be altered by nuclear accretion postnatally without reproductive problems. Again, we are beginning to understand more about the activation, proliferation, and differentiation of satellite cells, although specific physiological regulators have not yet been confirmed. Assuming that satellite cell activity could be altered and nuclear accretion in fibers could be influenced, satellite cells may be more receptive to manipulation efforts during certain periods of growth than others. Early postnatal growth is the time of greatest satellite cell activity and would correspond to the period of greatest sensitivity to hormones and growth factors. On the other hand, later phases of growth are marked by decreasing nuclear accretion rate; therefore, stimulating additional satellite cell proliferation and differentiation could result in an extension of the rapid muscle growth phase that is normally associated with muscle growth in younger animals.

Affecting changes in muscle growth by altering protein metabolism has been the most commonly considered avenue, primarily because of the erroneous assumption that cellularity does not change after birth. During normal growth, synthesis and degradation tend to move in parallel, with synthesis rate exceeding degradation rate. Consequently, accelerated growth rate is accompanied by an accelerated degradation rate; hence, there is no increase in efficiency of protein accretion. Because these processes seem to be coupled, Millward (1985) suggested that manipulating synthesis may be the most reasonable way to affect protein accretion. Specific alterations in synthesis await increased knowledge of the mechanisms of muscle protein gene regulation and the elucidation of hormones or other external signals, such as electrical stimulation or stretch, that modulate the expression of these genes.

Likewise, strategies for manipulating degradation rate in muscle will not progress beyond the empirical stage without a mechanistic understanding of the proteases involved and their regulation. Protein degradation is, however, an attractive target for postnatal growth manipulation. If degradation rate could be decreased, net rate of protein accretion would be accelerated and less energy would be expended on resynthesizing degraded protein.

To illustrate that the present level of cellular and molecular understanding of important regulatory events is grossly inadequate and, indeed, limiting, consider some current growth-manipulating techniques. Take three growth-altering treatments: growth hormone (GH), steroid hormones and their analogs, and beta-adrenergic agonists. With all three, scientists are still dependent on information that is often one or two decades old, or on empirical observations, the biology of which is still not fully understood. In these cases current biology is not leading the way to new applications; rather, new applications are leading basic biological investigation.

Let us begin with GH. Direct administration of GH to domestic meat animals was first reported in pigs by Truman and Andrews (1955), Henricson and Ullberg (1960), and Machlin (1972). Later, Chung et al. (1985) also reported direct administration of GH to pigs. Dramatic increases in muscle growth in GH-treated pigs (Etherton et al., 1986) could be the result of action at several sites, such as adipose tissue, where GH could be having an antilipogenic effect. If energy is not stored in adipose tissue, it may be more available for growth. It is also possible that when growth processes are stimulated, they demand more energy than do adipose tissue triglyceride storage activities. GH could also be having part of its effect by stimulating higher levels of somatomedins that are, in turn, stimulating satellite cells. Arguments can be made for increased muscle growth as a result of nuclear accretion and subsequent protein accumulation directed by new nuclei. Another plausible alternative may be somatomedinmediated depression of muscle protein degradation. Or, the net effect could be due to a combination of the above. The point is that it is application that is leading scientists to undertake basic biological research.

Next consider the steroid hormones and their analogs. Studies that were mostly empirical in nature gave us diethylstilbestrol. Its application has come and gone from agriculture, yet we still do not know its precise mode of action. Even the action of testosterone on muscle growth is unclear. Trenbolone acetate (TBA) is another ex-ample—it stimulates growth, but again, the mechanism is unknown. In terms of the biological events responsible for muscle growth, it must either directly or indirectly stimulate nuclear accretion, stimulate protein synthesis, or decrease protein degradation. At least one report suggests that protein synthesis is depressed but that protein degradation is depressed to a greater extent, thus leading to a net increase in rate of protein accretion as well as in efficiency of protein gain (Vernon and Buttery, 1976). This in vivo study was not able to address the direct or indirect nature of the action of TBA. In vitro studies are limited; however, TBA does not appear to have a direct effect on protein degradation in L6 muscle cells in culture (Ballard and Francis, 1983).

Another example of application leading basic investigation concerns a class of agents that has received a great deal of attention in recent years, the beta-adrenergic agonists. One of these—clenbuterol—was originally designed as a respiratory drug but was subsequently shown to have a stimulatory effect on rat growth. Since then, it has been used to stimulate growth and feed efficiency in poultry, sheep, and cattle (Baker et al., 1984; Dalrymple et al., 1984; Ricks et al., 1984). A great deal of effort is currently being devoted to understanding how it works. An obvious site of action would be as a lipolytic agent for adipose tissue; however, this alone could not explain the extreme muscle hypertrophy observed in sheep (Beermann et al., 1986). Recently, Kim et al. (1986) reported that the major effect appeared to be on hypertrophy of fast-twitch muscle fibers and that muscle DNA concentration actually decreased in the cimaterol-treated group. Beermann, however, indicated that a significant increase in DNA content was noted in 12-week studies with sheep but that DNA content increased after muscle hypertrophy (D. H. Beermann, personal communication, 1986). In another report, cimaterol was demonstrated to have an inhibitory effect on protein degradation in cultured myotubes from a rat muscle cell line (Forsberg and Merrill, 1986). Evidently, the beta-adrenergic agonists may have multiple sites of action, especially for protein degradation and adipose tissue metabolism, but this conclusion remains highly speculative.

These examples of a few of the most interesting agents currently being investigated for use in stimulating muscle growth not only demonstrate that application is leading investigation, they also provide striking demonstrations that muscle growth in meat animals can be manipulated to increase protein production and decrease triglyceride deposition beyond the normal physiological limits of a particular animal. They also suggest that the muscle growth processes mentioned earlier—protein synthesis and degradation and pre- and post-natal muscle cellularity alterations—repre-sent legitimate targets for growth-regulating strategies.

In the future, several approaches may be used to enhance rate and efficiency of muscle growth, but for now the most promising are administration of recombinant hormones. As indicated, recombinant GH has been shown to have impressive stimulatory effects on growth, feed efficiency, and carcass composition in pigs. Other hormones will surely be investigated in a similar manner. Based on recent research in muscle development, somatomedin-C/IGF-I is a logical choice for such application. New growth factors or combinations of growth factors that affect muscle development, such as fibroblast growth factor and IGF-II, are also candidates.

At present, GH administration entails regular injections during later stages in postnatal life. In a second generation of studies, researchers may wish to effect a permanent change in the cellularity of the animal, such as increased fiber number or myonuclei content. In contrast to approaches that are designed primarily to alter protein metabolism, cellular/developmental changes may only require acute treatments during early, critical stages of development. Therefore, the need for costly, labor-intensive administration schemes could be eliminated, as would potential questions about the presence of drug residues in the final product.

At another level of sophistication, transgenic animals may have a place in livestock production systems. Growth has already been accelerated in transgenic mice carrying a metallothionein-human growth hormone fusion gene (Palmiter et al., 1982) and in mice expressing the metallothionein-human growth hormone releasing factor minigene (Hammer et al., 1985). In addition, these genes have been shown to be transmittable to subsequent generations, although reproduction suffered in some of the initial studies (Hammer et al., 1985). These techniques will undoubtedly be applied in large domestic animals to produce new germplasm. Furthermore, it may be possible to construct and perpetuate the genes of important hormones that can be regulated by coupling the genes to promoters that can be turned on or off at critical periods through nutritional, pharmacological, or environmental manipulation. These approaches will obviously require a more detailed description of the significant regulatory events in muscle growth and the important factors that mediate these events so that appropriate molecular targets can be selected.

Conclusions

Major obstacles exist. New fundamental knowledge of cellular and molecular mechanisms of growth is desperately needed. Technical advances are also needed in the area of delivery systems for effectively administering exogenous agents at specific times and in appropriate amounts. Of critical importance are practical means for targeting the delivery of agents to specific tissues. It is conceivable that a factor could have a beneficial effect on one tissue or organ and a detrimental effect on another. This may be a major impediment to the application of certain hormones or growth factors. Technical advances are still needed in gene transfer and gene construct technology, but progress is occurring rapidly. These are only a few of the problem areas that need to be addressed.

Advances in the production of nutritious muscle protein foods will probably not come by altering the cellular composition of a muscle fiber. Membrane systems and myofibrillar proteins in muscle are highly conserved and may not be amenable to efforts to inflict gross alterations that would provide a more desirable balance of amino acids or reduced cholesterol content. An approach to improving the nutritional attributes of meat products that holds greater promise is one that attempts to reduce the amount of adipose tissue associated with meat products while maintaining palatability. Great advances can be made in the efficient production of muscle protein by providing a growing knowledge base in biology, by rapidly adopting new scientific technologies, and by fostering innovative applied research.

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