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If one were to compare today's animal growth research to research from a mere 50 yr ago, one would see programs with few similarities. The evolution of this research from whole-animal through cell-based and finally molecular and genomic studies has been enhanced by the identification, isolation, and in vitro evaluation of adipose- and muscle-derived stem cells. This paper will highlight the struggles and the milestones that make this evolving area of research what it is today. The contribution of adipose and muscle stem cell research to development and growth, tissue regeneration, and final carcass composition are reviewed.

This study examined the effects of basic fibroblast growth factor (bFGF), insulin-like growth factor I (IGF-I) and transforming growth factor beta (TGF-beta) on the proliferation and differentiation of primary bovine satellite cells (BSC) in vitro. Individually, these three factors had the following effects on satellite cells: bFGF stimulated proliferation (P less than .01) but inhibited differentiation (P less than .05); IGF-I had no effect on proliferation but stimulated differentiation (P less than .01); and TGF-beta inhibited both proliferation and differentiation (P less than .01). When combined, the following effects were observed: maximum stimulation of proliferation (P less than .01) occurred in the presence of bFGF and IGF-I and differentiation was not stimulated; TGF-beta and bFGF continued to inhibit differentiation (P less than .01), but in the presence of bFGF, TGF-beta stimulated proliferation (P less than .01). No stimulation was observed in the presence of TGF-beta and IGF-I. Bovine satellite cells respond to these three growth factors that have been shown to regulate the activity of other myogenic cells, and in most instances, the responses among cells from various species are similar. These experiments indicate that the interactions of growth factors may be critical in regulating bovine satellite cell activity.

Differentiation of rat skeletal muscle satellite cells was studied in vitro. Linoleic acid and insulin, two unrelated compounds that reportedly stimulate differentiation of other types of myogenic cells, were used to examine the regulation of differentiation in satellite cell cultures. As in cultures of chick embryo muscle cells, linoleic acid stimulated fusion but only at low serum concentrations or in defined medium without fibroblast growth factor (FGF). The effects of insulin on differentiation were quite variable, however; at very low cell densities no stimulatory effect was observed. In intermediate and, to a lesser extent, high density satellite cell cultures, the addition of insulin at concentrations between .01 and 1.0 microM stimulated satellite cell fusion. Whenever increases in fusion were observed, however, a parallel increase in cell number was also found. A closer examination of the relationship between differentiation and the presence or absence of mitogenic agents in the medium suggested that a mitogenic signal and the resultant proliferation of cells prevented differentiation. Subsequent experiments indicated that fusion could be induced by lower serum concentration or by removal of FGF, as long as linoleic acid was present in the medium. Therefore, proliferation and differentiation appear to be antagonistic processes in cultured satellite cells. If the rate of proliferation is depressed, either by mitogen removal or by increasing cell density, differentiation is favored. Differentiation can, therefore, be regulated and applied to in vitro studies of satellite cell activity.

Muscle growth of male obese (ob/ob) and lean mice at 2, 3, 5 and 8 wk were analyzed on the basis of weights of gastrocnemius, plantaris and soleus muscles from each hind leg. The carcasses (prepared by removing skin, viscera, head, feet and tail) were analyzed for fat content so that the effect of phenotype on the relationship between muscle weight and fat-free carcass weight could be assessed. For each age group the obese mice had less muscle relative to fat-free carcass weight than lean mice, with the difference being significant at 3 wk (P less than .05) and 8 wk (P less than .025). The proliferative activity of muscle satellite cells in 2- and 3-wk-old obese and lean mice was measured on isolated muscle fibers by autoradiography. Muscle fiber diameter and number of nuclei/unit length were unaffected by phenotype, but the proportion of muscle nuclei showing proliferative activity was lower (P less than .01) in obese than in lean mice at 2 wk (1.05 vs 1.93%, respectively) and 3 wk of age (.23 vs .59%, respectively). These results are consistent with the suggestion that muscle growth is limited by satellite-cell proliferative activity, although direct evidence for a cause and effect relationship is not provided.

Differences in total carcass bone, muscle and fat, and linear measurements of the tibia and radius were evaluated in barrows at 105 kg and boars at 105, 118, 132 and 145 kg live body weight. The carcasses of five replicates were physically separated into skin, bone and soft tissues, and the linear measurements of the tibia and radius were obtained on seven replicates. At live weight of 105 kg, boars did not differ significantly in fat-free muscle, but they had 33.2% less fat, 11% greater bone weight and 14% greater skin weight than barrows. At 145 kg, boars had total carcass fat weight comparable with 105-kg barrows. Fat-free muscle, bone and skin weight of boars increased at linear rates of .41, .083 and .104 kg/kg of body weight increase from 105 to 145 kg, respectively. At 105 kg, density and length of the tibia and radius did not differ between boars and barrows. The tibia of boars were heavier than those of barrows at 105 kg, resulting in a greater ratio of tibia weight to length (indirect measure of bone thickness). As boars increased in live weight from 105 to 145 kg, total weight and length of the tibia and radius increased linearly. The ratio of weight to length of the tibia and radius increased during this 40-kg weight gain, indicating that weight of both bones increased at a greater rate than length. These results indicate that boars and barrows have the same weight of total carcass fat when boars are 40 kg heavier than the barrows. The greater bone weight of boar carcasses compared with barrows is due to greater bone thickness.

Because most of the DNA in a mature muscle accumulates during postnatal life and is derived from satellite cells, cellular regulation of protein accumulation in muscle fibers originating from satellite cells is an important aspect of muscle growth control. These experiments were designed to study the modulation of a alpha-actin accumulation in satellite cell-derived myotubes by serum and two anabolic hormones commonly assumed to be involved in muscle growth regulation; growth hormone and testosterone. Satellite cells were cultured from rats ranging in age from 5 d to 1 yr. After fusion into myotubes, various levels of pig serum, 3 to 20%, were added to culture medium, and the amount of alpha-actin per myotube nucleus was determined 4 d later. The quantity increased with increasing percentages of serum in the medium. For an assessment of the extent to which serum stimulatory activity was due to growth hormone or testosterone in the serum, similar experiments were conducted with medium containing 10% pig serum plus various levels of porcine growth hormone or testosterone. Neither of these hormones stimulated actin accumulation at concentrations from 10(-9) to 10(-6) M, a range encompassing physiological levels for each hormone. These experiments do not support the premise that either growth hormone and (or) testosterone are the blood-borne agents that interact directly with muscle to stimulate protein accretion, because neither growth hormone nor testosterone acted directly on muscle cells in vitro to stimulate muscle protein accumulation.

The rate of gain, carcass measurements and three muscles were evaluated in 65 crossbred boars representing 13 litters that were allotted at 4 wk of age to slaughter weight and treatment groups as follows: 1) 105 kg, castrated; 2) 105 kg, intact; 3) 118 kg, intact; 4) 132 kg, intact and 5) 145 kg, intact. One barrow and four boars within a litter constituted a replicate and each replicate was penned separately. The growth rate of all boars to 105 kg constituted one group and was compared with the growth rate of barrows to 105 kg live body weight. Average daily gain from 4 wk until 105 kg did not differ significantly between boars and barrows. Growth rate of the boars continued at an increasing rate until they reached 87.3 kg live weight, while maximum daily gain of barrows occurred at 76.3 kg live weight or 11 kg less than that of boars. At 105 kg, boars had 31.3% less 10th rib backfat thickness and 2.9% greater carcass length than barrows, but longissimus muscle area did not differ. Barrows had greater backfat thickness at 105 kg than 145-kg boars. As live weight increased from 105 to 145 kg, carcass length, 10th rib backfat thickness and longissimus area of boars increased (P less than .01) linearly. Fat-free muscle weights of the brachialis (BR), semitendinosus (ST) and longissimus (L) did not differ between boars and barrows at 105 kg. Boars at 105 kg had 1.3 and 1.7% more moisture in the BR and ST, respectively, than barrows. Percentage protein, total intramuscular fat and fiber diameter in the BR, ST and L muscles did not differ between boars and barrows at 105 kg or with increasing live weight in boars. Total RNA increased linearly (P less than .05) in the BR and ST as boars increased in live weight from 105 to 145 kg.

Proliferation of rat skeletal muscle satellite cells was studied in vitro, and their ability to respond to a variety of protein hormones was examined, including: growth hormone, prolactin, luteinizing hormone, thyrotropin and fibroblast growth factor. All experiments were conducted in serum-free medium to avoid complicating interactions with serum constituents such as other hormones or binding proteins. Dose-response curves were constructed for each protein and encompassed the physiological range plus concentrations two to three orders of magnitude greater than the physiological range. Of the proteins examined, the only one shown to have the ability to directly stimulate satellite cell proliferation was fibroblast growth factor. None of the anterior pituitary protein hormones had the ability to interact directly with satellite cells to stimulate proliferation in vitro. Therefore, satellite cells seem to be selective in their response to protein hormones, and the classes identified thus far are hormones in the insulin-like growth factor family and fibroblast growth factor. These two classes of protein hormones are quite different and would not be expected to act through a common pathway. Consequently, we have proposed a dual regulatory system that may allow for local as well as systemic stimulation of satellite cells.