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ANIMAL PRODUCTS |
Swiss Federal Research Station for Animal Production, Posieux 1726, Switzerland
Abstract
Maternal nutrition and progeny birth weight affect muscle fiber development in the pig, thereby influencing early postnatal growth rate. The objective of the study was to determine the extent to which growth, morphometric characteristics, and area and distribution of slow-oxidative (SO), fast oxidative-glycolytic (FOG), and fast glycolytic (FG) fibers of three muscles (LM = longissimus muscle; RF = rectus femoris; ST = semitendinosus) of slaughter pigs were affected by DE intake level during the first 50 d of gestation. Multiparous Swiss Large White sows were assigned randomly to one of three energy intake treatments: 1) fed 2.8 kg/d of a standard diet (STD; n = 6) containing 10.7 MJ DE/kg; 2) fed 2.8 kg/d of a low-energy diet (LE; n = 5) containing 6.6 MJ DE/kg; or 3) fed 4.0 kg/d of a standard diet (HE; n = 5) containing 10.7 MJ DE/kg (as-fed basis). Sows were subjected to energy intake treatments for the first 50 d of gestation; however, from d 51 to parturition, sows received 2.8 kg/d of the standard diet, and the amount of feed offered each sow during lactation was adjusted according to the litter size. Sows farrowed normally and pig birth weights were recorded. Based on birth weight, the two lightest (1.27 kg; Lt) and two heaviest (1,76 kg; Hvy) barrows and gilts from the 16 litters (n = 64) were selected at weaning and were offered a fixed amount of feed (170 g x BW0.569/d) from 25 to 105 kg BW. Regardless of the birth weight, progeny from HE sows grew slower (P < 0.05) during lactation and the growing-finishing period, had a lower (P < 0.05) gain-to-feed ratios, and had higher (P < 0.05) percentages of adipose tissue than pigs born from LE sows. The ST was shorter (P = 0.03) in Lt than in Hvy pigs, and the ST of gilts was heavier (P = 0.01) and had a larger (P = 0.01) girth than the ST of barrows. Overall mean fiber area tended to be larger (P 
0.11) in the LM and light portion of the ST of Lt than in Hvy pigs, and was larger (P = 0.03) in the ST of gilts than barrows. The ST of progeny from LE sows had fewer (P < 0.10) FG fibers, which was compensated by either more (P < 0.05) FOG in the light portion of the ST, or more (P < 0.10) SO fibers in the dark portion, and these differences were more pronounced in Lt pigs than in Hvy pigs. Overall, maternal feeding regimen affected muscle fiber type distribution, whereas birth weight and gender affected muscle fiber area.
Key Words: Birth weight • Gender • Maternal Nutrition • Muscle Fibers • Pigs
Introduction
Developing muscle fibers can be classified into primary and secondary fibers (Swatland, 1975
). Primary fiber number is believed to be a fixed genetic effect and seems to be unaffected by conditions in utero (Dwyer and Stickland, 1991). Alternatively, secondary fiber formation has been shown to be determined by prenatal events, during fetal development (Dwyer and Stickland, 1994). Because muscle fiber hyperplasia is completed at birth, postnatal growth is hypertrophic, and ultimate mass depends, then, on muscle fiber number (Dwyer et al., 1993). Variations in postnatal development of muscle fibers appear to depend on the primary fiber number and on the ratio of secondary to primary fibers. However, comparison of large and small littermates selected by birth weight suggested that this difference is due to a lower secondary-to-primary fiber ratio in smaller littermates (Handel and Stickland, 1987).
Muscle fibers undergo a process of contractile differentiation during development that leads to the distribution pattern found in adulthood consisting of clusters of slow type I fibers surrounded by fast type II fibers (Suzuki and Cassens, 1980). Within the first 20 d of age, muscle fibers differentiate into oxidative, oxidative-glycolytic, or glycolytic fibers. At birth, the majority of fibers are oxidative but, with increasing age, muscle metabolism as a whole becomes more glycolytic (Lefaucheur, 1986).
These observations demonstrate that the prenatal and early postnatal periods affect the ratio of primary to secondary fibers and, therefore, the metabolic and contractile maturation of myofibers. Because it is possible to alter this ratio by nutrition during early gestation (Dwyer and Stickland, 1994) and because the ratio is affected by birth weight (Handel and Stickland, 1987), this study was conducted to determine the extent to which energy intake of the sow in early pregnancy and birth weight of the offspring affect their subsequent muscle fiber type composition at slaughter.
Materials and Methods
Animals and Treatments
Multiparous Swiss Large White sows (n = 16) were mated to two Swiss Large White boars by artificial insemination and were randomly assigned at mating to one of three experimental groups differing in daily DE intake. Sows were fed a standard gestation diet of either 2.8 kg (standard diet [STD]; n = 6) or 4.0 kg (high energy [HE]; n = 5) from d 0 to 50 of gestation (Table 1). The low-energy (LE; n = 5) diet was formulated to contain 60% of the DE content of the standard gestation diet, whereas the amount of feed offered in the HE treatment should allow the sow to ingest 40% more DE than the STD group (as-fed basis). Both diets contained the same amount (as-fed basis) of CP, lysine, methionine-cystine, threonine, and tryptophan (11.0, 0.50, 0.42, 0.42, and 0.12 g/MJ DE, respectively). From d 51 of gestation to parturition, all sows were fed 2.8 kg of the standard gestation diet, and, during the 35-d lactation period, sows were fed 4.5 kg of a standard lactation diet (Table 1). In litters with more than eight pigs, the sow’s feed allowance was increased by 200 g per additional pig. Sows were individually fed twice daily and had ad libitum access to water. Feed consumption for each sow was recorded during gestation and lactation, as well as the number of pigs born per sow (litters were not adjusted for size) and birth and weaning weights.
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). The pigs were individually penned when they achieved a BW of 25 kg. Pigs were then fed a standard growing-finishing diet (Table 1) twice daily until slaughter and had ad libitum access to water. Total daily feed allowance was adjusted weekly according to BW by the formula 170 g x BW0.569. The nursery, growing, and finishing diets were formulated to meet Swiss nutrient requirements for nursery and growing-finishing pigs (Boltshauser et al., 1993). The Swiss Federal Committee for Animal Care and Use approved all procedures involving animals.
Measurements and Tissue Sampling
Ultrasonic backfat thickness (5 cm from the midline and 5 cm caudal of the last rib) was measured in sows on d 1, 50, and 100 of gestation and at weaning. At the end of the growing-finishing period (104.6 kg), feed was withheld from pigs 12 h before transportation to the research station abattoir. Slaughter and dissection procedures were carried out according to the Swiss Pig Performance Testing Station (MLP, Sempach, Switzerland) meat cutting standards as previously described by Bee et al. (2002).
Within 30 min after exsanguination, three muscles were removed from the left side of each carcass, including the longissimus muscle (LM), semitendinosus (ST), and rectus femoris (RF). Weight, girth, and length of the ST, as well as the weight of the RF and LM area (at the 10th rib), were determined after muscle excision. Muscle samples for fiber determination were taken from the central region of the RF, from the center of the dark (STdark) and light portions (STlight) of the ST, and the center of the LM anterior to the 10th-rib location. A sample (approximately 1 cm x 1 cm x 3 cm) of each muscle was immediately secured to a labeled flat stick, rolled in talcum powder, and frozen in liquid nitrogen for subsequent analysis.
Histochemical Method
For fiber determination, frozen muscle samples were equilibrated to -25°C, and then a section of tissue was cut from the stick and trimmed to facilitate transverse sectioning. Samples were mounted on a cryostat chuck with a few drops of tissue-freezing medium (Tissue-Tek, Sakura Finetek Europe, Zoeterwoude, The Netherlands). Sections (10 µm in thickness) were cut using a Cryotome (Shandon, Inc., Pittsburgh, PA) and subsequently mounted on glass microscopic slides and allowed to air-dry for 30 min. Sections were then treated with the combination succinic dehydrogenase and acid myofibrillar ATPase staining procedure (Solomon and Dunn, 1988). Stained sections were observed at 125x with a Olympus BX50 microscope in transmitted light mode (Olympus Optical Co., Hamburg, Germany) equipped with a high-resolution charge-coupled device digital camera (ColorView12, Soft Imaging System GmbH, Münster, Germany). Muscle fibers were classified as slow oxidative (SO), fast oxidative-glycolytic (FOG), and fast glycolytic (FG) based on the stain reaction. The SO fibers showed the darkest and the FG showed the lightest staining intensity. Three random fields at different locations within a slide of each muscle sample were captured as TIFF files, and a minimum of 250 muscle fibers were analyzed with the analySIS 3.0 image analysis software (Soft Imaging System GmbH). To minimize the incidence of measuring intrafasicular terminations of myofibers in the LM and RF, only fibers larger than 700 µm2 were included in calculations. Fiber type distribution was calculated in two ways: 1) expressed as the percentage of fiber type to the total of all fibers (fiber number percentage; FNP) and 2) expressed as the percentage of the total measured area relative to the area of each fiber type (fiber area percentage; FAP). Total number of fibers in the ST was estimated by extrapolating the number of fibers of the STdark and STlight in a known area to the girth of the ST at the midpoint, whereas, in the LM, the total fiber number was calculated by the ratio of the LM area to the mean cross sectional area.
Statistical Analysis
All statistical analyses were conducted using PROC MIXED of the SAS System version 8.2 (SAS Inst. Inc., Cary, NC). Performance data of the sows were analyzed as one-way analysis of variance to study the effects of DE-intake in early gestation. Data for growth performance, carcass characteristics, morphometric muscle measurements, as well as for the area and distribution (FNP, FAP) of myofibers for each fiber type and muscle of the progeny, were analyzed as a 3 x 2 x 2 factorial split-split-plot design, and DE intake of the sows, birth weight, and gender of the progeny were the treatment variables. The DE intake level was considered the main plot, birth weight group the subplot, and gender the sub-subplot. Sow within DE intake level, sow within DE intake level x birth weight category, and the residual error were used as error terms to test the whole plot, subplot and sub-subplot effects, respectively. Differences with probability levels of P < 0.10 were considered significant, and means were separated using least significant difference test. Main effects were reported as least square means and significant two-way interactions were indexed. One light and one heavy castrate born from LE sows did not reach the slaughter weight due to low growth rate and leg-related problems and, therefore, were excluded from the data set.
Results
Sow Performance
From mating to d 50 of gestation, daily DE intake and BW were 38 and 72% higher (P < 0.05), respectively, in HE sows compared to the STD sows; however, backfat depth was not (P > 0.10) different between HE and STD sows (Table 2). Furthermore, LE sows consumed 38% less (P < 0.05) DE than STD sows, which resulted in less (P < 0.05) BW change and backfat in LE sows. From d 50 to parturition, when all sows were fed the standard gestation diet, neither BW nor fat depth change was different (P > 0.10) among treatments. From parturition to weaning, HE sows lost more (P < 0.05) BW than LE sows, whereas BW changes in STD sows were intermediate to HE and LE sows. When considering all progenies (data not shown), energy intake during early gestation did not (P > 0.10) affect average birth weight (1.41, 1.60, and 1.52 kg for HE, STD, and LE, respectively), weaning weight (13.7, 12.3, and 13.4 kg for HE, STD, and LE, respectively), number of pigs born alive (11.6, 10.6, and 10.4 for HE, STD, and LE, respectively), or number of pigs at weaning (9.0, 10.0, and 8.4 for HE, STD, and LE, respectively).
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. Contrary to the whole litter, mean birth weight of the four pigs per litter from HE sows tended to be lower (P < 0.10) compared to the selected progeny of STD and LE sows. During lactation, pigs from LE sows grew faster (P < 0.05) and, consequently, had heavier (P < 0.05) BW at weaning than pigs from HE and STD sows. The Lt pigs grew slower (P = 0.03) during lactation and BW at weaning was lower (P = 0.02) compared to Hvy pigs. However, ADG during the nursery period was not (P > 0.10) affected by sow treatment, birth weight group, or gender.
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). Conversely, ADFI was not (P > 0.10) affected by sow treatment, birth weight, or gender. Percentage of lean tissue was lower (P < 0.01), and percentage of omental fat was higher (P < 0.01), in barrows than gilts. Percentage of adipose tissue was affected by the sow treatment (P = 0.02), birth weight (P = 0.01), and gender (P < 0.01), being highest in pigs born from the HE sows, in pigs with a low birth weight and in barrows, respectively. Morphometric muscle measurements were not (P > 0.10) affected by early gestation treatments. The ST muscle was shorter (P = 0.03) in pigs with low birth weights. Compared to carcasses of barrows, carcasses of gilts had larger (P < 0.01) LM areas, and heavier (P = 0.01) ST with greater (P = 0.01) girth.
Muscle Fiber Area
Least square means for fiber area for the LM, RF, and ST muscles are summarized in Table 4. Plane of nutrition in early pregnancy did not (P > 0.10) affect fiber area of muscles of progeny. Pigs with low birth weight exhibited a larger (P 0.09) fiber area for SO in the LM and for FOG in the STlight, which was also reflected in larger (P 0.10) overall mean fiber areas for those muscles. For gilts, the SO, FOG, and overall mean fiber areas in the STdark and the FOG, FG, and overall mean fiber areas in the STlight were larger (P 0.05) than for barrows. In contrast, gilts had smaller (P 0.03) SO fibers in the LM and RF, and a tendency existed for FG fibers in the STdark to be smaller (P = 0.07). The differences between genders for the mean average area measurements of the ST closely paralleled the weight and girth of this muscle (Table 3). The tendency for a larger (P = 0.09) average fiber area in the LM of Lt pigs was not reflected in an increase in LM area.
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). These changes were compensated in the dark and light portions of the ST either by fewer SO or fewer FOG fibers, respectively. In the STlight, differences between the HE and LE treatments in the relative amount of FOG and FG fibers were more pronounced in pigs with a low birth weight (Figure 1). When fiber type distribution was expressed as relative fiber area (FAP) contribution, the same effects of plane of nutrition in early pregnancy were found in the STlight (Table 6). In addition, the area contribution of SO fibers in the RF was higher (P < 0.05) in pigs from HE compared to LE sows. The differences were more pronounced in the Lt compared to the Hvy group (DE intake x birth weight interaction; P < 0.05) and were more distinct in gilts than in barrows (DE intake x gender interaction; P < 0.05). Regardless of whether muscle fiber distribution was expressed as FNP or FAP, barrows had more FOG and fewer FG fibers in the RF (P 0.01) than gilts. Energy intake during early gestation had no (P > 0.10) effect on fiber distribution in the LM.
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Compared to the STD sows, BW gains of LE sows were 44% lower at d 50 of pregnancy, as might be expected from the reduction in DE intake (-38%), but backfat thickness accretion was markedly lower (-67%). However, contrary to the severe energy restriction throughout gestation reported by Pond et al. (1988), both BW and backfat depth of the LE sows increased. The present results suggest that dietary energy restriction was partly compensated by reducing adipose tissue accretion to overcome lean tissue growth. On the other hand, the 38% higher DE intake in the HE group compared to the STD group resulted in almost doubling the BW change but the increase in the backfat depth was less marked. These observations coincide with the fact that, when protein and amino acid supplies are not limiting as was the case in the present study in the HE treatment, elevating dietary energy intake increases protein retention during pregnancy (Dourmad et al., 1996) and does not affect backfat depth during lactation (Sinclair et al., 2001). By contrast, Prunier et al. (2001) reported that increased energy supply above requirements for maintenance, for the gravid uterus and for the developing fetuses throughout pregnancy, inhibits ad libitum feed intake during the lactation period, resulting in a marked decrease in BW and backfat depth. In the present study, feed intake from farrowing to weaning was not affected because feed intake in the lactation period was adjusted to the litter size weekly, and litter size did not differ among treatments. Nevertheless, BW loss and decrease in backfat depth during lactation were inversely correlated with the feed intake during gestation (higher in HE than LE sows), which is in accordance with the data reported in the aforementioned study.
The number of sows involved in the present experiment was too small to draw conclusions about the effects of dietary treatments on litter performance, and no data are available on the reproductive performance (return to estrus) of these sows. However, neither restricted nor increased supply of energy affected the number of progeny alive at birth, mean birth weight, or piglet BW at weaning. These results were not surprising because even very severe dietary restrictions throughout pregnancy have failed to affect litter performances (Pond et al., 1985).
At the end of nursery period, piglet BW was unaffected by the sow treatments and birth weight, but, at weaning, the BW of the selected pigs born from the HE sows were lower compared to those of the LE sows. This finding suggests that feed intake during lactation was probably limited in the progeny of HE sows compared to progeny of LE sows. The growth retardation was only partly compensated during the nursery period, when feed was supplied ad libitum. In the growing-finishing period (from 25 to 104 kg), ADG and gain-to-feed ratio were decreased in progeny born from HE sows compared to those from STD and LE sows. These findings are not in agreement with studies reported by Dwyer et al. (1994), Pond et al. (1985), and Pond and Mersmann (1988). Dwyer et al. (1994) showed significantly higher growth rates of progeny born from sows fed 5 kg/d of feed from d -25 to 80 of gestation compared to progeny of control sows fed 2.5 kg/d. Similarly, Pond et al. (1985) and Pond and Mersmann (1988) reported that the growth rate of progeny was significantly lower from wk 10 onward when maternal intake had been restricted during gestation. However, the present study differed from the previous experiments in several ways, namely, the magnitude of feed allowance during pregnancy, time and duration of treatment, composition of the gestation diet, and a restricted feeding regimen of progeny during the growing-finishing period. As expected, the lower gain-to-feed ratio of the progeny from HE sows was reflected in a higher percentage of adipose tissue.
Early gestation treatment affected neither average fiber area nor fiber area of individual fiber types in the LM, RF, and ST of the progeny. Because LM area, girth of the ST, and RF weight was not altered, it can be assumed that muscle fiber number was not affected by the sow treatment. This is contradictory to results showing that a doubling of the maternal feed intake immediately before fiber hyperplasia (d 20 to 50 of gestation) increased the mean number of secondary fibers (Dwyer et al., 1994), and there was a tendency for the total fiber number to also increase in the ST of pigs. A reduction of fiber numbers in the biceps brachii (Ward and Stickland, 1991; Dwyer and Stickland, 1992; Dwyer and Stickland, 1994) and extensor digitorum (Dwyer and Stickland, 1992) muscles of newborn guinea pigs was reported when maternal feed intake was limited to 60% of ad libitum in the first third of, or throughout gestation. In addition, the results of those studies indicated distinct effects on progeny birth weight, as well as on muscle weight. In an attempt to isolate the dietary component responsible for the reduction in fiber number, Dwyer and Stickland (1994) demonstrated that the reduction was noted only when maternal diets were deficient in both protein and carbohydrate. In addition to the species differences, the lack of response to maternal dietary restriction in the present study could be caused by the low-energy gestation diet, which was restricted only in the DE content but not in the amount of protein.
In the low-birth weight pigs, fiber area tended to be larger in the LM (SO and FG) and STlight (FOG and FG). Although not reaching significance, this was also true for all fiber types in the RF and STdark. Pearson correlation coefficient between average fiber area and BW at birth amounted to -0.34 and -0.31 for the LM and RF, respectively, but was not significant for the ST. Powell and Aberle (1981) and Hegarty and Allen (1978) reported greater fiber areas in the psoas major, ST, and semimembranosus muscles of runt pigs (birth weight <1,000 g) than in their heavier littermates at equal slaughter weights. Likewise, Handel and Stickland (1987) found significantly larger myofiber cross-sectional area in the ST and trapezius muscle of 128-d-old pigs when runts (776 g BW) were compared to small (1,144 g BW) and larger (1,544 g BW) littermates. Although, in the present study, girth and weight of ST did not differ between the two birth weight groups, estimated fiber number in the ST was lower (P = 0.04) in the Lt (819 x 103) compared to the Hvy pigs (911 x 103), confirming observations obtained by Wigmore and Stickland (1983). Similar results would be expected for the LM, because average fiber area tended to be smaller in high-birth-weight pigs but LM area was equal between the two birth weight groups. However, estimated fiber number in the LM did not differ (P = 0.38) between Lt and Hvy pigs (215 x 103 vs. 234 x 103, respectively).
The effect of gender on muscle fiber area is controversial. Some studies reported larger fibers in gilts than in barrows (Solomon et al., 1990; Larzul et al., 1997), whereas others failed to report any differences (Sosnicki, 1987; Ender, 1994). Furthermore, when gender effects (gilts vs. barrows) were detected for the same muscle, the affected fiber types were not the same. For example, Solomon et al. (1990) reported larger SO and FOG fibers in gilts compared to barrows, whereas Larzul et al. (1997) indicated that gilts had larger FOG and FG fibers than barrows. For the LM, the present results are consistent with those not reporting any gender differences; however, larger fibers were found in the ST of gilts compared to barrows, and, although not (P = 0.07) statistically different, the opposite effect was observed in the RF. In the ST, the gender-associated hypertrophy seemed to be dependent on the fiber type because the impact was greatest in the predominant fiber types of the respective portion (SO and FOG in the STdark, and FOG and FG in the STlight). The observed difference in the morphometric ST measurements (weight and girth) between gilts and barrows were due to the fiber hypertrophy because estimated fiber number revealed no (P = 0.67) differences between gilts and barrows (856 x 103 vs. 875 x 103, respectively). Estimated fiber number of the LM did not (P = 0.34) differ between gilts and barrows (2,351 x 103 vs. 2,134 x 103, respectively) even though the LM area was distinctly larger in gilts than barrows. These results illustrate the difficulty of estimating fiber number due to the impracticability of preparing and counting entire sections of large muscles of livestock species. It is well known that sampling procedures and subsampling techniques for myofiber counting is subjected to high levels of error in estimates of muscle fiber number.
The fiber distribution observed in adult pig muscles is primarily determined by the prenatal development of primary and secondary myofibers; the former, once developed, act as a framework on which myoblasts align and fuse to form the population of secondary fibers. By d 75 of gestation, primary fibers begin to express slow myosin heavy chain (MyHC) and mature into type I slow muscle fibers (Lefaucheur et al., 1995). When formed, most of the secondary fibers express fast MyHC and mature into type IIA (FOG), IIX, or IIB fibers (FG). Within the same muscle, prenatal nutrition and birth weight do not affect the number of primary fibers. One possible reason why primary fiber formation is spared from nutritional effects could be related to the timing of their formation, which occurs prior to secondary fibers, and when the fetuses are relatively small and making few demands on maternal nutrition (Ward and Stickland, 1991). In contrast, secondary fiber formation depends on the maternal plane of nutrition (Dwyer and Stickland, 1991). When muscle fiber distribution was expressed as numerical contribution (FNP; Table 5), the present results partly confirm those observations. Pigs born from the LE sows tended to have fewer FG fibers in the STdark and STlight compared to pigs from HE sows. In the STdark, the decrease was compensated by more SO fibers, suggesting a higher conversion rate of secondary (FG) to primary fibers (SO) during pre- or postnatal development. Although the number of SO fibers was similar, the number of FOG fibers was greater in the STlight of pigs from LE than from HE sows. Lefaucheur et al. (1995) indicated that, in the superficial part of the ST, primary and the majority of secondary fibers give rise to fast fibers, and, within the first 2 wk following parturition, the remaining secondary fibers mature to slow fibers. From the present results, it seems unlikely that in the STlight conversion from fast to slow fibers was affected by the sow nutrition. The differences in the FG and FOG distribution due to maternal nutrition strategy were primarily found in the Lt pigs but not in Hvy pigs. Lower birth weight is associated with an unfavorable intrauterine position, and small fetuses show signs of growth retardation consistent with prenatal malnutrition (Wigmore and Stickland, 1983). Therefore, the present observation in the STlight could indicate that, for small fetuses, a restricted nutritional supply of the sow amplifies the effect on muscle fiber development.
Although the composition of the LM and, in part, that of the RF was similar to the STlight, sow nutrition did not affect the fiber type distribution. The lack of effect on the LM compared to the ST could be explained by the developmental time gradient, which runs cephalocaudally and proximodistally (Ward and Stickland, 1991). Therefore, primary fibers in the less distal and caudal muscles like the LM would be developing when fetuses are small and less demanding. The reason why the fiber type distribution of the RF, despite its location more caudal than the ST, was neither affected by the maternal nutrition nor by the birth weight is unknown.
In conclusion, this study shows that maternal nutrition during gestation did not affect muscle fiber area in slaughter-weight pigs; however, gender and, to a lesser extent, birth weight influence fiber cross-sectional area. In contrast, low-energy supply during early gestation increased oxidative capacity and tended to decrease the estimated number of fibers in the ST. This may be due to a lower formation rate of secondary fibers.
Implications
Results of the present study indicate that both prenatal nutrition and birth weight could be determinants for skeletal muscle growth, and, consequently, affect growth performance and carcass characteristics. However, each factor seems to have specific effects on myofiber development. Prenatal nutrition modulated muscle fiber distribution, whereas birth weight and gender primarily affected myofiber hypertrophy. Furthermore, the effect of maternal nutrition and birth weight depended on the anatomical location of the muscle. The increased oxidative capacity in the semitendinosus muscle of progeny born from sows fed a low-energy diet suggests a delay in muscle maturation.
Footnotes
1 The author thanks M. D. Lindemann from the University of Kentucky for the review of the manuscript, and C. Keller, C. Biolley, G. Guex, G. Maïkoff, and P. Stoll for their excellent technical assistance. 
2 Correspondence: La Tioleyre, 4 (phone: +41-26-40-77-222; fax: +41-26-40-77-300; e-mail: giuseppe.bee{at}alp.admin.ch).
Received for publication February 12, 2003. Accepted for publication October 16, 2003.
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