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A Comparison of Breast Muscle Characteristics in Three Broiler Great-Grandparent Lines

V. E. MacRae^*,1, M. Mahon, S. Gilpin, D. A. Sandercock^*, R. R. Hunter^* and M. A. Mitchell^*

^* Roslin Institute (Edinburgh), Roslin, Midlothian, EH2 9PS, UK; and School of Medicine, Keele University, Staffordshire ST5 5BG, UK

¹ Corresponding author: vicky.macrae{at}bbsrc.ac.uk

	ABSTRACT

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION REFERENCES

 
Genetic selection of broiler chickens has led to a gross overdevelopment of the broiler breast muscle pectoralis major. This may have resulted in increased myopathy and detrimental effects on meat quality. The present study examined 3 commercial great-grandparent lines (lines A, B, and C). Lines A and B are female lines, and line C is a male line. The mean BW of line C (2.7 kg) was significantly greater than those of lines A and B (both 2.3 kg). However, the mean breast yield of both lines B and C (8.9 and 8.7%, respectively) was significantly greater than that of line A (6.9%). Line B therefore matched the meat yield of line A while maintaining a high reproductive capacity. The mean breast fillet weight of line A (169 g) was significantly lower than lines B (207 g) and C (235 g). No differences were observed between lines in either mean fiber size or amount of connective tissue. Therefore, additional fibers must provide the additional weight in the breast fillet of lines B and C, compared with A. Plasma creatine kinase activity, a commonly used marker of muscle damage, was significantly higher in line A (1368 IU/L) than in lines B (995 IU/L) and C (982 IU/L). However, qualitative evaluations of muscle pathology revealed no differences among lines. Selection for increased embryonic muscle fiber number, rather than for increased radial fiber growth, could improve growth potential and may also alleviate muscle damage.

Key Words: skeletal muscle • myopathy • broiler

	INTRODUCTION

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Genetic selection of broiler chickens for desirable production traits, including increased breast yield and reduced abdominal fat, has been extremely successful. However, the traditional selection for these traits may have been almost fully exploited; at 42 d of age an average broiler chicken may reach a BW of 2.2 kg, having consumed only 1.82 g of food per 1.0 g of BW gain (Mitchell, 1999). This genetic selection has led to a gross overdevelopment of the broiler breast muscle pectoralis major. This may have resulted in increased susceptibility to spontaneous and stress-induced myopathy (muscle damage; Mitchell, 1999), and may have induced detrimental effects on meat quality attributes (Sante et al., 1991; Le Bihan-Duval et al., 1999; Sandercock et al., 2001).

Commercial broiler chickens are typically derived from highly selected pedigree great-grandparent lines. In developing and maintaining a commercial strain of broilers from these great-grandparent lines, a balance of features related to growth and reproduction must be considered, which can be influenced by the genetic makeup of the bird (Leeson and Summers, 2000).

The present study compared the breast muscle characteristics of 3 commercial great-grandparent lines (lines A, B, and C). Although all lines have undergone intensive selection for growth-related production traits, distinct differences in their genetic makeup exist. For example, line A is a female line that has been selected for an earlier peak in egg production, but hens in this line do not lay for as long a time period as those in line B, which is a female line with a later egg production peak. Overall, both lines produce the same number of eggs. Line C is a male line and has been selected primarily for growth-related traits. This study compared pectoralis major breast muscle growth, pathology, and fiber morphometry in lines A, B, and C.

	MATERIALS AND METHODS

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Six female broiler great-grandparent birds from each of the lines (A, B, and C; 18 birds in total) were reared in 3.6-m² pens littered with wood shavings on a light regimen of 14 h light:10 h dark, at 21°C and 55% RH. During the first 3 wk, brooders were placed in the pens to provide a spot temperature of 38°C and ambient temperature of 21 to 25°C. Food and water were available ad libitum. The birds were fed a commercial broiler diet that contained 29.8% soy and 62.2% wheat (Roslin Nutrition, Midlothian, UK).

At 6 wk of age, the birds were weighed and the blood was sampled. The birds were then euthanized using an intravenous injection of sodium pentobarbitone. Each blood sample was transferred to a 5-mL blood collection tube (Teklab, Sacriston, Durham, UK) containing 50 units of Li-heparin anticoagulant and placed on ice. The samples were centrifuged at 1,500 x g (MSE-Mistral 2000R; Sanyo Gallenkamp plc, Leicester, UK) for 5 min. The plasma supernatant was pipetted into plasma tubes and immediately frozen at –20°C. Creatine kinase (CK) activity, a commonly used marker of muscle damage (Yasmineh et al., 1978; Ibrahim et al., 1981), was determined using a kinetic assay (Randox, Crumlin, County Antrim, UK), and was assessed following the manufacturer’s protocol.

Approximately 2 cm³ muscle samples of the pectoralis major breast muscle were removed from standardized regions of the left side of the bird. The samples were oriented for transverse fiber sectioning and mounted on cork disks using a viscous embedding compound (OCT Tissue-Tek; Sakura, Zoeterwoude, the Netherlands). The muscle blocks were then snap-frozen by immersion in liquid nitrogen-cooled Isceon⁴⁵ (Hotfrost, Edinburgh, UK) and stored at –70°C.

Serial 10-µm sections were cut from the frozen muscle samples on a cryostat (Bright Instruments, Huntingdon, Cambridgeshire, UK), stained with hematoxylin and eosin (H&E) and Masson’s trichrome, and reacted for reduced nicotinamide adenine dinucleotide, myofibrillar adenosine triphosphatase, or nonspecific esterase. A qualitative and descriptive evaluation of muscle pathology was performed on pectoralis major muscle sections from each bird, using adapted human biopsy evaluation forms (Cumming et al., 1994).

The radial muscle fiber size was estimated in sections stained with H&E by measuring the minimum fiber diameter (MFD), using image analysis software (Scion Image for Windows, Scion Corp., Frederick, MD). The MFD is defined as the distance across the narrowest part of the fiber profile passing through the centroid (Dubowitz and Brooke, 1973; Cumming et al., 1994). The amount of connective tissue in sections stained with H&E was estimated by point-counting stereology. However, the amount of connective tissue determined is approximate, because blood vessels and nerves would have been included in the estimation. The MFD (100 fibers) and amount of connective tissue (500 points) were estimated from pectoralis major muscle sections from each bird.

Two-way ANOVA was performed to determine the effect of line. A GLM analysis incorporating pairwise comparisons using Tukey’s test was undertaken to compare groups within the ANOVA model. Statistical significance was accepted at P < 0.05. This study was undertaken under an appropriate Home Office license and conformed to ethical standards of the Roslin Institute.

	RESULTS AND DISCUSSION

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Distinct differences in genetic makeup existed among the 3 great-grandparent lines examined in this study. Lines A and B are female lines, which are selected for differing egg production traits. Line C is a male line, and has been selected primarily for growth-related traits. The mean BW of the male line, line C (2.7 kg), was greater compared with the 2 female lines, lines A (2.3 kg) and B (2.3 kg; Table 1; P < 0.05). However, the mean breast yield of lines B (8.9%) and C (8.7%) was greater than that of line A (6.9%; P < 0.05). Line B is therefore notable in its ability to match the meat yield of the male line while maintaining a high reproductive capacity.

View larger version (41K): [in this window] [in a new window]   Figure 1. (A) Fiber size variation; (B) basophilic fiber; and (C) necrotic fibers with fatty tissue replacement. Hematoxylin and eosin stain. Bar = 50 µm.

It has been proposed that during the postnatal period, avian muscle growth occurs only by hypertrophy (increased radial growth of muscle fibers) and not by hyperplasia (increased muscle fiber number), with fiber number becoming fixed before or shortly after hatch (Goldspink and Yang, 1999). Genetic selection for increased growth of broiler and broiler great-grandparent muscles, in particular the pectoralis major breast muscle, is associated with increased hypertrophy (MacRae et al., 2006). Selection for increased hypertrophy may be associated with muscle damage caused by radial fiber growth outstripping the support systems and large fibers splitting because of metabolic stress (Mahon, 1999; Kranen et al., 2000; MacRae et al., 2006). However, genetic selection for increased broiler pectoralis major muscle growth also appears to have resulted in a larger muscle fiber number set and greater posthatch growth potential (Remignon et al., 1995). This was also observed in the broiler great-grandparent lines in the present study. The mean breast fillet weight of line A (169 g) was lower than that of lines B (207 g) and C (235 g; P < 0.05). Assessment of muscle sections revealed no differences between lines in either mean fiber size or amount of connective tissue (Table 2). Therefore, additional fibers must provide the additional weight in the breast fillet of lines B and C, compared with line A. Lines B and C would have attained a given breast weight in a shorter period of time than line A, and would have greater overall growth potential. Again, line B is notable in its ability to match the growth potential of the male line (line C) while maintaining a high reproductive capacity.

	ACKNOWLEDGMENTS

 
The authors wish to thank Cobb-Vantress Inc. for their generous support of this work.

Received for publication August 3, 2006. Accepted for publication October 9, 2006.

	REFERENCES

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