Intramuscular fat (IMF) content plays a key role in various quality traits of meat. IMF content varies between species, between breeds and between muscle types in the same breed. Other factors are involved in the variation of IMF content in animals, including gender, age and feeding. Variability in IMF content is mainly linked to the number and size of intramuscular adipocytes. The accretion rate of IMF depends on the muscle growth rate. For instance, animals having a high muscularity with a high glycolytic activity display a reduced development of IMF. This suggests that muscle cells and adipocytes interplay during growth. In addition, early events that influence adipogenesis inside the muscle (i.e proliferation and differentiation of adipose cells, the connective structure embedding adipocytes) might be involved in interindividual differences in IMF content. Increasing muscularity will also dilute the final fat content of muscle. At the metabolic level, IMF content results from the balance between uptake, synthesis and degradation of triacylglycerols, which involve many metabolic pathways in both adipocytes and myofibres. Various experiments revealed an association between IMF level and the muscle content in adipocyte-type fatty acid-binding protein, the activities of oxidative enzymes, or the delta-6-desaturase level; however, other studies failed to confirm such relationships. This might be due to the importance of fatty acid fluxes that is likely to be responsible for variability in IMF content during the postnatal period rather than the control of one single pathway. This is evident in the muscle of most fish species in which triacylglycerol synthesis is almost zero. Genetic approaches for increasing IMF have been focused on live animal ultrasound to derive estimated breeding values. More recently, efforts have concentrated on discovering DNA markers that change the distribution of fat in the body (i.e. towards IMF at the expense of the carcass fatness). Thanks to the exhaustive nature of genomics (transcriptomics and proteomics), our knowledge on fat accumulation in muscles is now being underpinned. Metabolic specificities of intramuscular adipocytes have also been demonstrated, as compared to other depots. Nutritional manipulation of IMF independently from body fat depots has proved to be more difficult to achieve than genetic strategies to have lipid deposition dependent of adipose tissue location. In addition, the biological mechanisms that explain the variability of IMF content differ between genetic and nutritional factors. The nutritional regulation of IMF also differs between ruminants, monogastrics and fish due to their digestive and nutritional particularities.

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The quality of grilled steaks was assessed by experienced panellists in longissimus thoracis (LT), semitendinosus (St) and triceps brachii (TB) muscles of Aubrac, Charolais, Limousin and Salers breeds raised in two production systems: 15-, 19- and 24-month-old bulls and 4-, 6- and 8-year-old cull cows.Scores for sensory ‘initial tenderness’, ‘overall tenderness’, ‘juiciness’, ‘residue after mastication’ and ‘flavour intensity’ for all 497 meats were pooled to derive three eating quality classes.Meats from the bulls and cows and from the four breeds were evenly distributed among the three eating quality classes. The highest quality class, representing one third of all the meats, contained 45% of the LT, 35% of the TB and 21% of the St muscles and one third of the meats from the 8-year-old cull cows. The meats in this class tended to have finer fibres, a greater proportion of slow oxidative fibres, slower post-mortem glycolysis, lower connective tissue and higher fat contents than those in the lower classes.Lipid content accounted for proportionately 0·56 of the variation in flavour intensity and pH at 3 h post mortem, 0·52 of the variation in tenderness due to muscle and slaughter age.Considering both young bulls and cull cows together, tenderness was highest in the meats from 15-month-old bulls and low in the meats from the intermediate age groups, and flavour and juiciness was highest in the meats from the oldest animals from each production system.

Marbling is an important criterion for beef quality grading in many countries. The purpose of the current study was to utilize the natural genetic variation to identify major metabolic indicators of marbling in cattle differing in genotypes. Rectus abdominis (RA, oxidative), semitendinosus (glycolytic), and longissimus thoracis (LT, oxido-glycolytic) muscles were taken from steers of different genotypes that expressed high [Angus, n = 16; and crossbred (Angus x Japanese Black), n = 10] or low (Limousin, n = 12) levels of marbling in their meat. Muscles from Angus and crossbred steers were characterized, as expected, by a greater triacylglycerol (TAG) content (P < 0.001) and also by greater protein contents of fatty acid-binding protein specific for heart and muscles (H-FABP; P < 0.001 for RA and P < 0.05 for LT muscle) or for adipocytes (A-FABP; P < 0.001 for RA and LT muscles). Moreover, oxidative enzyme activities (beta-hydroxyacyl-CoA dehydrogenase, citrate synthase, isocitrate dehydrogenase, cytochrome-c oxidase) were greater (P < 0.01 to 0.001) in the 3 muscles studied, whereas glycolytic enzyme activities (phosphofructokinase and lactate dehydrogenase) were lower (P < 0.001) in RA muscle in Angus and crossbred steers compared with Limousin steers. Significant correlations were observed between TAG content and H- and A-FABP protein contents, and oxidative (r > or = +0.55, P < 0.001) or glycolytic enzyme activities (r > or = -0.47, P < 0.001), when the 3 genotypes and muscles studied were considered as a whole. In addition, A-FABP protein content and some oxidative enzyme activities were significantly correlated with TAG content independently of the genotype and muscle effects. In conclusion, A-FABP protein content, as well as oxidative enzyme activities, may be used as indicators of the ability of steers from extreme genotypes to deposit intramuscular fat.

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Myogenesis is a complex process of which the underlying mechanisms are conserved between species, including birds and mammals. Despite a good understanding of the stages of myogenesis, many of the mechanisms involved in the regulation of proliferation of the successive myoblast generations, the cellular transitions cell proliferation/alignment of myoblasts/fusion of myoblasts into myotubes/differentiation of myofibres and the control of total myofibre number still remain unknown. An in vivo proteomic analysis of the semitendinosus muscle from Charolais foetuses, at three specific stages of myogenesis (60, 110 and 180 days postconception), was conducted using 2-DE and MS. Expression profiles of more than 170 proteins were revealed and analysed using two way hierarchical clustering and statistical analysis. Our studies identify, for the first time, distinct proteins of varied biological functions and protein clusters with myogenic processes, such as the control of cell cycle activity and apoptosis, the establishment of cellular metabolism and muscle contractile properties and muscle cell reorganisation. These results are of fundamental interest to the field of myogenesis in general, and more specifically to the control of muscle development in meat producing animals.

Some proteins have been revealed as biomarkers for beef tenderness by previous studies. These markers could be used in immunological tests to predict beef tenderness, in living animals as well as in carcasses. It is well known that rearing practices modify the amounts of mRNA and proteins. Therefore, the reliability of protein tests could be affected by livestock and biological effects such as production systems, breed, muscle and animal type. This study analysed the effects of animal and muscle type on 24 proteins. The animals studied were 67 young bulls and 44 steers of the Charolais breed, and muscles were Longissimus thoracis and Semitendinosus. Protein amounts were determined by Dot blot, an immunological technique. Results showed that expressions of 20 proteins were influenced by animal and/or muscle type. These results could lead to modifications and adaptations of prediction tests according to rearing practice, bovine breed and beef cut.

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