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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Marbling is an important meat quality trait, in that it contributes directly to the value of beef on international markets. The development of marbling is not well understood, though there have been some significant recent discoveries regarding adipogenesis in general. This article describes a working hypothesis around the early events of marbling. It attempts to rationalise findings from several mammalian experimental systems on hyperplastic growth of adipocyte precursor cells.

1. The supply and utilisation of acetate has been estimated simultaneously in the whole animal and tissues of sheep using a combination of isotopedilution and arteriovenousdifference techniques. Animals were made alloxandiabetic and acetate metabolism was compared when stabilized to normal metabolite levels with insulin (ITA sheep) and when food and insulin had teen withdrawn for 36 h (fasted, diabetic sheep).2. Acetate was simultaneously produced and utilized by all tissues. The exogenous (or gut) supply of acetate was the most important determiniint of circulating acetate level. Endogenous acetate was produced mainly in the liver; 77 and 94% in fasted. diabe1.ic and ITA sheep respectively. The production of endogenous acetate remained fairly constant and was not related to ketogenesis, which supports the idea that circulating acetate is largely a product of fermentation. The liver, gut and muscle utilized 17,25 and 54% respectively (96% total) of the acetate entry rate in ITA sheep; a similar percentage utilization was found in fasted, diabetic sheep.3. Acetate is largely oxidized to carbon dioxide in the gut and muscles of sheep and may account for 3 0 4 0 % of their oxidative metabolism. This figure is similar to that for the whole animal. The total acetate taken up by the liver could account for 30% of the oxygen consumption; however, the liver may not directly oxidize all the utilized acetate.4. The over-all conclusion froin this study is that acetate is largely of dietary origin and the major factor determining its rate of utilization is the arterial concentration.

1. The over-all and regional metabolism of non-esterified fatty acids (NEFA) was studied using a combination of isotopic and arteriovenous-difference techniques.2. There was a common linear relationship, whether stearic, palmitic or oleic acids were used as tracer, between the arterial NEFA concentration and the rates of entry and oxidation.3. Assuming that the tracer used reflected the metabolism of all the NEFA, the total entry rate in fed and fasted pregnant ewes was (mean+SE) 0.44k0.02 and 0.55 k0.07 mmol/h per kg body-weight respectively. Oxidation of NEFA contributed (rneanfs~) 34k 5 and 58*7% to the respiratory carbon dioxide in fed and fasted animals, this accounting for (mean ~S E ) 46 6 and 59 k 3% of the respective entry rates. 4.Hind-limb muscle both utilized and produced NEFA. The mean gross fractional extraction (calculated from isotopic uptake) was (mean + SE) 9 f 1 %. Gross utilization of any NEFA and appearance of 14C0, across the muscle were linearly related to the arterial concentration of tracer fatty acid, irrespective of whether this was oleate or stearate. The amount of 14C0, appearing was consistent with ( m e a n f~~) 54k 8% of the CO, produced by the hind-limb being derived from NEFA oxidation. 5. Infused NEFA were partly converted to ketone bodies. Uptake and oxidation in the hind-limb of ketones formed in the liver could account for approximately 20% of the "CO, apparently produced in muscle from NEFA. Correction for this reduces the proportion of CO, derived from NEFA to 43%. There was some indication that ketones were also produced from NEFA in the hind-limb.6. NEFA were not a significant energy source for the gravid uterus.7. An over-all view of energy sources for the whole animal and for hind-limb muscle in normal and fasted pregnant sheep was presented.