The growth and differentiation of porcine skeletal muscle fibre types and the influence of birthweight.

Muscle growth and development was studied in 49 Large White pigs from a total of 17 litters. Representative large (mean birthweight of 1544 g), small (1144 g) and runt (776 g) littermates were selected and slaughtered at the same age, ages ranging from birth to 128 days. Fresh frozen, serial transverse sections taken from the semi-tendinosus and trapezius muscles of these animals were stained for the histochemical demonstration of acid and alkaline pre-incubated adenosine triphosphatase, succinate dehydrogenase and glycogen phosphorylase. Profiles of the muscle fibre types were compiled for each animal. In both muscles the number of slow oxidative (SO) fibres, that were arranged together in groups within 'metabolic bundles', increased with growth. The transverse sectional area (TSA) of the semitendinosus muscle increased with the 2/3 power of liveweight whereas the area occupied by SO fibres increased at a rate significantly greater than 1.0 (P less than 0.01). Regression analysis revealed that the area of this muscle occupied by SO fibres was greater (P less than 0.001) in runt and small littermates relative to their large littermates when they were compared at an equal liveweight. This greater TSA of the semitendinosus classified as 'SO' in lower birthweight pigs was the result of a combination of higher percentages (P less than 0.05) of SO fibres and significantly greater (P less than 0.001) SO fibre mean TSAs. The mean TSAs of all myofibre types were similar between littermates of the same age but most types were of greater TSA in the lower birthweight littermates when compared (by regression analysis) at the same liveweight suggesting that fibre TSA was age- rather than weight-related. The higher percentage of SO fibres in the low birthweight pigs, when compared at an equivalent liveweight to their large littermates, appeared to be related to their affected secondary/primary fibre number ratio. This phenomenon, plus the data on the number of slow fibres per metabolic bundle, indicated that it was apparently the number of slow fibres per metabolic bundle which was regulated with liveweight gain rather than the resultant percentage of slow fibres within the muscle.