The effects of low birthweight on the ultrastructural development of two myofibre types in the pig.

An ultrastructural investigation of the postnatal development of oxidative and non-oxidative fibres from the deep and superficial portions of the semitendinosus muscle, respectively, was undertaken on 32 pure bred Large White pigs from a total of 11 litters. This study quantifies the changes in mitochondrial, lipid droplet and myofibrillar content of these two myofibre types between birth and 84 days of age, and evaluates differences between the largest male (mean birthweight of 1559 g), smallest normal male (1147 g), and runt (758 g) littermates. The oxidative and non-oxidative fibres, as well as possessing different complements of mitochondria, lipid droplets and myofibrils, showed different rates of myofibrillar accumulation. The relatively small postnatal change in the percentage volume of myofibrils of oxidative fibres, as opposed to the high change within the non-oxidative fibres, presented a cytological basis by which to explain the differential effects of growth retardation on these fibre types. The ultrastructural composition of myofibres was not impaired by reduced birthweight except when, as in two extreme cases, birthweight was severely reduced. In these instances the myofibrillar percentage volume of the non-oxidative fibres was greatly affected.