"Giant" muscle fibres in skeletal muscle of normal pigs.

Observations made during a growth and development study of the semitendinosus and trapezius muscles of 49 purebred Large White pigs between birth and 128 days of age revealed the presence of giant fibres. The occurrence, histochemical and ultrastructural properties of these giant fibres were investigated. A high proportion of the pigs (85 per cent) contained giant fibres in their muscles but these giant fibres usually represented less than 1 per cent of the total myofibre population. Giant fibres possessed enhanced adenosine triphosphatase activity and a high capacity for oxidative metabolism (indicated by succinate dehydrogenase activity) which was reflected ultrastructurally by the greatly heightened electron density of myofibrils and by an abnormally high percentage of mitochondria and lipid droplets. These deviations from normal muscle fibre composition, together with the reduced percentage volume of sarcoplasmic reticulum, were consistent with changes seen in functionally over-loaded muscle. It appears that giant fibre anomalies occur through increased activity stimulated in occasional muscle fibres, perhaps by a structural defect, such as an inadequate amount of sarcoplasmic reticulum, which causes hyper-contractile activity within the fibres and associated compensatory adaptations. Giant fibres did not appear to represent fibres undergoing degenerative changes.