Fibre type differentiation during postnatal development of miniature pig skeletal muscles.

Histochemical differentiation of 12 skeletal muscles with a different fibre type composition was studied in miniature pigs from 80 d of gestation to 1 year of age. Two fetal myofibre types were distinguished at 100 d of gestation by the mATPase reaction after acid preincubation. The staining for oxidative enzyme activities showed no conspicuous differences between fibres up to the 6th day after birth. Starting from this age it was possible to distinguish 3 fibre categories: SO (slow-twitch oxidative); FOG (fast-twitch oxidative-glycolytic); and FG (fast-twitch glycolytic). A characteristic cluster distribution of the 3 fibre types was observed in all studied muscles with the exception of the masseter muscle which consisted only of the type SO and FOG fibres with a mosaic arrangement. The frequencies of both SO and FG fibre types increased and the proportion of type FOG fibres decreased during the postnatal period. These changes could be explained by developmental transformations among the individual fibre types. The type FOG fibres converted preferably to the fibre type (SO or FG) that prevailed in the muscles of adult animals.