Postnatal development and differentiation of myofibres in functionally diverse porcine skeletal muscles.

Marked changes in muscle function occur after birth, with the response being dependent on developmental stage. Therefore, postnatal cellular ontogeny of functionally distinct skeletal muscles was investigated in the pig, a large mammal born at a relatively advanced stage of development. Assessment of myofibre contractile (type I slow/type II fast) and metabolic (oxidative/glycolytic) properties at Days 0, 2, 5 and 14 revealed type-specific differences in hypertrophy and differentiation. Type I fibre proportions increased significantly in soleus and diaphragm, especially between Days 0 and 5, and rhomboideus showed a similar trend, but in longissimus there was a slight decrease during Days 0-2. Cytochrome oxidase activity was relatively high and similar among myofibres in all muscles at birth, and fibres with low activity were not detected until Day 5. In contrast with previous reports, glycolytic fibres were present in all muscles at birth; postnatal changes in alpha-glycerophosphate dehydrogenase activity were both muscle- and myofibre-specific. Hence, although myosin ATPase activity and metabolic properties of porcine myofibres are well developed at birth, they continue to mature postnatally. This suggests that postnatal muscle development can be modulated by extrinsic factors, even in mammals born at a relatively advanced stage of development.