The numbers and types of muscle fibres in large and small breeds of pigs.

M. semitendinosus and m. trapezius (portion) were removed from eight miniature pigs ranging from 21 to 160 days of age and eight age-control as well as eight weight-control commercial Large White pigs. Complete transverse frozen sections were obtained for each muscle sample and stained for various enzyme activities including myosin adenosine triphosphatase activity which enabled the identification of 'metabolic bundles'. This in turn enabled conclusions to be made about the prenatal development of the muscle in terms of primary and secondary myofibres. The Large White pigs contained 173% more muscle fibres in m. semitendinosus than did the miniature pigs. Primary myofibre number was found to be about four times more important than secondary to primary myofibre ratios in determining myofibre number in the two breeds of pigs. Both primary myofibre number and secondary to primary myofibre ratios were, however, significantly greater in Large White than in miniature pigs. When the age- and weight-control Large Whites were compared with the miniature pigs it was found that at any given live weight the miniature pigs had thicker myofibres whereas at the same age there was no significant difference. The total area of m. semitendinosus occupied by slow myofibres was about three times greater in the Large White pigs; the functional aspects of this are discussed. It was concluded that genetically smaller animals develop fewer muscle fibres in their muscles by a different mechanism to that exhibited by animals which are smaller due to nutritional deprivation in utero.