Muscle cellularity and birth weight

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Abstract
A study of the effects of low birth weight on muscle cellularity was performed on 48 pedigree Large White pigs selected, from a total of 17 litters, on the basis of their weight at birth. Where possible, the largest male (mean birth weight of 1544 g), smallest male (1135 g), and runt (776 g) littermates were chosen. Fresh frozen, whole mid belly, sections of m. semitendinosus and samples of m. trapezius from each animal were stained for the demonstration of acid pre-incubated myosin adenosine triphosphatase. The use of this stain demonstrated groups of positively stained, slow-contracting, myofibres which were each surrounded by a compliment of negatively stained, fast-contracting, fibres which together constituted ‘metabolic bundles’. The positions of metabolic bundles are indicative of the presence of single primary myofibres in the foetal muscle, all the other myofibres in the metabolic bundles being derived from subsequently formed secondary fibres. Determination of total myofibre number and primary fibre number were made for m. semitendinosus together with an estimation of the secondary to primary fibre-number ratios for both this muscle and for m. trapezius. Low birth weight was associated with a permanently reduced total muscle fibre number, proportionately in the order of 0·19 (P < 0·001) between large and runt littermates. A reduced muscle fibre number was not always associated with low birth weight, but when this was the case it was generated through a reduced secondary to primary fibre-number ratio (P < 0·01). Primary fibre number was not significantly affected in low birth-weight pigs except in extreme cases of runting.