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The major component of a given muscle is the constituent muscle fibres. Lean growth and ultimate muscle mass are therefore largely determined by the number of muscle fibres and the size of those fibres. During myogenesis, myoblasts develop from mesenchymal precursor cells by proliferation and myogenic commitment. Myoblasts subsequently fuse to form multinucleated myofibres. Postnatal growth of skeletal muscle is mainly realised through increases in length and girth of the muscle fibres, but not by increases in muscle fibre number. Postnatal fibre hypertrophy, associated with accumulation of myonuclei (satellite cell proliferation) and muscle-specific proteins, is inversely correlated with the number of prenatally formed muscle fibres. On the other hand, both fibre number and fibre thickness are positively correlated with muscle mass and lean meat percentage. Both fibre number and fibre size are influenced by selection as shown by differences between breeds and correlated responses to (lean) growth selection. Increases in muscle mass solely by fibre hypertrophy, as observed particularly in meat-type pigs and chickens, may be associated with problems in stress adaptability and ultimate meat quality. Genetic variability and heritability of muscle fibre number and size are sufficiently high to include these traits in farm animal selection in addition to commonly used selection criteria for improving lean meat content and meat quality. |
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