Myogenesis and postnatal skeletal muscle cell growth as influenced by selection

The major component of a given muscle is the constituent muscle fibres. Lea n growth and ultimate muscle mass are therefore largely determined by the n umber of muscle fibres and the size of those fibres. During myogenesis, myo blasts develop from mesenchymal precursor cells by proliferation and myogen ic 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 fi bre number. Postnatal fibre hypertrophy, associated with accumulation of my onuclei (satellite cell proliferation) and muscle-specific proteins, is inv ersely correlated with the number of prenatally formed muscle fibres. On th e other hand, both fibre number and fibre thickness are positively correlat ed with muscle mass and lean meat percentage. Both fibre number and fibre s ize are influenced by selection as shown by differences between breeds and correlated responses to (lean) growth selection. Increases in muscle mass s olely by fibre hypertrophy, as observed particularly in meat-type pigs and chickens, may be associated with problems in stress adaptability and ultima te meat quality. Genetic variability and heritability of muscle fibre numbe r and size are sufficiently high to include these traits in farm animal sel ection in addition to commonly used selection criteria for improving lean m eat content and meat quality. (C) 2000 Elsevier Science B.V. All rights res erved.