POSTNATAL GROWTH OF MUSCLE

• Animals are born with a certain number of muscle fibers in each muscle.
• Fiber contraction speed, metabolism and growth pattern are controlled by motor neurons.
• Radial growth involves the longitudinal splitting and proliferation of myofibrils.
• Longitudinal growth involves the formation of new sarcomeres at each end of the fiber.
• Real fiber number and length are very difficult to estimate unless all fibers run from one end of a muscle to the other, but this does not happen in major carcass muscles because of intrafascicularly terminating muscles fibers.
• The apparent number of fibers is seen in a cross section of a muscle and depends on the fraction of the real fiber number that runs through the cross section. There is no guarantee that this fraction remains constant during development, and often the apparent fiber number may change.
• Muscle fibers are multinucleate at birth and new nuclei are added during postnatal development, derived from the mitotis of myoblasts trapped on the muscle fiber surface as satellite cells.