Variation of Muscle Length and Sarcomere Length during Postnatal Growth in Mice

Variation of the length of skeletal muscle fibers and sarcomere lengths were investigated in extensor digitorum longus (EDL) and soleus (SOL) muscles in mice (ICR) during postnatal growth in the range from 2 to 12 weeks after the birth. EDL and SOL were fixed in 2.5% gultaraldehyde contained Lock\u27s solution at right angle position of articulatio genus and talocruralis. Single muscle fibers were dissected and lengths of fibers were measured under the dissecting microscope. For measuring sarcomere lengthes, laser diffraction technique was used. Following results were obtained ; (1) the length of muscles and muscle fibers increased rapidly up to 8 and 5 weeks in EDL and SOL respectively ; (2) relative muscle and fiber lengths at 12 weeks against 2 weeks were 1.72 and 1.30 in EDL, 1.60 and 1.32 in SOL ; (3) sarcomere lengths at both ends of the fiber showed no significant change during growth ; (4) on the other hand, sarcomere lengths at mid point showed maximum values at 8 and 5 weeks in EDL and SOL respectively. These observations suggest that the sarcomere are stretched especially at mid point when lengths of muscle fibers increase rapidly. It would be possible to consider that this changes of the sarcomere length during postnatal growth affect the contractile properties of muscles. (J. Physical Fitness Japan 1983, 32 : 134～139