The direct determination, by counting, of the number of sarcomeres in series along the length of single teased muscle fibres taken from mice of different ages showed that the increase in fibre length during normal growth is accompanied by a large increase in sarcomere number. The greatest increase occurs during the 3 weeks after birth. By counting the number of muscle fibre nuclei in single teased fibres it was shown that the number of nuclei per fibre increases with age, and that this increase continues beyond the point at which the fibres have ceased to grow in length. It is suggested that post-natal increase in nuclear number is associated with both increase in length and increase in girth of the muscle fibres.
By injecting tritiated adenosine into young mice, an attempt was made to label newly formed actin filaments and ribosomes and thus to determine the region where new sarcomeres are laid down during increase in fibre length. Using autoradiography and scintillation counting it was shown that the radioactive label was incorporated more into the ends than into the middle regions of the muscles. The implication of these findings is that new sarcomeres are added on serially at the ends of the muscle fibres. An investigation, at the ultrastructural level, of muscle fibres taken from foetal and newborn mice indicates that the end of the fibre is a region of active development. This area is characterized by numerous ribosome formations and by myofilaments which are not organized into myofibrils. Cells which can occasionally be seen fusing with the end regions of young muscle fibres indicate a possible way in which nuclei are added to the growing fibre.
Immobilization experiments have shown that it is possible to alter both the rate and the extent of the post-natal increase in sarcomere number. Immobilization of limb joints, by means of plaster casts, so that the muscle is held in either the extended or the shortened position results in the number of sarcomeres along the fibres falling far short of that in the fibres from control muscles. Removal of the restriction is followed by a rapid increase in the number of sarcomeres in series and a return to the normal level within a period of about 4 weeks. These experiments indicate that for normal growth to occur, it is important for a muscle to be able to contract isotonically.