AGE-RELATED-CHANGES IN REPLICATION OF MYOGENIC CELLS IN MDX MICE - QUANTITATIVE AUTORADIOGRAPHIC STUDIES

Cell replication in muscle was measured by tritiated thymidine (H-3-Td R) incorporation and autoradiography, in mdx mice from 2-44 weeks of a ge. Pre-mitotic labelling (within 1 h of H-3-TdR injection) was determ ined in 16 mice aged from 15 to 300 days. In 30 further mdx mice, one leg was irradiated 1 h after H-3-TdR injection to block DNA synthesis. Post-mitotic labelling was measured in both legs 10-15 days later. Be tween 20 and 60 days of age a very high proportion (up to 2%) of muscl e (satellite cell) nuclei were replicating pre-mitotically; from 80-30 0 days cell replication was detectable but at much lower levels. Centr ally placed nuclei within muscle fibres appeared at 24 days, increased rapidly to 50% by 50-100 days, declining thereafter to 25% at 300 day s. In post-mitotic samples, labelled myotubes and labelled peripheral muscle nuclei (satellite cell nuclei and myonuclei) appeared at 28 day s and were present in the mdx muscles through to 310 days, indicating continued cell replication and muscle regeneration. Myogenic cell repl ication was both retarded and inhibited by irradiation. These data dem onstrate that muscle cell replication in mdx mice commences at about 3 weeks of age, is maximal at 4-8 weeks, but continues at lower levels until at least 44 weeks.