DNA Synthesis in the Myocardium of Growing, Mature, Senescent and Dystrophic Mice

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Abstract
Heart muscle from mice injected with thymidine-H3 was radioautographed and the distribution of radioactive nuclei was quantitated. Comparison of various techniques demonstrated that when fibers were outlined by the periodic acid-Schiff staining technique, and only areas with fibers cut in cross section were counted, the lowest number of radioactive cardiac muscle fiber nuclei was found, thus indicating that less rigid methods of identification led to the inclusion of connective tissue nuclei by mistake. In 9 day old mice, the 2 cell types were radioactive in the ratio of 9 cardiac muscle nuclei to 819 connective tissue nuclei. Onlythe latter were labeled in cardiac muscle from mice 21 or 30 days old. In old mice, no radioactive cardiac muscle nuclei were found, but in one of these mice a large number of connective tissue cells were radioactive. DNA synthesis was not detected in cardiac muscle fibers of dystrophic mice.