Wound repair and proliferation were examined in the injured newt atrium with light- and electron-microscopic techniques including autoradiography. Hearts were injured by removing a piece approximately 0.5 mm2of the atrial wall. The five-day wound was an endothelial and mesothelial-lined blood clot bordered by a 150-μm necrotic zone. Repair progressed from the periphery inward with areas of macrophage activity replaced by fibroblasts and connective tissue. The wound at 25 days consisted of a scar with few myocytes. There was no difference in the proliferative behavior between the right and left atria. Proliferative cells were localized to a 500-μm reactive zone surrounding the wound. The maximum mesothelial cell thymidine-labeling index of 20.5 and mitotic index of 1.4 were seen 5 days after injury. The peak connective tissue cell thymidine-labeling index of 10.2 and mitotic index of 0.4 were seen 10 days after wounding. The peak thymidine-labeling index of 9.8 for myocardial cells was recorded 10 days after injury with a mitotic index of 0.2. Proliferation returned to control levels by 25 days post-injury. Electron microscopy demonstrated that myocytes engaged in DNA synthesis were indistinguishable from control myocytes. Z-band material was not observed in mitotic myocytes, but myofilaments and junctions were presen
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