Radioautographic and cytometric analysis of myocyte growth and myocardial regeneration in rats.

During postnatal histogenesis of the rat myocardium DNA synthesis and myocyte proliferation are essentially completed by the 20th-25th day of development, while growth of the muscle cells continues after the 30th day. On the first days following injury of the myocardium of newborn and four-day-old rats the index of labelled nuclei and the mitotic index decrease considerably. Later in myocardial regions adjacent to and remote from the focus of the injury the changes in both indices were identical. No such phenomena were observed in 14-day-old, 30-day-old, and adult rats. After injury in all stages of development a scar formed in the myocardium. It is concluded that regeneration (secondary morphogenesis) does not occur in the myocardium, as assumed earlier, but the process of natural development which had been disturbed by the trauma continues. In the myocardial region remote from the site of the trauma this process, especially in four-day-old rats, is of a compensatory nature. The trauma briefly stimulates myocyte proliferation of the stumps of 14-day-old rats. In the center of the focus of the injury even after the repeated injection of H3-thymidine labelled nuclei were not found in the muscle fibers. This raises doubt as to the possibility of proliferative morphogenesis of muscle fibers located in this region.