Postnatal DNA synthesis in hearts of mice: autoradiographic and cytofluorometric investigations

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Abstract
Proliferative activity of cardiomyocytes in ICR mice aged from 1 to 200 days old was studied by 3H-thymidine autoradiography. To determine the outcome of cardiomyocytic DNA synthesis, the DNA content of the cardiomyocytic nuclei in different age groups was also examined by a cytofluorometric method. Flash labelling experiments with 3H-thymidine showed that the cardiomyocytes synthesised DNA most actively in the first 14 days after birth; the label was equally distributed in various parts of the heart and seen in about 8.8% of the cardiomyocytic nuclei on the tenth day. Cumulative labelling experiments for seven and 14 days showed that about 90% of the cardiomyocytic nuclei became labelled, indicating that most cardiomyocytic nuclei of young growing mice developed in this period. Twenty days after birth DNA synthesis of the cardiomyocytes rapidly decreased, the label being evident in less than 0.2%. From 100 to 200 days DNA synthesising cardiomyocytes were seen occasionally in the subendocardial region of the left ventricle (labelling index 0.04%). The cytofluorometric study showed that under normal conditions cardiomyocytes in mice contained mostly diploid nuclei, with a few double diploid nuclei considered to be premitotic nuclei. DNA synthesised cardiomyocytes in adult mice hearts usually became binucleated cells.

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