Trypsin-dissociated atrial cardiocytes from adult rats were exposed to [3H]thymidine for sequential 24-hour periods from day 2 to day 12 of culture. On day 3 and each day thereafter, cells were prepared for ultrastructural radioautography and examined with an electron microscope. Maximal incorporation occurred on day 5, when 63 percent of the cardiocytes were labeled. Mitotic activity was never present in more than 0.5 percent of the cardiocytes examined. Incorporation of [3H]thymidine and mitosis occurred only in immature cardiocytes characterized by subsarcolemmal primary filaments and Z bands with or without specific granules; more mature cardiocytes were never labeled.

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