Abstract
Trypsin-dissociated adult rat atrial cardiocytes were exposed to [3H]thymidine for sequential 24-hr periods from day 3 to day 13 of culture. Following preparation for ultrastructural autoradiography, 1000 cells (at each daily interval) were examined with an electron microscope. Maximal incorporation occurred on day 5 when 63% of the cells were labeled. Small secondary peaks of incorporation occurred on days 8 and 11 following previous exposure to fresh serum. Mitotic activity never exceeded 0.5% of all cardiocytes examined. DNA synthesis and mitoses occurred only in immature cardiocytes characterized by subsarcolemmal filaments and Z-bands with or without specific granules; more mature cells were never labeled.
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