Fig. 2. Cardiomyocytes undergo DNA synthesis and mitosis. Cells were pulsed with BrdU for 9 hours at various times after plating and then fixed and stained for BrdU, MyHC and DNA. Parallel wells were stained for phosphorylated histone H3 (P-H3), MyHC and DNA. (A,B) Labelling of cardiomyocytes with BrdU. Confocal micrograph of two cells stained for BrdU (red), DNA (Hoechst 33258, blue) and MyHC (green). Note that the lower cardiomyocyte has incorporated BrdU. (C) Labelling of cardiomyocytes with an antibody against phosphorylated histone H3 (red) and MyHC (green). Myofibrils are excluded from the central region where mitosis is occurring. (D) Time course of entry into S phase and mitosis. Note that there is a peak of cells undergoing S phase and mitosis at 10 days. The results are the average of duplicate dishes. (E1-E4) Time lapse of a cardiomyocyte undergoing mitosis. Both daughter cells were contracting at 145 minutes (E4). An outline of the cell is seen in the right, lower side of each picture. The time elapsed is shown in the left top side. Note the myofibrils (yellow arrow) and the chromosomes (blue arrow). Red arrow points to the cleavage furrow; N, nuclei. Scale bar: 50 µm (in C and E4).

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