Fig. 1.

A and B. Comparison of mouse and human ES-cell derived cardiomyocyte proliferation. Mouse ES cells were differentiated for 12 days (A) and human ES cells were differentiated for 14 days (B). The cultures were pulsed with BrdU and sections from cell pellets were immunostained for myosin heavy chain (mouse) or sarcomeric actin (human) to detect cardiomyocytes; cell nuclei were counterstained with methyl green. BrdU+ cardiomyocytes (brown cytoplasm, blue nuclei) were rarely seen in differentiating mouse ES cultures, whereas numerous BrdU+ cardiomyocytes were detected in differentiating human ES cultures, as noted by the arrows. Scale bar = 50 µm. C and D. Mitotic figures and BrdU incorporation in human ES cell-derived cardiomyocytes. H7 human ES cells were differentiated for 19 days before Percoll separation and fraction IV cells were cultured for an additional 4 days with 20% FBS. Cells were stained for sarcomeric actin (red) and BrdU (brown), and nuclei were counterstained with hematoxylin (blue). Multiple BrdU+ cardiomyocytes were evident; mitotic cardiomyocytes in metaphase (C) and telophase (D) are shown by the arrows.