Figure 1.

The cell cycle and myogenesis. Cell cycle: the G1 phase commits cells to enter the cell cycle and prepare them to duplicate their DNA during the S phase. After DNA duplication, cells enter G2 phase that prepares for mitosis (M phase), where chromatids and daughter cells separate. After M phase cells can enter G1 or G0, the latter being non-proliferating and metabolically quiescent phase. In satellite cells upon exit from G0/G1, the duration of the first cell cycle in mice is much longer than in cycling cells. The length of the first cell cycle is longer in the satellite cell subpopulation with higher stemness, dormant properties (Pax7^Hi). Fused nuclei that constitute the differentiated muscle cell have irreversibly exited the cell cycle. In the muscle fiber, a few satellite cells are maintained in G0 (two cells are represented in green in the myotube), ready to re-enter the cell cycle upon injury. In a variety of cells and organisms, DNA damage or adverse survival conditions (see text) may induce arrest at other phases of the cell cycle than in G0, namely in G1 or G2 (also indicated as G1/S or G2/M transition, respectively). This is the case for embryonic myogenesis in zebrafish that halts stem cells in G2 (see text). SC, stem cell.