Figure 2.

Myogenesis and clonal drift in the zebrafish embryo. (A) Muscle stem cells blocked in G2 by Meox1 maintain the capacity of self-renewal and when the cell cycle blockage is lifted they re-enter the cell cycle (activated) and, after mitosis, the daughter cells have either a differentiated fate (purple arrows), or a stem cell fate (green arrows). In the first case, cells enter the cell cycle, proliferate, differentiate and generate muscle fibers. In the second case cells blocked in G2 become quiescent thereby, maintaining stemness and the capacity of self-renewal upon re-entering the cell cycle. They can undergo multiple rounds of this process (two cycles, N and N+1 are represented here). This process takes place in muscle stem cells that express Meox1 (represented in the scheme). Muscle stem cells that do not express Meox1 primarily undergo proliferation and differentiation but have reduced or lost capacity of self-renew; (B) stem cells heterogeneity in the early phases and, as a consequence, over successive self-renewal events (N, N+1, … N + x), the dominance of a single stem cell clone in the growth of individual myotomes (clonal drift).