Fig 3.

Proposed model for the significance of Pax7:MRF protein ratios and satellite cell fate decisions. Modified from Olguin et al. (2007). Satellite cells (Pax7+/MyoD−/myogenin−) must commit to proliferate, differentiate or renew the progenitor population to maintain muscle function. Extrinsic signalling activates satellite cells and up-regulate MyoD (orange) with a concomitant decline in Pax7 expression (grey). Upon commitment to terminal differentiation, up-regulation of myogenin (red) directly or indirectly down-regulates Pax7. In a small subset, Pax7 down-regulation is prevented by unknown mechanisms resulting in MyoD down-regulation, blocking myogenin induction and eventually leading to the commitment to a quiescent, undifferentiated phenotype (yellow cell, black nucleus). In this model, the Pax7:MyoD expression ratio is regulated primarily by post-translational regulation of Pax7 and MyoD. Black arrowheads indicate putative control points (discussed in the text) where regulation of Pax7 activity/stability may participate in the control of satellite cell fate decisions.