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. 2007 Jun 4;177(5):769–779. doi: 10.1083/jcb.200608122

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Copyright © 2007, The Rockefeller University Press

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Figure 6. — Working model: reciprocal regulation of Pax7 and MRFs during myogenic cell fate commitment. Satellite cells (Pax7⁺/MyoD⁻/myogenin⁻) must commit to proliferate, differentiate, or renew the progenitor population to maintain muscle function. We propose that commitment to proliferate requires environmental cues (gray arrows) that activate satellite cells and up-regulate MyoD (blue) with a concomitant decline in Pax7 expression (green). Upon commitment to terminal differentiation, up-regulation of myogenin (red) down-regulates Pax7. In a small cell population, up- regulation of myogenin is prevented; Pax7 is up-regulated by unknown mechanisms, resulting in MyoD down-regulation (green nucleus and dashed cytoplasm cell) leading to the commitment to a quiescent, undifferentiated phenotype. In this model, the Pax7/MRF expression ratio is critical and integrates with environmental signals (gray arrows) to regulate cell fate commitment.