Figure 7. Return to quiescence and self-renewal is temporally instructed in a subpopulation of cycling Pax7 satellite cells.

(A) Cartoon depicting the TM strategy to induce Cre activation in Control (Pax7-CreER^tm;Spry1^WT/WT) and SC-Null (Pax7-CreER^tm;Spry1^flox/flox) mice 14 days prior to injury) or at distinct phases of muscle regeneration. Muscles were analyzed 50 days after muscle injury. (B) Muscle sections from experiments depicted in (A) were stained with anti-Pax7, MyoD, Ki67, laminin and DAPI. Histogram shows the average number of Pax7⁺ cells per muscle section in regenerated SC-Null muscle expressed relative to the number of Pax7⁺ cells in the uninjured contra-lateral muscle. Data were averaged over a minimum of 4 mice per group and expressed as mean ± sem. (C) Muscles from Spry1^+/+ and Spry1^lacZ/lacZ adult mice were injured and left to regenerate for 13 days. Mice were treated with U0126 or vehicle (DMSO) via IP injection 10 and 11 days after injury. Histograms show the number of quiescent Pax7⁺ satellite cells located underneath the basal lamina of muscle sections. (D) Mice were subjected to two rounds of injury (1° and 2°) and 50 days of repair. Fifty days after the second injury muscle was analyzed for the number of Pax7⁺ cells in satellite cell position in Spry1^+/+ and Spry1^lacZ/lacZ (left panel) and TM-treated SC-Null and littermate controls (right panel). (**,p<0.01). (E) The average muscle fiber size from Control and SC-Null muscles from (D).