Figure 4: Testing satellite cell role in maintaining size of limb myofibres.

(a) Schematic showing ablation of satellite cells at 6 months and harvested at either 12 or 20 months. (b–f) Representative cross-sections showing outlined laminin+ myofibres in limb muscles EDL, TA, gastrocnemius, plantaris and soleus at 12 and 20 months in Pax7^CreERT2/+;Rosa^DTA/+ mice (see Fig. 3 for representative cross-sections of Pax7^+/+;Rosa^DTA/+ mice). Histograms generated using MuscleQNT show relative frequency of myofibres sizes in muscles at 12 months (dark blue, control Pax7^+/+, n=7, for each muscle; red satellite cell-ablated Pax7^CreERT2/+, n=8 for each muscle) and 20 months (light blue, control Pax7^+/+, n=4 for EDL, n=5 for other muscles; pink, satellite cell-ablated Pax7^CreERT2/+, n=6 for each muscle). Scale bar, 100 μm for all panels. (g) Additional histograms for the soleus show frequency of fast MyHCI− and slow MyHCI+ myofibres. Permutation tests (see Methods) were conducted to determine whether counts of myofibres were significantly different in a particular bin between control and satellite cell-ablated muscles. Black asterisks indicate an empirical P-value of <0.05 and grey asterisks a value of <0.10 (see Methods). Note that the MuscleQNT parameters slightly differ between 12- and 20-month-old TA, gastrocnemius, plantaris and diaphragm muscles; to explicitly compare the CSA of 12- and 20-month-old control muscles see Fig. 3. (h) Myonuclei are significantly longer (two-tailed student t-test, **P<0.01) and number of myonuclei are significantly reduced in 20-month versus 12-month-old EDL, but not in plantaris (two-tailed student t-test, *P<0.05).