Figure 1. Heterochronic transplantation of aged satellite cells (SCs) to local or systemic young environment fails to rescue age-associated phenotypes.

(a) (left) Young and aged myofiber-associated SCs cultured for 96 h and stained with Pax7, MyoD and Myogenin. Scale bar, 50 μm. (right) Percentages of Syndecan-4⁺ (Sdc4⁺) SCs at 96 h that are quiescent (Pax7⁺), proliferating myoblasts (Pax7⁺/MyoD⁺ or MyoD⁺) or differentiating myocytes (MyoD⁺/Myogenin⁺ or Myogenin⁺). Chart area scaled to Sdc4⁺ SCs per myofiber length (n = 3, ≥ 20 myofibers/condition. P < 0.05 for 96 h Young vs. Aged cells/myofiber length and Pax7^–/MyoD⁺. P < 0.001 for MyoD^–/Myogenin⁺, t-test.). (b–f) (b) Schematic for heterochronic, βActGFP SC and wild-type myofiber co-culture. GFP⁺Sdc4⁺ SCs (^) and endogenous Sdc4⁺ SCs (*) after (c) 24 h or (d) 72 h in culture. Scale bar, 50 μm. Average number of GFP⁺/Sdc4⁺ SCs per myofiber length at (e) 24 h, and (f) fold increase in donor SCs per myofiber at 72 h in culture (n = 3, ≥ 20 myofibers per condition. *P < 0.05, t test). (g-j) (g) Schematic for heterochronic myofiber transplantation to young muscle hosts. (h) Muscle sections harvested at 30 d or 60 d (not shown) post-transplantation . Insets depict donor-derived SCs (^ = GFP⁺/Sdc4⁺) and contralateral, uninjured (UI) muscle. Scale bars, 50 μm or 5 μm (SCs). (i) Average number of donor-derived SCs per field (j) and Log₂ fold-change in donor-derived SCs per field comparing 60 d vs. 30 d post-transplantation (n = 3 to 5 transplant recipients. *P < 0.05 for Aged 30 d vs. 60 d; ***P < 0.001 for Young vs. Aged, t test). Mean ± s.e.m. for all.