Fig. 5: Fibrillar ECM persisted in the muscle belly of Pax3^Cre/Cre mutants but MTJ-specific ECM did not.

A–F WT or Pax3^Cre/Cre decellularized E13.5 triceps tendon (arrowheads) and muscle belly fibers were COL1⁺, COL5⁺, and FBN2⁺. G–I’ In WT decellularized E13.75 forelimbs, the COL22A1⁺ MTJ (green) and WGA⁺ ECM fibers (magenta) elongated under tension and were restored to original configuration when tension was removed, as indicated by the fiducial markers (dotted outline). * denotes an unresolved fiducial marker. G’–I’ The MTJ exhibited elongation in under tension (dotted line outline). J Quantification of average COL22A1⁺ MTJ fiber length revealed MTJ length significantly increased when stretched under tension and returned to the relaxed configuration for N = 3 independent limbs *: P-value < 0.05 Tukey’s post-hoc test. K, L The MTJ of decellularized WT but not Pax3^Cre/Cre forelimbs was marked by COL22A1⁺ (green) at the end of the WGA⁺ tendons (magenta, arrowheads). Magnification = 20× (A–F), 25× (G–K). Z-projection: z = 230 µm (A), 122 µm (B), 24 µm (C), 23.6 µm (D), 230 µm (E), 115 µm (F), 96.3 µm (G, G’, H, H’), 96.5 µm (I, I’), 3D rendering: z = 110 µm (J, K), scale bars = 100 µm (A–K), 25 µm (G’–I’). Representative images from N = 3 independent biological replicates.