Collagenase type 2

For AMCMs, mice were heparinized and anesthetized using deep isoflurane (5%) anesthesia. The hearts were surgically isolated, washed in ice‐cold cardiomyocyte isolation buffer (CIB; 120 mm NaCl, 5.4 mm KCl, 0.5 mm MgSO4, 0.33 mm NaH2PO4, 25 mm NaHCO3, 22 mm glucose, 25 mm HEPES, 10 mm BDM, and 30 mm taurine), cannulated via the ascending aorta and digested through perfusion with an enzyme CIB buffer containing 1 mg mL⁻¹ Type II Collagenase (BioFroxx, Germany) and 0.6 mg mL⁻¹ Type IV Collagenase (BioFroxx, Germany) for 15 min at 37 °C using a Langendorff perfusion system. The digested hearts were mechanically shredded and dissociated in Minimum Essential Medium Eagle (Sigma‐Aldrich, USA) supplemented with 10% bovine serum albumin (BSA; Sigma‐Aldrich, USA). After filtration with a 100 µm mesh filter and centrifugation at 500 rpm for 5 min, harvested AMCMs were reintroduced with calcium in four concentration gradients, ranging from 0 to 900 µm.
For NMCMs, newborn C57BL/6J mice (within 72 h) were euthanized by decapitation and the hearts were collected. NMCMs were isolated as previously described^[57] and identified by immunofluorescence of cTnT.

Gelatin (porcine skin, type A), dopamine hydrochloride, 2-(dimethylamino)ethyl methacrylate (DMAEMA), polyethylene glycol diacrylate (PEGDA), visible light initiator lithium phenyl(2,4,6-trimethylbenzoyl) phosphinate (LAP), DCFH-DA, Hoechst 33342, β-sodium glycerophosphate, l-ascorbic acid and dexamethasone were purchased from Sigma Aldrich (St. Louis, MO, USA). Heparin sodium salt (Mn = 1.25 kDa, >189 U/mg, Sinopharm Chemical Reagent Co. Shanghai, China), 2, 2-diphenyl-1-picrylhydrazyl (DPPH) (Mackline, China), Recombinant Human/Murine/Rat BMP-2 (E.coli derived, PeproTech, USA), Human/Murine/Rat BMP-2 Standard TMB ELISA Development Kit (PeproTech, USA), Live/Dead Staining Kit (Beyotime, China), CCK8 assay (ApexBio, USA), fetal bovine serum (FBS; HyClone, USA), α-MEM (HyClone, USA), 1% penicillin-streptomycin solution (HyClone, USA), trypsin (Gibco, USA), type II collagenase (BioFroxx, Germany), DAPI (Abcam, USA), Alizarin Red S (ARS) Staining Kit (Beyotime, China), Alp staining (Beyotime, China), Hematoxylin and Eosin (H&E) Staining Kit (Solarbio, China) and Masson's Trichrome Stain Kit (Solarbio, China) were used for in vitro and in vivo study. Unless otherwise stated, all other regents and solvents were used as received without further purification or modification.

The attached adipose tissue was removed before harvesting the skin wound tissue and the skin samples were sliced and digested with the Opti-MEM (Invitrogen) solutions containing collagenase type I (0.5 mg/ml, BioFroxx, China), collagenase type II (0.5 mg/ml, BioFroxx, China), collagenase type IV (1 mg/ml, BioFroxx, China), hyaluronidase (1 mg/ml, BioFroxx, China), and deoxyribonuclease I (0.02 mg/ml, Biosharp, China) for 30 min on a shaker at 37 °C. The tissue was mechanically ground using a Tissue Grinder with Pestle (YiXi Bio, China), washed with a complete medium containing 10% FBS, and the red blood cells were lysed.
Cell surfaces were stained in the dark on ice for flow cytometry analysis. The cells were stained with antibodies against the surface antigens CD4 (MultiSciences, China), CD25 (MultiSciences, China), CD8 (Proteintech, China), CD69 (Proteintech), and CD103 (Proteintech). Cells were fixed and permeabilized and then incubated with anti-CD16/32 (MultiSciences, China) to block nonspecific binding. Staining of intracellular markers, such as the Treg marker FoxP3 (MultiSciences, China), were performed on ice. The cells were then analyzed by flow cytometry.