Age-Dependent Fibroblast Programs Govern Regenerative and Fibrotic Tendon Repair

Tendon injuries often result in fibrosis, compromising function and predisposing to re-injury. Here, we used a full-width, non-repair Achilles tendon transection model in young (3 weeks old) and adult (18-20 weeks) rats to elucidate the cellular mechanisms governing regenerative versus fibrotic healing. Functional and biomechanical analyses revealed that young tendons recovered motion and load-bearing capacity more rapidly but exhibited more fibrotic early healing. Single-nuclei RNA sequencing identified seven major cell populations within the connective tissue compartment. Adult tendons maintained a "synthetic fibroblast" population marked by upregulated ECM synthesis, reduced stress-related gene expression, whereas young tendons favored expansion of Cxcl12/Lrp6/Gas6 fibrotic fibroblasts linked to oxidative and pro-angiogenic signaling. The young group showed sustained activation of Nox4-Gas6 pathways driving a self-reinforcing fibrotic circuit. These findings define a fibroblast lineage bifurcation that dictates oxidative stress signaling as a key regulator of fibrotic remodeling, highlighting potential therapeutic targets to promote regenerative tendon repair.