Scleraxis Lineage Cells Contribute to Organized Bridging Tissue During Tendon Healing, and Identifies Subpopulations of Resident Tendon Cells

During tendon healing, it is postulated that intrinsic tendon cells drive tissue regeneration while extrinsic cells drive pathological scar formation. Intrinsic tendon cells are frequently described as a homogenous, fibroblast population that is positive for the marker Scleraxis. It is controversial whether intrinsic Scleraxis localize within the forming scar tissue during adult tendon healing. We have previously demonstrated that calcium binding protein S100a4 is a driver of tendon scar formation and marks a subset of intrinsic tendon cells. However, the relationship between Scleraxis and S100a4 has not been explored. In this study, we aimed to investigate the localization of Scleraxis lineage cells following adult murine flexor tendon repair and to establish the relationship between Scleraxis and S100a4 throughout both homeostasis and healing. We have shown that adult Scleraxis lineage cells localize within the scar tissue and organize into a highly aligned cellular bridge during tendon healing. Additionally, we demonstrate that markers Scleraxis and S100a4 label distinct populations in tendon during homeostasis and localize differently within tendon scar tissue, with Scleraxis found specifically in the organized bridging tissue and S100a4 localized throughout the entire scar region. These studies define a heterogeneous tendon cell environment and demonstrate discreet contributions of subpopulations during healing. Taken together, these data enhance our understanding and ability to target the complex cellular environment of the tendon during homeostasis and healing.