Scleraxis lineage cells contribute to organized bridging tissue during tendon healing and identify a subpopulation of resident tendon cells.

During tendon healing, it is postulated that tendon cells drive tissue regeneration, whereas extrinsic cells drive pathologic scar formation. Tendon cells are frequently described as a homogenous, fibroblast population that is positive for the marker Scleraxis (Scx). It is controversial whether tendon cells localize within the forming scar tissue during adult tendon healing. We have previously demonstrated that S100 calcium-binding protein A4 (S100a4) is a driver of tendon scar formation and marks a subset of tendon cells. The relationship between Scx and S100a4 has not been explored. In this study, we assessed the localization of Scx lineage cells (Scx) following adult murine flexor tendon repair and established the relationship between Scx and S100a4 throughout both homeostasis and healing. We showed that adult Scx localize within the scar tissue and organize into a cellular bridge during tendon healing. Additionally, we demonstrate that markers Scx and S100a4 label distinct populations in tendon during homeostasis and healing, with Scx found in the organized bridging tissue and S100a4 localized throughout the entire scar region. These studies define a heterogeneous tendon cell environment and demonstrate discrete contributions of subpopulations during healing. These data enhance our understanding and ability to target the cellular environment of the tendon.-Best, K. T., Loiselle, A. E. Scleraxis lineage cells contribute to organized bridging tissue during tendon healing and identify a subpopulation of resident tendon cells.