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

Abstract

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.

The cellular basis of fibrotic tendon healing: challenges and opportunities

“…A similar proliferative response is seen in the endotenon in an equine collagenase-induced flexor tendon injury model, however the identity of these cells and how they contribute to the healing response has yet to be determined (92). During both murine flexor and patellar tendon healing, cells derived from the epitenon/paratenon appear to migrate into the injury area (29,95). Taylor et al demonstrated that the murine tendon epitenon is positive for the basal lamina-related protein laminin during homeostasis (44), and Gelberman et al found that epitenon cells express elevated levels of fibronectin compared to the tendon body shortly following injury (94).…”

“…Chemotactic signaling from a number of important growth factors, including TGF-β family members (28), platelet-derived growth factor BB (PDGF-BB) (90, 91), insulin-like growth factor-1 (IGF-1) (92), and basic fibroblast growth factor (bFGF) (93) are thought to stimulate the proliferation and migration of various fibroblast populations, with the majority of the early healing response appearing to originate from cells derived from the epitenon and not from the tendon body itself. Shortly following injury, epitenon cells adjacent to the injury site begin to proliferate and form a thickened epitenon layer in both canine and murine models (29,94,95). A similar proliferative response is seen in the endotenon in an equine collagenase-induced flexor tendon injury model, however the identity of these cells and how they contribute to the healing response has yet to be determined (92).…”

“…Tracing the activity of Scx-expressing cells in mouse models has provided perhaps the best insight to the fate of tenocytes following injury, though the process is still not well understood. Scx is expressed by the majority of adult tenocytes in the tendon body, but is not expressed by epitenon or paratenon cells, during homeostasis (28,95,104). The localization of Scx+ cells during healing is controversial and likely context dependent, with differential results observed between different models of healing.…”

“…The localization of Scx+ cells during healing is controversial and likely context dependent, with differential results observed between different models of healing. Studies using a patellar window defect (29), partial Achilles transection (28), and complete flexor tendon transection and repair (95) demonstrate localization of these cells within the scar tissue. In contrast, complete transection without repair of the Achilles tendon demonstrates a lack of Scx+ cells in the bridging scar tissue (26).…”

“…Moreover, Scx+ tenocytes located within the tendon body adjacent to the injury site do not appear to proliferate following injury in adults (26,28,29). As to the origin of Scx+ cells, Dyment et al suggested that the Scx+ cells located within the scar tissue may be derived from migrating epitenon cells that turn on Scx expression following injury (29), however there is also evidence that Scx-lineage cells migrate to the injury site from within the tendon parenchyma (95). Combined, these studies indicate that the localization of Scx-expressing cells is likely tendon-, context-, and time-dependent.…”