Zonal Characterization and Differential Trilineage Potentials of Equine Intrasynovial Deep Digital Flexor Tendon-Derived Cells

Abstract Background: Intrasynovial deep digital flexor tendon (DDFT) injury is a common cause for forelimb lameness in horses and are typically associated with poor outcomes. Intrasynovial tendon contains a superficial fibrocartilage in response to compressive forces from an opposing boney prominence, and is critical for tendon gliding function. Characterization of tendon-derived cells (TDC) from intrasynovial tendon fibrocartilage has not been conducted as for prototypical extrasynovial tendon and is necessary for developing improved therapeutic strategies. Results: In this study, we successfully isolated homogenous TDC from the fibrocartilaginous (fTDC) and tendinous (tTDC) zones of equine forelimb intrasynovial DDFT via low-density plating method. During monolayer passage, both TDC subpopulations exhibited clonogenicity, high in vitro proliferation rate, and fibroblast-like morphology. Third passage fTDC and tTDC were positive for cell surface antigens CD90 and CD29 and negative for CD44 and CD45. There were no significant differences in the basal tenogenic, osteogenic and chondrogenic marker expressions of fTDC and tTDC. Trilineage differentiation demonstrated that fTDC were largely restricted to chondrogenic differentiation; whereas, those from the tendinous zone underwent osteogenic and chondrogenic differentiation. Both TDC subpopulations displayed weak adipogenic differentiation potentials. Conclusions: These results provide a foundation for studies exploring cell-based therapies for intrasynovial tendon repair as these TDC are potential targets to enhance intrinsic repair capacity. Pending further investigation, promoting chondrogenic properties in cells administered exogenously into the intrasynovial space may be beneficial for intrasynovial tendon regeneration.