Abstract

BackgroundTendon injury is a significant clinical problem, and regenerative treatments are limited by our understanding of endogenous tendon stem cells. Recent evidence suggests that tendon stem cells are diverse in phenotypic character, and may in fact exist on a spectrum of differentiation capacities. However, the functional significance of each differentiation phenotype is poorly understood. Toward this end, we performed a comprehensive assessment of differentiation capacity toward four connective tissue lineages (adipose, bone, cartilage and tendon) with clonal tendon stem cell lines to: 1) evaluate the differences, if any, in tenogenic potential, and 2) identify the relationships in differentiation phenotype and proliferation capacity. MethodsTendon stem cells were derived from whole equine flexor tendons for this study (N=3). Clonal tendon stem lines were generated by low-density cell plating, and subjected to standard assays of tri-lineage differentiation and population doublings. For tenogenesis, a previously engineered three-dimensional hydrogel construct was incorporated. Differentiation was quantified by the relative gene expression of lineage-specific markers, and confirmed with lineage-specific cell staining. Tenogenesis was further analyzed by hydrogel contraction and histomorphometry. Statistical significance was determined using analysis of variance and post-hoc Tukeys tests. ResultsThree distinct tendon stem cell phenotypes were identified, namely differentiation toward: 1) adipose, bone, cartilage and tendon, 2) bone, cartilage and tendon, or 3) adipose, cartilage and tendon. Further, a positive correlation was found in the ability to differentiate toward all four lineages and the generation of a robust, composite tendon-like construct upon tenogenesis, manifested by the strongest expressions of scleraxis and mohawk, and parallel alignment of tenocyte-like cells with elongated cell morphologies. Significantly increased numbers of cumulative cell population doublings were seen in the absence of adipogenic potential in clonal tendon stem cell lines. ConclusionsOur study strengthens reports on the heterogeneous character of tendon stem cells and identifies key differences in their differentiation and proliferative potentials. Isolation of potent tendon stem cell populations from tendon stromal fractions may yield improved therapeutic benefits in clinical models of repair and promote a native, regenerative phenotype in engineered tendons. Future studies may be targeted to understanding the functional contributions of each tendon stem cell phenotype in vivo, and identifying additional cell phenotypes.