Tendon Regeneration: Roles of Growth Factors and Phenotypic Diversity in Tendon Stem Cells

Tendon injuries significantly impact quality of life and are often career ending. Mesenchymal stem cell (MSC) therapy is known to augment intrinsic tendon healing, however, little is known of the stem cells endogenous to tendon, the microenvironmental cues that induce tendon differentiation, and whether individual cells in an inflammatory milieu respond differently to these cues. To address these questions, a three-dimensional tenogenesis assay was developed as an efficient and reproducible metric of cellular capacity to differentiate toward tendon. In contrast to more complex assays of tenogenesis, this design incorporates a simple apparatus using commercially available plasticware for the application of uniaxial static strain in in a type I collagen cell-seeded hydrogel construct. Tendon-related gene expression, glycosaminoglycan levels, elongated cell morphologies and parallel cell alignments are enhanced with BMP-12 induction over ten days of culture. This dissertation provides novel insight to the roles of growth factors in MSC tenogenesis.

Tendon healing in vivo is dependent on endogenous tendon stem cells (TSC) that mediate the inflammatory response to injury and promote synthesis of collagen and matrix remodeling, among other extracellular processes. Recent evidence suggests that these cells exist on a spectrum of differentiation potencies, and may be differently committed to the tendon fate. Individual stem cells were isolated from the tendon, and their capacities for proliferation, tri-lineage differentiation and tenogenesis were evaluated. Three distinct TSC phenotypes were revealed, and significant, positive correlations were found in quadra-differentiation potency (toward four lineages) and the expression of a strong, composite tendon phenotype.

These studies have important implications in the current standard-of-care in regenerative therapies for tendon. Our benchtop tenogenesis assay can be used to determine the therapeutic potential of allogeneic MSC lines and MSCs from novel sources for 'off-the-shelf' treatments. Our study of TSCs lends valuable insight to the diversity of cell phenotypes found in tendon, and the potential contributions of each phenotype to tendon healing and homeostasis. These results further strengthen the status of tendon as a superior source of stem cells for tendon repair.