Defining the Role of Elastic Fibers in Tendon Mechanics

Tendons serve as a linking component of the musculoskeletal system by transferring forces between muscle and bone. As such, the structural proteins of the tendon extracellular matrix are of vital importance for the tissue to function properly and maintain its mechanical integrity. Collagen is the principal constituent of tendon and makes up its aligned hierarchical organization. Other structural proteins, such as elastin, are in comparison understudied and not well understood in relation to tendon function. Elastin, the main component of elastic fibers, has unique mechanical properties including high extensibility, fatigue resistance, and elasticity; these properties are important for elastin-rich tissues such as blood vessels and the lungs that experience constant cyclic loading. In tendon, elastin only makes up 1-2% of the tissue by dry weight, yet prior work has demonstrated a significant impact of elastin on tendon mechanics despite its low content. Moreover, this work is motivated clinically by elastinopathic heritable disorders such as Williams-Beuren syndrome, cutis laxa, and Weill-Marchesani syndrome that cause musculoskeletal abnormalities, and by elastin degradation resulting from aging which may lead to increased risk of injury or chronic pain. This work sought to further refine the understanding of how elastic fibers contribute to tendon mechanics by taking advantage of genetically modified mouse models, which had not previously been used to investigate elastic fibers in tendon, and also by developing and implementing innovative techniques to incorporate into more traditional model systems. Two elastinopathic mouse models were utilized for this research: elastin haploinsufficient mice, which have reduced elastin content, and fibulin-5 knockout mice, which have disorganized elastin. Both of these mouse models had elevated stiffness or modulus at higher strains in comparison to wild-type controls. Analysis of collagen realignment using polarized light imaging during mechanical testing demonstrated that the elastinopathic tendon had greater changes in alignment, suggesting that lack of a full elastin network may result in greater collagen engagement and thus a stiffer tendon. Within these mouse models, there was a generally greater effect of elastin observed in tendons with an energy-storing function compared to tendons with a more positional role. In addition, a three-dimensional fiber analysis algorithm was developed to quantify the structural organization of the elastic fiber network; results showed that there was much greater connectivity and lower alignment of elastic fibers within the interfascicular matrix compared to elastic fibers in tendon fascicles, but differences between tendon type and species were limited. Because mouse tendon does not have the fascicular structure of larger tendon, a comparison of the effect of elastin degradation on tendon mechanics across tendons both with and without interfascicular matrix was performed. Within these experiments, elastin degradation had a universal effect on the magnitude and non-linearity of the elastic response while having a tendon-specific effect on viscoelastic properties, which corresponded to differences in elastin content between tendons. These data suggest that the elastic fibers within the interfascicular matrix do little to contribute to purely tensile mechanics of tendon, although they still are likely important contributors to shear or other off-axis loading. In summary, this work shows that elastic fibers affect the tensile response of tendon by regulating collagen reorganization within tendon fascicles during loading. Changes to elastic behavior caused by full elastin degradation were consistent across species and tendon types, while energy-storing tendons, that had greater elastin content, also demonstrated altered viscoelastic properties and were more affected by smaller changes to the elastic fiber network