The Connection of Composition, Structure, and Dynamic Processes to Tendon Mechanics: Structure-Function Relationships in Collagen V Deficient Tendons

Tendons are able to withstand the broad range of stresses and strains via their finely tuned composition and structure. In addition, tendons undergo a coordinated set of dynamic responses, specifically collagen uncrimping, re-alignment, sliding and deformation, within the matrix. To date, a complete understanding of the hierarchical structure-function relationships in tendon is lacking. Therefore, the overall goal of this thesis was to measure tendon structure and function in a mouse supraspinatus model of altered structure, and to analyze links between mechanical properties, dynamic processes and composition/structure using a series of statistical analyses. In the studies presented here, we used novel and established methods to measure the multi-scale composition, structure and mechanical function of mouse supraspinatus tendons from wild type, collagen V heterozygous and collagen V null mice. Overall, we found that the experimental groups were mechanically inferior to the wild type group, with larger changes in both macroscale function and the dynamic responses (re-alignment, crimp, deformation, sliding). In addition, while fibril morphology was altered at both locations, the insertion site also exhibited alterations in cell and fiber morphology as well as extracellular matrix composition. Finally, using a novel regression approach, we found that the contribution of composition and structure as well as the contribution of dynamic processes to determining macroscale mechanical function was highly dependent on location and that the dynamic processes were significant mediators of the relationship between composition/structure and mechanical properties. Overall, we conclude that although collagen V is a quantitatively minor component in mature tendon/ligament, it is a major regulator of composition and structure during development which ultimately leads to mechanical function. Furthermore, we conclude that the dynamic responses to load are crucial factors in ultimately determining regionally-dependent mechanical function. This information will help to guide clinicians in developing preventative techniques and appropriate rehabilitation strategies, as well as help to define the appropriate and important parameters on which to base tissue engineering efforts for tendon augmentation or replacement. Finally, this work presents a strong foundation on which to develop future experimental and modeling efforts in order to fully understand the complex structure-function relationships present in tendon

The effect of myostatin deficiency on achilles tendon structural and material behavior in male mice

Myostatin (MSTN) is a secreted protein that acts as a negative regulator of skeletal muscle growth. Deficiencies in this protein have been shown to increase muscle mass in a variety of animal models. Consequently, clinical suppression of myostatin is now being pursued as a therapeutic strategy to counteract the muscle wasting that occurs in patients with degenerative neuromuscular diseases such as Duchenne Muscular Dystrophy (DMD). Although research supports the use of myostatin suppression therapy to increase muscle mass, investigation into the effects of myostatin on tendon is limited. The aim of this study was to investigate the effects of myostatin deficiency by characterizing the structural, material and compositional properties of the Achilles tendons from mature (16 week old) male myostatin deficient (MSTN-/-), wild type (MSTN+/+), and heterozygous (MSTN+/-) mice. Specifically, we tested the hypothesis that myostatin deficiency is associated with stiffer and stronger tendons, that these effects are dose dependent, and that structural and material differences can be explained by differences in tendon composition. The experimental model consisted of sixty male mice, thirty of which were used for Achilles tendon tensile mechanical testing and thirty for tendon biochemical compositional analysis. Results demonstrated that there were no significant differences in tendon geometric, structural or material properties. There was a statistically significant difference in total body mass, with a larger mass in the myostatin null group, but there was no difference in Achilles tendon wet weight itself. DNA, glycosaminoglycan (GAG), and hydroxyproline (indicative of total collagen) content were also assessed. Myostatin null animals were found to have less DNA per wet weight, but more GAG per cell than their wild type counterparts. There were no significant differences in collagen between any of the genotypes. These data do not support the conclusion that myostatin deficiency causes stiffer and stronger tendons, but suggest that myostatin levels have no effect on mature mouse Achilles tendon mechanical properties. Further study is necessary to better understand the role of myostatin in tendon composition and mechanical function