Peterson, B. E. | Search bioRxiv

bioRxiv2023

Structural Determinants of Tendon Function During Development and Their Sensitivity to Mechanical Stimulation

The load-bearing capabilities of tendon are acquired during neonatal stages of development, characterized by an abrupt increase in multiscale mechanical properties. While prior work has identified numerous changes within the collagenous structure during these developmental periods, the primary structural elements that give rise to this abrupt mechanical functionality, and their mechanobiological sensitivity, remains unclear. To address this gap in knowledge, we leveraged a combination of ultrastructural imaging, biochemical/thermodynamic assays, multiscale mechanical testing, and shear lag modeling to probe the dynamic structure-function relationships and establish their sensitivity to mechanical stimulation during tenogenesis. Mechanical testing and modeling suggested that the rapid increase in multiscale mechanics can be explained by a increasing fibril length and intrafibrillar crosslinking. To test this, we inhibited collagen crosslinking during development and observed a drastic reduction in multiscale mechanical capabilities that was explained by a reduction in both fibril modulus and length. Using muscle paralysis to investigate mechanosensitivity, we observed a significantly impaired multiscale mechanical response despite small changes in fibril diameter and fibril area fraction. While there was no change in crosslinking density, there was a decrease in thermal stability with flaccid paralysis, and our shear-lag model suggested that flaccid paralysis produces a reduction in fibril length and intrafibrillar crosslinking. Together, these data suggest that both intrafibrillar crosslink formation and fibril elongation are critical to the formation of load-bearing capabilities in tenogenesis and are sensitive to musculoskeletal activity. These findings provide critical insights into the biological mechanisms that give rise to load-bearing soft tissue.

bioengineering↗

bioRxiv2021

Mechanical Stimulation via Muscle Activity is Necessary for the Maturation of Tendon Multiscale Mechanics during Embryonic Development

During embryonic development, tendons transform into a hypocellular tissue with robust tensile load-bearing capabilities. Previous work suggests that this mechanical transformation is due to increases in collagen fibril length and is dependent on mechanical stimulation via muscle activity. However, the relationship between changes in the microscale tissue structure and changes in macroscale tendon mechanics is still unclear. Additionally, the specific effect of mechanical stimulation on the multiscale structure-function relationships of developing tendons is also unknown. Therefore, the objective of this study was to measure the changes in tendon mechanics and structure at multiple length scales during embryonic development with and without skeletal muscle paralysis. Tensile testing of tendons from chicken embryos was performed to determine the macroscale tensile modulus as well as the magnitude of the fibril strains and interfibrillar sliding with applied tissue strain. Embryos were also treated with either decamethonium bromide or pancuronium bromide to produce rigid or flaccid paralysis. Histology was performed to assess changes in tendon size, spacing between tendon subunits, and collagen fiber diameter. We found that the increase in the macroscale modulus observed with development is accompanied by an increase in the fibril:tissue strain ratio, which is consistent with an increase in collagen fibril length. Additionally, we found that flaccid paralysis reduced the macroscale tendon modulus and the fibril:tissue strain ratio, whereas less pronounced effects that were not statistically significant were observed with rigid paralysis. Finally, skeletal paralysis also reduced the size of collagen fibril bundles (i.e., fibers). Together, these data suggest that more of the applied tissue strain is transmitted to the collagen fibrils at later embryonic ages, which leads in an increase the tendon macroscale tensile mechanics. Furthermore, our data suggest that mechanical stimulation during development is necessary to induce structural and mechanical changes at multiple physical length scales. This information provides valuable insight into the multiscale structure-function relationships of developing tendons and the importance of mechanical stimulation in producing a robust tensile load-bearing soft tissue.

developmental biology↗