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    Tendinopathy, degeneration of the tendon that leads to pain and dysfunction, is common in both sports and occupational settings, but multi‐scale mechanisms for tendinopathy are still unknown. We recently showed that micro‐scale sliding (shear) is responsible for both load transfer and damage mechanisms in the rat tail tendon; however, the rat tail tendon is a specialized non‐load‐bearing tendon, and thus the load transfer and damage mechanisms are still unknown for load‐bearing tendons. The objective of this study was to investigate the load transfer and damage mechanisms of load‐bearing tendons using the rat plantaris tendon. We demonstrated that micro‐scale sliding is a key component for both mechanisms in the plantaris tendon, similar to the tail tendon. Namely, the micro‐scale sliding was correlated with applied strain, demonstrating that load was transferred via micro‐scale sliding in the plantaris and tail tendons. In addition, while the micro‐scale strain fully recovered, the micro‐scale sliding was non‐recoverable and strain‐dependent, and correlated with tissue‐scale mechanical parameters. When the applied strain was normalized, the % magnitudes of non‐recoverable sliding was similar between the plantaris and tail tendons. Statement of clinical significance: Understanding the mechanisms responsible for the pathogenesis and progression of tendinopathy can improve prevention and rehabilitation strategies and guide therapies and the design of engineered constructs. © 2019 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 37:1827–1837, 2019

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  2. Abstract

    Rodent tendons are widely used to study human pathologies such as tendinopathy and repair, and to address fundamental physiological questions about development, growth, and remodeling. However, how the gross morphology and multi‐scale hierarchical structure of rat tendons, such as the tail, plantaris, and Achilles tendons, compare with that of human tendons are unknown. In addition, there remains disagreement about terminology and definitions. Specifically, the definitions of fascicle and fiber are often dependent on diameter sizes, not their characteristic features, and these definitions impair the ability to compare hierarchical structure across species, where the sizes of the fiber and fascicle may change with animal size and tendon function. Thus, the objective of the study was to select a single species that is commonly used for tendon research (rat) and tendons with varying mechanical functions (tail, plantaris, Achilles) to evaluate the hierarchical structure at multiple length scales using histology,SEM, and confocal imaging. With the exception of the specialized rat tail tendon, we confirmed that in rat tendons there are no fascicles and the fiber is the largest subunit. In addition, we provided a structurally based definition of a fiber as a bundle of collagen fibrils that is surrounded by elongated cells, and this definition was supported by both histologically processed and unprocessed samples. In all rat tendons studied, the fiber diameters were consistently between 10 and 50 μm, and this diameter range appears to be conserved across larger species. Specific recommendations were made highlighting the strengths and limitations of each rat tendon as a research model. Understanding the hierarchical structure of tendon can advance the design and interpretation of experiments and development of tissue‐engineered constructs.

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