Comparative multi‐scale hierarchical structure of the tail, plantaris, and Achilles tendons in the rat

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.

Multi‐Scale Loading and Damage Mechanisms of Plantaris and Rat Tail Tendons

“…Tendinopathy, degeneration of the tendon that leads to pain and dysfunction, is common in both sports and occupational settings, but the mechanisms for tendinopathy are still unknown. The mechanisms of tendinopathy have been attributed to overloading that leads to inferior mechanical and microstructural degradation, and these occur across multiple hierarchical scales . Rodent tendon models of overloading have frequently been used to study tendinopathy; however, the multi‐scale mechanisms of normal load transfer and damage are not well understood.…”

“…Using the rat tail tendon, we recently showed that micro‐scale sliding is a mechanism for load transfer, and non‐recoverable micro‐scale sliding in response to high tensile loading leads to tendon damage . Yet, the rat tail tendon is specialized and is a relatively non‐load‐bearing tendon, and thus the load transfer and damage mechanisms are still unknown for load‐bearing rat tendons. Despite some differences between the rat tail tendon and load‐bearing tendons, we hypothesize that micro‐scale sliding is a mechanism for both tendons since they both have similar fiber structures .…”

“…Yet, the rat tail tendon is specialized and is a relatively non‐load‐bearing tendon, and thus the load transfer and damage mechanisms are still unknown for load‐bearing rat tendons. Despite some differences between the rat tail tendon and load‐bearing tendons, we hypothesize that micro‐scale sliding is a mechanism for both tendons since they both have similar fiber structures . We chose rat plantaris and Achilles tendons to study these mechanisms.…”

“…We chose rat plantaris and Achilles tendons to study these mechanisms. The plantaris tendon bears high‐stress in vivo and has a relatively simple structure . The Achilles tendon also bears high‐stress and is a highly specialized tendon with a complex structure, where three sub‐tendons fuse together to form a single tightly bound tendon …”

“…The objective of this study was to investigate the multi‐scale load transfer and damage mechanisms of load‐bearing tendon rat plantaris and to evaluate if the mechanisms are similar to the mechanisms demonstrated in the rat tail tendon. Recently published data from the tail tendon, which has the simplest structure, was reanalyzed and included for comparison . We hypothesize that the micro‐scale sliding will increase with applied strain and will be non‐recoverable when loaded to high strain in the plantaris tendon.…”