Comparative Multi-scale Hierarchical Structure of the Tail, Plantaris, and Achilles Tendons in the Rat

Abstract

Rodent tendons are widely used to study human pathology, such as tendinopathy and repair, and to address fundamental physiological questions about development, growth, and remodeling. However, how the gross morphology and the multi-scale hierarchical structure of rat tendons, such as the tail, plantaris, and Achillles tendons, compare to that of human tendons are unknown. In addition, there remains disagreement about terminology and definitions. Specifically, the definition of fascicle and fiber are often dependent on the diameter size and not their characteristic features, which impairs the ability to compare across species where the size of the fiber and fascicle might change with animal size and tendon function. Thus, the objective of the study was to select a single species that is widely used for tendon research (rat) and tendons with varying mechanical functions (tail, plantaris, Achilles) to evaluate the hierarchical structure at multiple length scales. This study was designed including, histology, SEM, and confocal imaging. We confirmed that rat tendons do not contain fascicles, and thus the fiber is the largest tendon subunit in the rat. 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 tendons. In all rat tendons studied, the fiber diameters were consistently 10-50 {micro}m, and this diameter appears to be conserved across larger species. Specific recommendations were made for the strengths and limitations of each rat tendon as tendon research models. Understanding the hierarchical structure of tendon can advance the design and interpretation of experiments and development of tissue engineered constructs.