Abstract
The transmission of mechanical muscle force to bone for musculoskeletal stability and movement is one of the most important functions of tendon. The load-bearing tendon core is composed of highly aligned collagen-rich fascicles interspersed with stromal cells (tenocytes). Despite being built to bear very high mechanical stresses, supra-physiological/repetitive mechanical overloading leads to tendon microdamage in fascicles, and potentially to tendon disease and rupture. To date, it is unclear to what extent intrinsic healing mechanisms of the tendon core compartment can repair microdamage. In the present study, we investigated the healing capacity of the tendon core compartment in an ex vivo tissue explant model. To do so, we isolated rat tail tendon fascicles, damaged them by applying a single stretch to various degrees of sub-rupture damage and longitudinally assessed downstream functional and structural changes over a period of several days. Functional damage was assessed by changes in the elastic modulus of the material stress-strain curves, and biological viability of the resident tenocytes. Structural damage was quantified using a fluorescent collagen hybridizing peptide (CHP) to label mechanically disrupted collagen structures. While we observed functional mechanical damage for strains above 2% of the initial fascicle length, structural collagen damage was only detectable for 6% strain and beyond. Minimally loaded/damaged fascicles (2–4% strain) progressively lost elastic modulus over the course of tissue culture, despite their collagen structures remaining intact with high degree of maintained cell viability. In contrast, more severely overloaded fascicles (6–8% strain) with damage at the molecular/collagen level showed no further loss of the elastic modulus but markedly decreased cell viability. Surprisingly, in these heavily damaged fascicles the elastic modulus partially recovered, an effect also seen in further experiments on devitalized fascicles, implying the possibility of a non-cellular but matrix-driven mechanism of molecular repair. Overall, our findings indicate that the tendon core has very little capacity for self-repair of microdamage. We conclude that stromal tenocytes likely do not play a major role in anabolic repair of tendon matrix microdamage, but rather mediate catabolic matrix breakdown and communication with extrinsic cells that are able to effect tissue repair.
Highlights
A third of all musculoskeletal clinical visits relates to tendon and ligament disorders, with tendinopathy being the most common of these complaints [1,2]
The load-bearing tendon core is composed of highly aligned collagen-rich fascicles interspersed with stromal cells
Despite being built to bear very high mechanical stresses, supra-physiological/repetitive mechanical overloading leads to tendon microdamage in fascicles, and potentially to tendon disease and rupture
Summary
A third of all musculoskeletal clinical visits relates to tendon and ligament disorders, with tendinopathy being the most common of these complaints [1,2]. Multiple fibers together with resident tenocytes form a fascicle, which is the fundamental functional unit of the tendon and often considered to represent the so-called tendon core [2]. In their role as the main load-bearing tendon component, fascicles are subjected to very high mechanical stress and accumulate mechanical damage in the ECM [6,7,8]. Only two tropocollagen molecules are linked in the fibrils of positional tendons With increasing strain, these immature, divalent crosslinks are relatively unable to prevent collagen molecular sliding and molecular extension [24,25,26,27]. We exploited tendon fascicles to characterize the ability of the intrinsic tendon stromal compartment for selfrepair after varying degrees of mechanically imposed matrix damage
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