Distribution of proteins within different compartments of tendon varies according to tendon type.

Chavaunne T. Thorpe,Kabelan J. Karunaseelan,Jade Ng Chieng Hin,Peter D. Clegg,Helen L. Birch,Hazel R. C. Screen,Graham P. Riley

doi:10.1111/joa.12485

Abstract

Although the predominant function of all tendons is to transfer force from muscle to bone and position the limbs, some tendons additionally function as energy stores, reducing the energetic cost of locomotion. To maximise energy storage and return, energy‐storing tendons need to be more extensible and elastic than tendons with a purely positional function. These properties are conferred in part by a specialisation of a specific compartment of the tendon, the interfascicular matrix, which enables sliding and recoil between adjacent fascicles. However, the composition of the interfascicular matrix is poorly characterised and we therefore tested the hypothesis that the distribution of elastin and proteoglycans differs between energy‐storing and positional tendons, and that protein distribution varies between the fascicular matrix and the interfascicular matrix, with localisation of elastin and lubricin to the interfascicular matrix. Protein distribution in the energy‐storing equine superficial digital flexor tendon and positional common digital extensor tendon was assessed using histology and immunohistochemistry. The results support the hypothesis, demonstrating enrichment of lubricin in the interfascicular matrix in both tendon types, where it is likely to facilitate interfascicular sliding. Elastin was also localised to the interfascicular matrix, specifically in the energy‐storing superficial digital flexor tendon, which may account for the greater elasticity of the interfascicular matrix in this tendon. A differential distribution of proteoglycans was identified between tendon types and regions, which may indicate a distinct role for each of these proteins in tendon. These data provide important advances into fully characterising structure–function relationships within tendon.

Highlights

Energy-storing tendons such as the human Achilles and patellar tendons have an important role in reducing the energetic cost of locomotion by stretching and recoiling with each stride to store and return energy (Lichtwark & Wilson, 2005; Malliaras et al 2015)
Our previous studies have demonstrated that the matrix interspersing fascicles [the interfascicular matrix (IFM)] is more extensible and elastic in energy-storing tendons than in positional tendons, providing the capacity for sliding and recoil between fascicles and enabling the greater extensions required by energy-storing tendons (Thorpe et al 2012, 2015b)
The finding that elastic fibres are localised to the IFM in the superficial digital flexor tendon (SDFT), supports previous studies that have demonstrated that the IFM is rich in elastin, both in tendon (Grant et al 2013) and ligament (Smith et al 2011)

Summary

Introduction

Energy-storing tendons such as the human Achilles and patellar tendons have an important role in reducing the energetic cost of locomotion by stretching and recoiling with each stride to store and return energy (Lichtwark & Wilson, 2005; Malliaras et al 2015). To enable this function, they have distinct mechanical properties such as greater extensibility and elasticity leading to improved energy storage, when compared with tendons that are purely positional in function, such as the anterior tibialis tendon. The composition of the IFM, is poorly characterised, and very little is known about compositional specialisation and how this results in the distinct mechanical properties seen in the IFM of energy-storing tendons

Methods

Results

Discussion

Conclusion