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

While 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 enable maximum 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 (IFM), which enables sliding and recoil between adjacent fascicles. However, the composition of the IFM is poorly characterised, therefore we 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 (FM) and the IFM, with localisation of elastin and lubricin to the IFM. Protein distribution in the energy storing equine superficial digital flexor tendon (SDFT) and positional common digital extensor tendon (CDET) was assessed using histology and immunohistochemistry. The results support the hypothesis, demonstrating enrichment of lubricin in the IFM compared to the FM in both tendon types, where it is likely to facilitate interfascicular sliding. Elastin was also localised to the IFM, specifically in the energy storing SDFT, which may account for the greater elasticity of the SDFT IFM. A differential distribution of proteoglycans was also 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