Abstract
Introduction
Energy storing tendons such as the equine superficial digital flexor tendon (SDFT) stretch and recoil with each stride and therefore require a high degree of compliance compared to tendons with a purely positional function, such as the equine common digital extensor tendon (CDET). This extra extensibility is provided by a specialised interfascicular matrix (IFM), which provides greater sliding and recoil between adjacent fascicles in energy storing tendons. However, the composition of the IFM remains largely undefined. We hypothesised that the IFM in the SDFT has a distinct composition, with a greater abundance of proteoglycans and elastin which facilitate extension and recoil.
Materials and Methods
Transverse and longitudinal sections were cut from the mid-metacarpal regions of SDFTs and CDETs from 5 horses aged 3–7 years. Sections were stained using Alcian blue/Periodic acid Schiff to detect proteoglycans, elastic Van Giesson's to detect elastin, and immunohistochemistry was performed using antibodies for decorin, biglycan, fibromodulin, lumican and lubricin. Resultant images were graded by blinded observers to assess staining intensity in the IFM and fascicular matrix (FM), and statistical significance determined using ANOVA.
Results
Overall proteoglycan abundance was significantly greater in the SDFT than the CDET (p<0.0001). In the SDFT, overall proteoglycan staining was greater in the FM than the IFM (p<0.0001). Elastin content was greater in the SDFT, where it was predominantly localised to the IFM. Fibromodulin staining was significantly greater in the CDET than the SDFT (p<0.05), whereas decorin staining was greater in the SDFT (p<0.05). In the SDFT, lubricin and biglycan staining were significantly greater in the IFM than the FM (p<0.05). Lumican staining was significantly greater in the CDET IFM than in the SDFT IFM (p<0.01).
Discussion
As hypothesised, the IFM has a specialised composition in the SDFT, with a greater abundance of elastin, lubricin and biglycan. It is likely that the greater abundance of lubricin facilitates sliding between fascicles, whereas the localisation of elastin to the IFM is likely to provide the superior ability to recoil. The differential abundance of decorin, fibromodulin and lumican across tendon regions and types may reflect the different roles of these proteoglycans in tendon.
