Age-related changes to the molecular and cellular components of equine flexor tendons

Specific tendons show a high incidence of partial central core rupture whic h is preceded by degeneration. In the performance horse, the superficial di gital flexor tendon (SDFT) is most often affected. We have described previo usly the molecular changes that are associated with degeneration in the cen tral core region of the equine SDFT. The pathophysiological mechanism leadi ng to change in synthetic activity of central zone cells in degenerated ten dons is not known. In this study, we test the hypothesis that ageing result s in matrix composition changes within the central zone of the SDFT. Extrac ellular matrix composition and cellularity were analysed in equine SDFTs co llected from Thoroughbred horses and compared with a flexor tendon which ra rely shows degenerative change and subsequent injury (deep digital flexor t endon, DDFT). Data were examined for age-related changes to central and per ipheral zone tissue of the SDFT and DDFT. Ageing in both tendons (SDFT and DDFT) resulted in a significant increase in collagen-linked fluorescence an d a decrease in cellularity in the DDFT but not the SDFT. The central zone tissue from the SDFT had a significantly higher proportion of type III coll agen than the peripheral zone of the tendon. The highest level of type III collagen was found in the central zone tissue of the SDFT from the older gr oup of horses and this may represent the early stages of a degenerative cha nge. Collagen content did not differ between the 2 flexor tendons; however, there were differences in collagen type and organisation. The SDFT had a h igher type III collagen content, higher levels of the mature trifunctional collagen crosslink hydroxylysylpyridinoline, lower total chondroitin sulpha te equivalent glycosaminoglycan content, smaller diameter collagen fibrils and a higher cellularity than the DDFT. In conclusion, differences in macro molecular composition exist between the flexor tendons and ageing contribut es to a tendon specific change in composition.