Chondrocyte Phenotype and Ectopic Ossification in Collagenase-induced Tendon Degeneration

We report chondrocyte phenotype and ectopic ossification in a collagenase-induced patellar tendon injury model. Collagenase or saline was injected intratendinously in one limb. The patella tendon was harvested for assessment at different times. There was an increase in cellularity, vascularity, and loss of matrix organization with time after collagenase injection. The tendon did not heal histologically until week 32. Ectopic mineralization as indicated by von Kossa staining started from week 8. Tendon calcification was mediated by endochondral ossification, as shown by expression of type X collagen. viva CT imaging and polarization microscopy showed characteristic bony porous structures and collagen fiber arrangement, respectively, in the calcific regions. Marrow-like cells and blood vessels were observed inside calcific deposits. Chondrocyte-like cells as indicated by morphology, expression of type II collagen, and sox 9 were seen around and embedded inside the calcific deposits. Fibroblast-like cells expressed type II collagen and sox 9 at earlier times, suggesting that erroneous differentiation of healing tendon fibroblasts may account for failed healing and ossification in collagenase-induced tendon degeneration. Because this animal model replicates key histopathological changes in calcific tendinopathy, it can be used as a model for the study of its pathogenesis at the patellar tendon.