Histological examination of tendon tissue from patients with knee joint osteoarthritis (OA) and controls. (a) Haematoxylin and eosin staining of tendons from OA patients revealed characteristic changes of tendon degeneration such as disruption of the tissue structure with fibrillations, inhomogeneous fiber composition and loss of cellularity. (b) In contrast, tendon tissue from controls showed the typical structure of normal tendon tissue. (c)–(f) Immunohistochemistry revealed expression of Fas in all samples. There was a scattered staining for Fas throughout the OA (c) and non-OA (d) tissues. Staining was slightly more intense in the OA samples but there was a clear, characteristic dominance of extracellular collagen matrix over cellular structures as well as some tissue disruption following pretreatment. Pancreatic tissue (e) was used as positive control. There was no staining in the negative controls where the primary antibody was replaced by an isotype matched immunoglobulin (f).