Expression of a wide range of extracellular matrix molecules in the tendon and trochlea of the human superior oblique muscle.

PURPOSE: To show that the molecular composition of the extracellular matrix of the trochlea and its associated tendon may explain the link between some cases of acquired Brown syndrome and rheumatoid arthritis.
METHODS: One trochlea and its tendon from 11 dissecting-room cadavers were fixed in methanol, cryosectioned, and immunolabeled with a panel of monoclonal antibodies against types I, II, III, V, and VI collagens, chondroitin-4 and -6, keratan and dermatan sulfates, aggrecan, link protein, versican, and tenascin. Labeling was detected with an avidin-biotin-peroxidase detection kit.
RESULTS: The trochlea had a central core of hyaline cartilage surrounded by a band of fibrocartilage, but the tendon had no cartilage cells. Significantly, however, both structures immunolabeled for aggrecan, link protein, and type II collagen-molecules typical of articular cartilage.
CONCLUSIONS: The presence of aggrecan, link protein, and type II collagen may account for the coincidence between transient attacks of acquired Brown syndrome in patients with juvenile and adult forms of chronic rheumatoid arthritis. Cleavage of aggrecan by aggrecanase in articular cartilage characterizes cartilage degeneration in rheumatoid arthritis. Thus, it is possible that aggrecan cleavage also occurs in the trochlea and tendon and contributes to their degeneration or to a local inflammatory reaction that may swell and thicken the tendon. In this context, it is also significant that link protein and type II collagen are now regarded as relevant antigenic targets for autoimmune responses in rheumatoid arthritis.