OBJECTIVE--To locate fibronectin ultrastructurally in the most superficial layer of normal articular cartilage of rabbits, in order to clarify its role in joint physiology. METHODS--Articular cartilage was obtained from the femoral condyle of seven normal adult rabbits and prepared by a method that included tannic acid fixation. Polyclonal antibodies against rabbit fibronectin were used in an immunohistochemical electron microscopic study, without any enzymic digestion but with a pre-embedding method for the transmission electron microscopy. RESULTS--The cartilage surface was successfully preserved by tannic acid fixation. The most superficial layer in electron photomicrographs was approximately 200-300 nm thick, cell free, and appeared to have two parallel components: the more superficial lamina and the deeper lamina. Gold labelled fibronectin lined this layer in immunohistochemical electron photomicrographs. CONCLUSIONS--Fibronectin covering the surface of the articular cartilage may have a role in joint lubrication and protection of the cartilage by binding with the collagenous matrix and hyaluronic acid in synovial fluid. Chondroitin sulphates may act as a charge barrier in close relationship with the collagen fibrils in the deeper lamina. Significant alteration in these functions may be one of the first causal steps leading to destruction of the articular cartilage.
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