Alterations in articular cartilage of the rabbit mandibular condyle following surgical induction of anterior disc displacement: light and electron microscopic immunocytochemistry using colloidal gold conjugate

Abstract

The purp?se ; ~f this study ~as to test the hypothesis that surgical ' . induction of anterior .disc displacement (ADD) in the rabbit craniomandibular . , .. '' .. · jo~nts (CMJ) will lead to degenerative osteoarthritic changes detect,~ble at the molecular, subcellular and cellular levels in the articular cartilage of the rabbit mandibular condyle. Ultrastructural features of the normal rabbit mandibular condyle were com pared to those of experimental condyles at two weeks· following i~duction of ADD. The quantities of type-VI and -IX collagens, as well· as the components of proteoglycans, such as .chondroitin-4~sulfate (C4S), chondroitin-6-sulfate (C6S), keratan s-u~fate (KS) and link protein (LP) were measured using immunogold labeling technique at the light and the electron microscopic levels. The right joint of each of 20 rabbits was exposed surgica~ly, and all discal attachments were severed except for the posterior attachment. The disc was then displaced anteriorly and sutured to the zygomatic ·arch. · The left joint served as a sham-operated control.. Ten additional joints were used as non-. operated controls. Deeply anesthetized rabbits were perfused with 2% buffe~ed formalin two weeks after surgery. The mandibular condyles were excised and decalcified in ethylenediaminetetraac~tic acid (EDTA). Paraffin embedded tissues were sectioned at 5 J.Lm for light microscopic study, while water-soluble plastic .embedded sections were used for electron microscopy. Sections were incubated in monoclonal antibodies directed against C4S, C6S, KS and LP, and in polyclonalantibodies.against type-VI and -IX collagen~. Mter incubation in the appropriate colloidal gold conjugated secondary antibodies, tissue sections were studied with light and electron microscopes. In addition; immunostaining for proliferating cell nuclear antigen (PCNA) was ~erform~d using paraffin sections, and the PCNA, indices of. control and experimentfll condyles were determined. Pathological alterations were obvious in the experimentB;l condyles, and I appeared to be characteristic oste~arthritic changes. These i~clude .cartilage . . . I neovascularization, chondrocyte clustering, vacuolation, loss of extracellular matrix next to the membranes of chondrocytes, and an increase in number of apoptotic chondrocytes. Increased numbers of PCNA-positive cells in the osteoarthritic cartilage of the experimental group .indicated an active chondrocytic proliferation. Ultrastructural changes in injured chondrocytes included increased amounts of RER and Golgi, suggesting an increase in the synthesis and secretion of possibly degradative enzymes with a decrease in the normal secretory products. The results of the immunocytochemistry using c~lloidal gold conju~ates both at the light and electron microscopic levels showed statistically significant depletion of C4S, C6S, KS, LP, type-VI collagen and type-IX collagen in the osteoarthritic cartilage (P < 0.05). The _reduction of binding molecules such as LP, type-VI and type-IX collagens suggest a possible mechanism for the observed loss of integrity of the extracellular matrix. It is concluded that surgical induction· of ADD in the rabbit CMJ leads to molecular, cellular and extracellular alterations in the articular cartilage of the mandibular condyle similar to those described previously in human ADD and in osteoarthritis of other synovial joints. The results of this study provide evidence that the loss of the. shock absorber function of the disc, and the exposure of the condyles to overloading inay cause the _injured chondrocytes to secrete degenerative cytokines as indicated by the loss of proteoglycans, binding collagens and LP. These molecular changes are expressed at the sub.cellular and cellular levels as osteoarthritis or degenerative: joint disease.