CHONDRONS FROM ARTICULAR-CARTILAGE .5. IMMUNOHISTOCHEMICAL EVALUATION OF TYPE-VI COLLAGEN ORGANIZATION IN ISOLATED CHONDRONS BY LIGHT, CONFOCAL AND ELECTRON-MICROSCOPY

The pericellular microenvironment around articular cartilage chondrocytes must play a key role in regulating the interaction between the cell and its extracellular matrix. The potential contribution of type VI collagen to this interaction was investigated in this study using isolated canine tibial chondrons embedded in agarose monolayers. The immunohistochemical distribution of an anti-type VI collagen antibody was assessed in these preparations using fluorescence, peroxidase and gold particle probes in combination with light, confocal and transmission electron microscopy. Light and confocal microscopy both showed type VI collagen concentrated in the pericellular capsule and matrix around the chondrocyte with reduced staining in the tail region and the interconnecting segments between adjacent chondrons. Minimal staining was recorded in the territorial and interterritorial matrices. At higher resolution, type VI collagen appeared both as microfibrils and as amorphous deposits that accumulated at the junction of intersecting capsular fibres and microfibrils. Electron microscopy also showed type VI collagen anchored to the chondrocyte membrane at the articular pole of the pericellular capsule and tethered to the radial collagen network through the tail at the basal pole of the capsule. We suggest that type VI collagen plays a dual role in the maintenance of chondron integrity. First, it could bind to the radial collagen network and stabilise the collagens, proteoglycans and glycoproteins of the pericellular microenvironment. Secondly, specific cell surface receptors exist, which could mediate the interaction between the chondrocyte and type VI collagen, providing firm anchorage and signalling potentials between the pericellular matrix and the cell nucleus. In this way type VI collagen could provide a close functional interrelationship between the chondrocyte, its pericellular microenvironment and the load bearing extracellular matrix of adult articular cartilage.