The synovial surface of the articular cartilage.

The articular cartilage has been the subject of a huge amount of research carried out with a wide array of different techniques. Most of the existing morphological and ultrastructural data on the this tissue, however, were obtained either by light microscopy or by transmission electron microscopy. Both techniques rely on thin sections and neither allows a direct, face-on visualization of the free cartilage surface (synovial surface), which is the only portion subject to frictional as well as compressive forces. In the present research, high resolution visualization by scanning electron microscopy and by atomic force microscopy revealed that the collagen fibrils of the articular surface are exclusively represented by thin, uniform, parallel fibrils evocative of the heterotypic type IX-type II fibrils reported by other authors, immersed in an abundant matrix of glycoconjugates, in part regularly arranged in phase with the D-period of collagen. Electrophoresis of fluorophore-labeled saccharides confirmed that the superficial and the deeper layers are quite different in their glycoconjugate content as well, the deeper ones containing more sulfated, more acidic small proteoglycans bound to thicker, more heterogenous collagen fibrils. The differences found between the synovial surface and the deeper layers are consistent with the different mechanical stresses they must withstand.