Ultrastructure of canine articular cartilage: comparison of normal and degenerative (osteoarthritic) hip joints.

Normal canine hip cartilage was compared with cartilage from the degenerative lesions found in young dogs with canine hip dysplasia. The upper 0.5 mm of normal cartilage was characterized. Four distinct layers or zones were found: a layer of fine fibrous material covering the surface, a layer (surface layer) of small (32 nm diameter or less) collagen fibrils tightly packed in bundles and oriented parallel to the surface, a layer (upper layer) or less tightly packed collagen fibrils oriented mostly parallel to the surface with about 33% of the fibrils 64 nm or more, and a layer (intermediate layer) of randomly oriented fibrils with more than 50% of the fibrils 64 nm or larger. Fibril density was high in the surface layer and decreased with depth into the cartilage. In a moderately advanced lesion of degenerative cartilage, there was a layer of amorphous material over the surface. The tightly packed surface layer of small fibrils was absent. The surface itself was uneven and fissued. At depths from the surface comparable to the upper and the intermediate layers in normal cartilage, the proportion of large fibrils was less than in normal cartilage. The overall density of fibrils in degenerative cartilage increased with depth into the tissue. Cells flattened parallel to the surface, with relatively large nuclei, were found in the upper layer of normal cartilage. Cells in the intermediate layers were larger and round. The oblong cells of the upper layer of normal cartilage were not found in any layer of degenerative cartilage. Differences between cells in other layers of normal and degenerative cartilages were minimal. A model for the arrangement of chondrocytes and collagen fibrils for normal and degenerative cartilage was proposed. Ultrastructural changes in degenerative cartilage were prominent in the upper 0.5 mm of cartilage. These changes were changes in the number of collagen fibrils/mum-2 and a change from a characteristic pattern of collagen fibril diameters and orientation found in normal tissue.