Objective. To investigate the appearance of hypertrophic chondrocytes in osteoarthritic (OA) cartilage, using type X collagen as a specific marker.

Methods. The biosynthesis of type X collagen was examined by metabolic labeling of freshly isolated articular chondrocytes with 3H-proline, immunoprecipitation, and sodium dodecyl sulfate–polyacrylamide gel electrophoresis of the synthesized collagens. Extracellular deposition of types X and II collagen was analyzed immunohistochemically.

Results. Immunostaining revealed an irregular distribution of type X collagen, which was localized around chondrocyte clusters in fibrillated OA cartilage, but was absent from the noncalcified region of normal articular cartilage. Freshly isolated OA chondrocytes synthesized predominantly type X collagen, while control chondrocytes synthesized mostly type II collagen.

Conclusion. Our findings indicate focal premature chondrocyte differentiation to hypertrophic cells in OA cartilage.