[Ultrastructure of hyaline cartilage and articular capsule tissue in alkaptonuric ochronosis].

The biochemical defect leading to alcaptonuria is well known. However, the pathogenesis of ochronotic arthropathy is poorly understood. The present ultrastructural study elucidated the localization of ochronotic pigment in the articular cartilage and evidence was found that suggests a pathway for cartilage destruction and further degradation. The amount of ochronotic pigment enlarged with increasing distance from chondrocytes. Pigment was neither seen in chondrocytes nor in the immediate vicinity of the chondrocytic lacunae. The interfibrillar localization as well as the preferential deposits in the deeper zone of the cartilage may indicate a binding of ochronotic pigment to proteoglycans of the cartilage. It is assumed that altered proteoglycans are responsible for the increased fragility of articular cartilage leading to cartilage fragmentation. Fragments of ochronotic cartilage are phagocytosed by connective tissue cells. Incomplete digestion of ochronotic pigment results in the formation of ochronotic residual bodies in these cells.