ULTRASTRUCTURAL IMMUNOLOCALIZATION OF BONE SIALOPROTEIN IN GUINEA-PIGOSTEOARTHRITIS

In diarthrodial joints, bone and cartilage are structurally and functi onally inseparable as shown in osteoarthritis (OA), where subchondral bone changes are integral in the disease process. By ultrastructural i mmunohistochemistry using polyclonal antibodies against guinea-pig bon e sialoprotein (BSP), we investigated the distribution of this matrix protein at the osteocartilaginous interface in Hartley guinea-pig knee s at different stages of primary osteoarthritis. Between 6 and 12 mont hs they developed moderate osteoarthritic changes predominantly in the medial condyle, progressing to severe OA at 30 months. In all age gro ups BSP labeling was concentrated to the osteocartilaginous interface at a 1 mu m narrow zone at the interface. In the medial osteoarthritic condyle, BSP was increased as compared with the lateral nonosteoarthr itic condyle, but only at 30 months, when cartilage fibrillation corre lated to BSP. Our observations suggest that altered BSP abundance may be a potential bone marker for late stage OA, while early events in bo ne cannot be monitored. BSP is expressed early in osteogenesis and may have a role in biological mineralization and growth. Since a sharp zo ne of intense BSP labeling remains at a remarkably constant level thro ughout life in guinea-pigs, BSP may have an important structural and/o r regulating role at the interface. The protein may act as an anchor o f calcified articular cartilage to subchondral bone or by regulating m ineralization at the osteocartilaginous interface.