Distribution and Expression of Cartilage Oligomeric Matrix Protein and Bone Sialoprotein Show Marked Changes During Rat Femoral Head Development

Distribution and sites of synthesis of a cartilage extracellular matrix protein, cartilage oligomeric matrix protein (COMP), and of a bone extracellular matrix protein, bone sialoprotein (BSP), were studied in the femoral head of growing Wistar rats from day 14 to day 60 by immunocytochemistry and in situ hybridization. This period includes formation of the secondary ossification center and differentiation of articular cartilage. At early stages, immunoreactivity for COMP was pronounced throughout the cartilage. The localization of COMP was predominantly territorial in the center of the immature femoral head and in the growth plate at all ages studied. In the superficial parts, a shift from a uniform extracellular matrix staining at day 14 to an interterritorial localization at day 33 to day 60 was seen, apparently concurrent with formation of articular cartilage. COMP staining, representing cartilage remnants, also extended into the center of the trabecular bone in the primary spongiosa. In the secondary ossification center, the staining for COMP decreased at the onset of calcification. The protein was only synthesized by chondrocytes, as shown by in situ hybridization. The highest level of COMP mRNA was detected in chondrocytes in the central region of the growth plate. In the layer corresponding to the articular cartilage of the femoral head, mRNA levels for COMP were low from day 14 to day 33 but were increased on day 60. This shows substantial synthesis in the developing articular cartilage. Immunoreactivity for BSP was detected in bone trabeculae of primary spongiosa. In situ hybridization showed the highest levels of BSP mRNA in regions of newly formed bone. BSP mRNA was detected in hypertrophic chondrocytes in the secondary ossification center as early as day 18, well before the appearance of immunochemically detectable BSP. Interestingly, simultaneous expression of COMP and BSP mRNA was seen after day 18 in hypertrophic chondrocytes of the growth plate and later also in hypertrophic chondrocytes close to the mineralization zone of the articular cartilage.