Changes in osteonectin distribution and levels are associated with mineralization of the chicken tibial growth cartilage.

Osteonectin is a calcium-binding matrix protein thought to play a role in regulating calcium distribution in a variety of biologic processes. To examine its role in endochondral bone formation, we examined the distribution of the protein during mineralization of the chicken tibial growth cartilage, using immunohistochemistry and immunoelectron microscopy. The synthesis of osteonectin was also determined in chondrocyte populations isolated from premineralizing and mineralizing regions of growth cartilage and assayed in short-term culture. The results show that a very low level of osteonectin is detectable in the resting, proliferating, and early hypertrophic zones of growth cartilage; in these zones, osteonectin is largely cell-associated. In contrast, a large amount of osteonectin is present in the mineralizing zone where it is associated with the matrix. Biosynthetic data from short-term culture experiments indicate, however, that osteonectin is synthesized and secreted by chondrocytes from both premineralizing and mineralizing zones. As indicated by immunoprecipitation, Northern hybridization, in vitro translation of hybrid-selected messenger RNA (mRNA), and electrophoretic analysis, osteonectin synthesized by chondrocytes of the premineralizing zones is not obviously different in structure from that synthesized by chondrocytes of the mineralizing zone. We conclude that osteonectin is a product of chondrocytes in each zone of growth cartilage but accumulates only in the mineralizing zone. The high affinity of the protein for calcium could favor its retention in calcifying matrix.