CHARACTERIZATION OF AGGREGATING PROTEOGLYCANS FROM THE PROLIFERATIVE, MATURING, HYPERTROPHIC, AND CALCIFYING ZONES OF THE CARTILAGINOUS PHYSIS

The introduction of a new microanalytical approach has made it possible to investigate the molecular structure and content of proteoglycans within the extracellular matrix of the physis when the volumes of the matrix and the cells change before mineralization of the extracellular matrix. By using extracts of serial frozen sections with a combination of gel chromatography and chemical, radioimmunoassay, and morphometric analyses, we demonstrated that, in the bovine fetal physis, the content of large aggregating proteoglycans reaches a maximum in the extracellular matrix of the hypertrophic zone, in the region where calcification begins, at the time when calcification starts. Morphometric analyses revealed that, at this time, the volume of matrix is reduced to a minimum. There was no evidence that the remaining proteoglycans were reduced in size, nor was aggregation changed at the time when calcification was initiated. Thus, contrary to earlier reports, there is no indication that these molecules need to be degraded and removed for calcification to begin, although a progressive loss of proteoglycans clearly occurs as the volume of the matrix is reduced. CLINICAL RELEVANCE: The observations that are described in this report aid in the improved understanding of the role of proteoglycans in the calcification of the extracellular matrix in the physis. As we and others have suggested, in view of their capacity to bind calcium and their reported concentration in focal sites of calcification, proteoglycans may act as sites for the nucleation of calcification. A molecular defect may occur in some chondrodysplasias and could result in pathological changes in skeletal growth and the repair of fractures, both of which involve endochondral ossification. Abnormalities of proteoglycans may therefore lead to abnormalities of calcification of cartilage, causing impaired skeletal growth and repair.