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The Assembly and Remodeling of the Extracellular Matrix in the Growth Plate in Relationship to Mineral Deposition and Cellular Hypertrophy: An In Situ Study of Collagens II and IX and Proteoglycan
Author(s) -
Mwale Fackson,
Tchetina Elena,
Wu C. William,
Poole A. Robin
Publication year - 2002
Publication title -
journal of bone and mineral research
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.882
H-Index - 241
eISSN - 1523-4681
pISSN - 0884-0431
DOI - 10.1359/jbmr.2002.17.2.275
Subject(s) - extracellular matrix , in situ , proteoglycan , extracellular , muscle hypertrophy , microbiology and biotechnology , chemistry , matrix (chemical analysis) , biology , endocrinology , organic chemistry , chromatography
The recent development of new specific immunoassays has provided an opportunity to study the assembly and resorption of type II and IX collagens of the extracellular matrix in relationship to endochondral calcification in situ. Here, we describe how in the bovine fetal physis prehypertrophic chondrocytes deposit an extensive extracellular matrix that, initially, is rich in both type II and type IX collagens and proteoglycan (PG; principally, aggrecan). The majority of the α1(IX)‐chains lack the NC4 domain consistent with our previous studies with cultured chondrocytes. During assembly, the molar ratio of type II/COL2 domain of the α1(IX)‐chain varied from 8:1 to 25:1. An increase in the content of Ca 2+ and inorganic phosphate (Pi) was initiated in the prehypertrophic zone when the NC4 domain was removed selectively from the α1(IX)‐chain. This was followed by the progressive loss of the α1(IX) COL2 domain and type II collagen. In the hypertrophic zone, the Ca 2+ /Pi molar ratio ranged from 1.56 to a maximum of 1.74, closely corresponding to that of mature hydroxyapatite (1.67). The prehypertrophic zone had an average ratio Ca 2+ /Pi ranging from 0.25 to 1, suggesting a phase transformation. At hypertrophy, when mineral content was maximal, type II collagen was reduced maximally in content coincident with a peak of cleavage of this molecule by collagenase when matrix metalloproteinase 13 (MMP‐13) expression was maximal. In contrast, PG (principally aggrecan) was retained when hydroxyapatite was formed consistent with the view that this PG does not inhibit and might promote calcification in vivo. Taken together with earlier studies, these findings show that matrix remodeling after assembly is linked closely to initial changes in Ca 2+ and Pi to subsequent cellular hypertrophy and mineralization. These changes involve a progressive and selective removal of types II and IX collagens with the retention of the PG aggrecan.