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

Authors

  • Fackson Mwale,

    1. Joint Diseases Laboratory, Shriners Hospitals for Children and Division of Surgical Research, Department of Surgery and Department of Medicine, McGill University, Montreal, Quebec, Canada
    Current affiliation:
    1. Orthopedics Research Laboratory, Jewish General Hospital, Lady Davis Institute for Medical Research, Division of Orthopedic Surgery, McGill University, Montreal, Quebec, Canada
    Search for more papers by this author
  • Elena Tchetina,

    1. Joint Diseases Laboratory, Shriners Hospitals for Children and Division of Surgical Research, Department of Surgery and Department of Medicine, McGill University, Montreal, Quebec, Canada
    Search for more papers by this author
  • C. William Wu,

    1. Joint Diseases Laboratory, Shriners Hospitals for Children and Division of Surgical Research, Department of Surgery and Department of Medicine, McGill University, Montreal, Quebec, Canada
    Current affiliation:
    1. Arthritis Unit, Department of Medicine, Massachusetts General Hospital, Boston, Massachusetts, USA
    Search for more papers by this author
  • A. Robin Poole Ph.D., D.Sc.

    Corresponding author
    1. Joint Diseases Laboratory, Shriners Hospitals for Children and Division of Surgical Research, Department of Surgery and Department of Medicine, McGill University, Montreal, Quebec, Canada
    • Joint Diseases Laboratory Shriners Hospitals for Children and Division of Surgical Research Department of Surgery and Department of Medicine McGill University 1529 Cedar Avenue Montreal, Quebec H3G1A6, Canada
    Search for more papers by this author

  • The authors have no conflict of interest

Abstract

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 Ca2+ 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 Ca2+/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 Ca2+/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 Ca2+ 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.

Ancillary