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Introduction
  • Bone is made up of
    • organic component
      • 40% of dry weight
    • inorganic component
      • 60% of dry weight
Organic component
  • Components include
    • collagen
      • 90% of organic component
      • primarily type I collagen
      • provides tensile strength
      • it is a triple helix composed of one alpha-2 and two alpha-1 chains
    • proteoglycans
      • responsible for compressive strength
      • inhibit mineralization
      • composed of glycosaminoglycan-protein complexes
    • matrix proteins
      • includes noncollagenous proteins
      • function to promote mineralization and bone formation
        • three main types of proteins involved in bone matrix
          • osteocalcin
            • most abundant non-collagenous protein in the matrix (10%-20% of total) 
              • produced by mature osteoblasts 
            • function
              • promotes mineralization and formation of bone
                • directly involved in regulation of bone density
                • attracts osteoclasts
            • signaling
              • stimulated by 1,25 dihydroxyvitamin D3
              • inhibited by PTH
            • clinical application
              • marker of bone turnover
                • can be measured in urine or serum
          • osteonectin
            • secreted by platelets and osteoblasts
            • function
              • believed to have a role in regulating calcium or organizing mineral in matrix
          • osteopontin
            • function
              • cell-binding protein
    • cytokine and growth factors
      • small amounts present in matrix
      • aid in bone cell differentiation, activation, growth, and turnover
      • include
        • IL-1, IL-6, IGF, TGF-beta, BMPs
Inorganic component
  • Components include
    • calcium hydroxyapatite (Ca10(PO4)6(OH)2  
      • provides compressive strength
    • osteocalcium phosphate (brushite)
 

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