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Slides - Bone Remodeling and Ossification

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					                                                  Ossification
Bone Remodeling and           • Bone formation
                              • Begins 6th – 7th week of embryonic development
    Ossification                from mesenchyme.
                                 – Mesenchyme is embryonic CT derived from the
                                   mesoderm
Building and sculpting bone
                              • Intramembranous ossification
                                 – Bone forms directly or within loose fibrous connective
                                   membranes formed by mesenchyme (no cartilage stage)
                                    • E.g. flat bones of cranium




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           Ossification, cont’d                         Epiphyseal line
• Endochondral ossification                                     • Remnant of epiphyseal
   – Replacement (or intracartilaginous ossification)             plate in long bone
     of cartilage by bone.
   – Best observed in a long bone
   – e.g. tibia (shin bone), femur (thigh bone)




              Epiphyseal plate
• The hyaline cartilage
  plate between the
  epiphysis and
  diaphysis that is
  responsible for the
  lengthwise growth of
  long bones
   – Sometimes called
     epiphyseal disks




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              Bone remodeling                                         Blood calcium level
•    Bones are remodeled continuously in                    • Low blood Ca2+ levels
     response to change in two factors                        – Parathyroid secretes PTH (parathyroid
    1. Blood calcium level                                      hormone), which activates osteoclasts to break
    2. Pull of gravity and muscles on skeleton                  down bone matrix releasing Ca2+ into blood.
                                                            • High blood Ca2+ levels
                                                              – Ca2+ is deposited in bone matrix as hard
                                                                calcium salts.




                                                            • Ca3(PO4)2(OH)2
       Gravity and muscle force                               – Hydroxyapatite (tri-calcium phosphate)
                                                                 • predominant crystallized mineral in bone.
• As long as we remain active (and have a                   • CaCO3
  good diet), bones tend to stay healthy and                  – Calcium carbonate
  strong.                                                        • Another common mineral in bone
• Bones become thicker where bulky muscles
  are attached and in areas of greatest stress
    – Due to osteoblasts laying down new matrix and
      getting trapped.
       • Once trapped they become osteocytes, mature bone
         cells




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posted:8/12/2012
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