Bones: Making and breaking

• Major functions of bones:

  • Support: provides framework and attachment points for muscles and tendons
  • Protection: contains valuable organs
  • Assistance in movement: muscles provide power to the skeleton
  • Mineral storage and release
  • Blood cell production
• Anatomy of a long bone:

  • Endochondral Ossification: process that forms long bones
• Bone within bone:

  • Cortical bone: compact
  • Trabeculae numbers become affected in osteoporosis
• Strength through organization:

  • Collagen acts as scaffolding
  • Trabecular bone is woven into the middle
  • Compact bone resembles a cone shell
• Hematopoiesis:

  • Takes place at the ends of long bones, pelvis, ribs, and skull
• Origins of bone cells:

  • Haemopoietic cells: continuously renewing stem cells, short-lived, simple structure
  • Mesenchymal cells: stem cells not continuously renewing, long-lived, complex structure
• MSC Differentiation:

  • Once committed to being a bone cell, they go through this process
  • Different types of stem cells involved
• Major bone cells origins:

  • Osteoblasts derived from MSCs
  • Osteocytes are the fate of osteoblasts when trapped within the matrix
  • Osteoclasts derived from HSCs
• Bone remodelling:

  • Formation and resorption of bone occurs normally or due to pathology
  • Factors involved: Parathyroid hormone, RANKL, TGF-ꞵ, M-CSF, OPG, Oestrogen
• Influencing pathways in bone remodelling:

  • The RANK/RANKL pathway
  • The OPG competition pathway
• Endochondral ossification:

  • Cartilage is replaced by bone
  • Examples: most bones of the body
  • Involves a 6-step process
• Intramembranous Ossification:

  • Simpler process than endochondral ossification
  • Examples: flat bones of the skull
  • Involves a 4-step process