Histo.5

• Bone


  A specialized connective tissue with a rich vascular supply, composed of calcified extracellular material (bone matrix) and three major cell types: osteocytes, osteoblasts, osteoclasts
• Bone


  • Supports the body
  • Protects many internal organs
  • Acts as the body's Ca2+ reservoir
• Osteoblasts


  Originate from mesenchymal stem cells, produce the organic components of bone matrix including type I collagen, proteoglycans, glycoproteins
• Bone formation by osteoblasts

  1. Deposition of organic components

  2. Deposition of inorganic components (calcium salts)

  3. Osteoid formation

  4. Calcification of matrix
• Osteocytes


  Cells of mature bone, lie in lacunae, represent osteoblasts entrapped in matrix during bone formation, have dendritic processes in canaliculi
• Osteocytes


  • Maintain integrity of lacunae and canaliculi
  • Play a role in removal or deposition of matrix and calcium
• Osteoclasts


  Bone removing cells, found at sites of bone resorption, have a ruffled membrane, remove bone by demineralization and matrix removal
• Stimulants of bone removal by osteoclasts


  • Factors secreted by osteoblasts
  • Factors secreted by macrophages
  • Factors secreted by lymphocytes
  • Parathyroid hormone
• Components of bone matrix


  • Inorganic salts (65% dry weight)
  • Organic ground substance and collagen fibres (35%)
• Organic matrix


  Ground substance containing glycosaminoglycans, proteoglycans, water, glycoproteins osteonectin and osteocalcin, collagen fibres (type I)
• Inorganic ions in bone

  • Calcium
  • Phosphorus/Phosphate
  • Magnesium
  • Carbonate
  • Hydroxyl
  • Chloride
  • Fluoride
  • Citrate
  • Sodium
  • Potassium
• Periosteum


  Outer fibrous layer of dense connective tissue, contains collagen bundles (Sharpey's fibres) that penetrate bone, has inner layer with osteoprogenitor cells
• Endosteum


  Thin layer covering small trabeculae in marrow cavities, contains osteoprogenitor cells, osteoblasts, bone lining cells
• Woven bone


  Nonlamellar, random disposition of collagen fibres, higher proportion of osteocytes, lower mineral content, forms more quickly but has less strength than lamellar bone
• Lamellar bone


  Organized in multiple parallel layers (lamellae) of calcified matrix, contains osteons with concentric lamellae around a central canal
• Osteon


  • Complex of concentric lamellae around a central canal containing blood vessels, nerves, endosteum
  • Lacunae with osteocytes interconnected by canaliculi
  • Cement line at outer boundary
• Compact bone


  Contains osteons and also parallel external and internal circumferential lamellae
• Cancellous (spongy) bone


  Made up of a meshwork of bony plates or rods (trabeculae) enclosing spaces filled with bone marrow, receives nutrition from blood vessels in marrow