CBH 23 bones

• Bone


  Specialized type of CT with a calcified extracellular matrix in which cells are embeded
• Functions of bones

  To support vital organs, and fleshy structures( hematopoietic organs)to provide storage for phosphate and calciumto regulate blood calcium levels via input from 2 separate non-bony tissues (stomach and parathyroid gland)
• bone matrix

  Inorganic part: calcium hydroxyapatite, bicarbonate, citrate, and magnesium organic part: type I collagen, multi adhesive glycoproteins(osteonectin), and calcium-binding proteins(osteocalcin)
• Bone cells

  Osteoclasts , osteoblast , osteocytes

  
• osteoblasts


  Growing cells that synthesize and secrete the organiccomponent of the matrix
• Osteocytes


  Bone cells in cavities (lacunae) between matrix layers(lamellae). They present cytoplasmic processes in small canaliculi that extend into the matrix
• osteoclasts


  Giant multinucleated cells involved in removing calcified bonematrix and remodelling bone tissue.
• periosteum


  The layer of non-calcified connective tissue covers bone on its external surfaces, except at synovial articulations and muscle attachments.‐ An outer dense fibrous collagenous layer and an inner cellular osteoprogenitor (osteogenic) layer.‐ Sharpey fibers (type I collagen) bind the periosteum to the bone surface To distribute blood vessels to bone.
• Endosteum


  Thin specialized connective tissue that lines the marrow cavities.‐ Contains osteoprogenitor cells and osteoblasts for bone growth and repair
• Types of bones anatomically

  Long , flat , sesamoid , short
• Types of bones macroscopically

  Compact/cortical boneCancellous/trabecular bone
• Types of bones microscopically 1

  ‐ Most bone in adults is organized as lamellar bone, characterized by multiple layers or lamellae of calcified matrix. They can be organized:‐ as parallel sheets concentrically around a central canal - Osteon (or Haversian system
• Types of bones microscopically 2

  ‐ Woven bone is nonlamellar and characterized by random disposition of type I collagen fibers. It is the first bone to appear in embryonic development and in fracture repair.It forms more quickly than a lamellar bone but it has less strength. It is usually temporary
• Compact osteonal bone

  Osteocytes in lacunae, communicate with each other through numerous processes located within fine canaliculi
• Osteoclasts


  Giant multinucleated cells are involved in removing calcified bone matrix and remodelling bone tissue.Derived by the fusion of several monocytes
• Endochondral ossification

  Bone is formed from a cartilage precursor, bones of extremities and parts of axial skeleton

  
• Intra membranous ossification


  Bone is formed from mesenchymal stem cells directly: flat bones of skull and face, mandible and clavicle
• Intra membranous ossification process

  Mesenchymal cells group into clusters, differentiate into osteoblasts, and ossification centres form. 2. Secreted osteoid traps osteoblasts, which then become osteocytes (small irregular areas of woven bone).3. Continued matrix secretion and calcification enlarges these areas and leads to fusion of neighbouring ossification centres forming trabecular bone.4. Compact bone develops superficial to the trabecular bone, and crowded blood vessels condense into red bone marrow
• Endochondral ossification process

  1. Mesenchymal cells differentiate into chondrocytes that produce a cartilage model of the future bony skeleton.2. A bone collar develops around the middle of the cartilage model causing chondrocyte hypertrophy in the underlying cartilage.3. Capillaries penetrate cartilage and deposit bone inside the cartilage model, forming the primary ossification centre.4. The secondary ossification centres begin to develop in the epiphyses.5. Bone replaces hyaline cartilage except at the epiphyseal growth plate (for continued bone elongation) and at articular cartilage.6. The epiphyseal plate disappears when full stature is achieved (becomes epiphyseal line)
• Zones of epiphyseal growth plate

  
• Resting zone or reserve cartilage


  Thin layer of non-dividing chondrocytes
• Proliferative zone


  Rapidly dividing chondrocytes that organize into distinct columns
• Zone of hypertrophy

  Chondrocytes cease dividing and grow in size. the cartilage matrix linear bands between the columns of hypertrophied cells
• Zone of calcification

  The cartilage matrix becomes calcified inhibiting the diffusion of nutrients. The dying chondrocytes are removed leaving longitudinal spicules of calcified cartilage (intensely basophilic)
• Zone of ossification

  Osteoprogenitor cells migrate into the cavities with the new blood vessels. New bone (eosinophilic) forms on the scaffold of calcified cartilage (basophilic). Osteoblasts ‐ condense on the spicules of calcified cartilage and produce new bone (osteoid). Osteoclasts ‐ large, multinucleated cells that remove bone tissue (both mineralized matrix and type I collagen).