skeletal system

• functions of bone

  • support
  • protection e.g. skull encloses the brain
  • movement e.g. muscle uses bone as levers
  • storage of minerals e.g. calcium and phosphate
  • blood cell production in the bone marrow
• bone components

  • 30% organic matrix - flexibility, tensile strength
  • collagen fibers
  • ground substance: proteoglycans, glycoproteins
  • bone cells
  • 70% inorganic salts - hardness
  • hydroxyapatites, calcium phosphates: lie in and around collagen fibers
• composition of bone

  • extracellular matter
  • organic matrix (osteoid tissue) - collagen fibrils bound by amorphous mucopolysaccharide substance
  • inorganic matrix (bone mineral) - calcium phosphate (hydroxyapatite)
  • cellular component
  • osteoblasts
  • osteoclasts
  • osteocytes
• bone formation

  the process by which bone forms is called ossification, occurs in 4 principle situations:

  1. initial formations of bones in an embryo and fetus, between 6th and 7th week of embryonic life
  2. the growth of bones during infancy, childhood and adolescence until their adult sizes are reached
  3. the remodeling of bone (replacement of old bones by new bone tissues throughout life) 
  4. the repair of fractures (breaks in the bone) throughout life
• intramembranous ossification


  this process forms bones directly from fibrous membranes, such as the flat bones of the skull, the mandible, and the clavicles. It begins when mesenchymal cells differentiate into osteoblasts, which can secrete an extracellular matrix that hardens into bone
• endochondral ossification


  this process forms bones by replacing hyaline cartilage and is responsible for most of the skeleton’s bones. It occurs in three main areas: the physis, the epiphysis, and the cuboidal bones of the carpus and tarsus
• bones role in calcium homeostasis

  • bone is the major reservoir of calcium in the body
  • parathyroid hormone (PTH) secreted by the parathyroid  glands increases blood calcium level. Calcitonin (CT) from the thyroid gland has the potential to decrease blood calcium level
  • vitamin D enhances absorption of calcium and phosphate and thus raises the blood levels of these substances
• exercise and bone tissue

  • mechanical stress increases bone strength by increasing deposition of mineral salts and production of collagen fibers
  • removal of mechanical stress weakens bone through demineralization and collagen fiber reduction
• ageing and bone tissue

  • the principal effect of ageing is demineralization, a loss of calcium from bones, which is due to reduced osteoblast activity
  • another effect of decreased production of extracellular matrix proteins (mostly collagen fibers), which makes bones more brittle and this more susceptible to fracture
• bone diseases - osteoporosis

  • osteoclast activity increases
  • loss of bone mass
  • compact bone thins
  • spongy bone - bigger gaps
  • risk of fractures
  • common in elderly women due to lack of oestrogen
  • other risk factors: smoking, low BMI, diet, corticosteroids