Bone Response to Damage

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Normal structure

Microscopic bone (Courtesy of RVC Histology images)

Periosteum and blood supply

• Periosteum
  • Specialised sheath of connective tissue covering bone except at the articular surfaces
  • Loosely attached except at tendon insertions and boney prominences (associated with major blood vessels penetrating bone)
  • Histologically:
    • Outer layer - fibrous for support
    • Inner layer - osteogenic
      • Contains osteoblasts and osteoprogenitor stem cells in young animals and in adults with fractures or disease
  • Rich supply of nerves and lymph vessels
  • Damage to periosteum:
    • Invokes a hyperplastic reaction of the inner layer
    • Is painful
    • Exostoses can remodel or remain
  • Lifting of periosteum:
    • Causes new bone formation below
  • Circumferential incision (e.g. during fracture)
    • Longitudinal bone growth results
    • May be only on one side where periosteum is damaged
      • Used by surgeons to treat angular limb deformities
• Blood vessels
  • Nutrient, metaphyseal, periosteal arteries
  • Normal flow of blood from medulla to periosteum due to higher pressures in medulla
  • Young animals have greater blood supply

• Endosteum lines the marrow cavity

Bone development

• Two main types of bone development:
  • Endochondral ossification (cartilage model)
    • Long bones mainly - physis and metaphysis
    • Mesenchymal cells differentiate into chondroblasts
      • Produce scaffold of mineralised cartilage on which osteoblasts deposit bone
    • Vascularised
    • Developed centres of ossification
      • Primary (diaphyseal)
      • Secondary (epiphyseal)
  • Intramembranous ossification
    • Flat bones mainly (e.g. skull), shaft of long bones
    • Mesenchymal cells differentiate into osteoblasts
    • No cartilage precursor template

Physis (Growth plate)

Growth plate (Image sourced from Bristol Biomed Image Archive with permission)

Growth plate magnified(Image sourced from Bristol Biomed Image Archive with permission)

• Originates from the cartilage model that remains only at the junction of the diaphyseal and epiphyseal centres

• Cartilage of metaphyseal growth plate is divided into: (from right to left on the magnified image)
  • - Resting (reserve) zone
  • - Proliferative zone
  • - Hypertrophic zone

• Site of many congenital or nutritional bone diseases in the growing animal
• Open in neonates and growing animals
  • Chondrocyte proliferation balances cell maturation and death
• Closes and ossifies at maturity
  • Regulated by androgens
• If growth teporarily stops -> layer of bone seals the growth plate -> moves into metaphysis when growth resumes -> forms Harris lines

Bone resorption

• Mediated by two hormones:
  • Parathyroid hormone (PTH)
    • Produced by chief cells in the parathyroid glands in response to decreased serum calcium
    • In response, osteoclasts increase in number and resorb mineralised matrix - increase Ca in blood
  • Calcitonin
    • Produced by C-cells in the thyroid glands in response to increased serum calcium
    • Inhibits osteoclasts

Bone dynamics

• Bone growth and maintenance of normal structure are directly related to mechanical forces
• Mechanical forces generate bioelectrical potentials (piezoelectricity)
  • These potentials strengthen bone
  • Inactivity reduces the potentials -> bone loss

• In neonates:
  • Bone growth predominates
  • Modelling is important
• In adults:
  • Formation of bone is balanced by resorption - remodelling
  • Continues throughout life under the influence of hormones and mechanical pressure
  • Bone resorption may exceed formation in pathological states (hormonal, trauma, nutritional) or in old age and disuse

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