Difference between revisions of "Bone Response to Damage"

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<big><center>[[Bones|'''BACK TO BONES''']]</center></big>
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==Changes to normal structure==
  
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===Damage to Periosteum===
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*Invokes a hyperplastic reaction of the inner layer
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*Is painful
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*Exostoses can remodel or remain
  
===Introduction===
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Lifting of periosteum causes new bone formation below
 
 
*Bone is a hard, highly specialised connective tissue
 
*Consists of interconnected cells embedded in a calcified, collagenous matrix
 
*Living, dynamic, responsive tissue, growing and remodelling throughout life
 
*Pathogenesis of many bone diseases is complex
 
**May involve genetic defects, diet or infection or a combination of these
 
*'''Function''':
 
**Support/protection
 
**Movement
 
**Stem cell storage
 
**Mineral storage
 
 
 
 
 
===Normal structure===
 
[[Image:Bone micro structure.jpg|right|thumb|100px|<small><center>Microscopic bone (Courtesy of RVC Histology images)</center></small>]]
 
 
 
*'''Cells'''
 
**Osteoblasts, osteocytes, chondroblasts and chondrocytes are derived from stromal fibroblastic system ('''osteoprogenitor cells); osteoclasts from haematopoietic system
 
**Ischaemia and hypoxia favour development of cartilage
 
**High oxygen tension and good blood supply favour bone development
 
**'''Osteoblasts'''
 
***Mesenchymal cells
 
***Arise from bone marrow stroma
 
***Histologically:
 
****Plump and cuboidal when active
 
****Basophilic cytoplasm
 
****When inactive - less cytoplasm -> flattened
 
***Produce bone matrix = '''osteoid''' - uncalcified
 
****Homogeneous substance
 
****Stains light pink with H&E
 
***Cell membranes are rich in alkaline phosphatase (ALP)
 
****Possibly involved in pumping calcium across membranes
 
***Promoted by growth factors
 
***Have receptors for [[Bones - normal#Bone resorption|PTH]]
 
****They contract in response -> space for osteoclasts to attach
 
**'''Osteocytes'''
 
***Osteoblasts that have become surrounded by mineralised bone matrix
 
***Occupy cavities called '''lacunae'''
 
***Contact osteoblasts and each other with cytoplasmic processes
 
****Reach through canaliculi in mineralised bone matrix
 
***Regulate composition of bone fluid
 
**'''Osteoclasts'''
 
***Histologically:
 
****Large, often multinucleated cells
 
****Acidophilic cytoplasm
 
***Derived from haematopoietic stem cells
 
***Responsible for bone resorption
 
****Firstly dissolve mineral followed by collagen
 
****Use brush border for this
 
***Sit in bone surface depression - '''Howship's lacuna'''
 
***Do <u>not</u> have receptors for PTH
 
***Have receptors for [[Bones - normal#Bone resorption|calcitonin]]
 
****Involute their brush border in response
 
****Detach from bone surface
 
***Respond to vitamin D by increasing their numbers and activity (parathyroid independent)
 
*'''Matrix''':
 
**Osteoid
 
***Type I collagen forms the backbone of the matrix (90%)
 
****Molecules in staggered rows
 
***Non-collagenous protein forming amorphous ground substance (10%)
 
**Mineral
 
***Crystalline lattice of calcium phosphate and calcium carbonate
 
***Also contains Mg, Mn, Zn, Cu, Na, F
 
***Accounts for 65% of bone
 
 
 
===Bone organisation===
 
 
 
*Patterns of collagen deposition:
 
**'''Woven bone''':
 
***"Random weave" which is only a normal feature in the foetus
 
***Coarse collagen fibres
 
***Later removed by osteoclasts and replaced by lamellar bone
 
***In adults it is a sign of a pathological condition (e.g. fracture, inflammation, neoplasia)
 
**'''Lamellar bone''':
 
***Orderly layers which are much stronger than woven bone
 
***Fine collagen fibres in concentric or parallel laminae
 
***Two main types:
 
****'''Compact bone (cortical)'''
 
*****Forms 80% of total bone mass
 
*****Consists of cells and interstitial substance - 30% ossein (type of collagen) and 70% minerals, especially calcium phosphate
 
*****Forms the shell of long bone shafts - contain [[Haversian systems]]
 
****'''Cancellous bone (spongy or trabecular)'''
 
*****Made up of plates, tubes or bars arranged in lines of stress
 
*****In vertebrae, flat bones and epiphyses of long bones
 
*****Contains no Haversian systems
 
**'''Laminar bone'''
 
***Formed on periosteal surface of diaphysis
 
***Accomodates rapid growth of large dogs and farm animals
 
***Plates of woven bone from within the periosteum
 
***Concentric plates
 
***As it forms, it fuses with the bone surface
 
 
 
 
 
===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 [[Bones - fractures|fracture]])
 
***Longitudinal bone growth results
 
***May be only on one side where periosteum is damaged
 
****Used by surgeons to treat [[Bones - developmental#Angular limb deformity|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
 
  
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Circumferential incision (e.g. during [[Bones Fractures - Pathology|fracture]])
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*Longitudinal bone growth results
 +
*May be only on one side where periosteum is damaged
 +
**Used by surgeons to treat [[Angular Limb Deformity|angular limb deformities]]
  
 
===Physis (Growth plate)===
 
===Physis (Growth plate)===
 
 
 
[[Image:Growth plate.jpg|left|thumb|100px|<small><center>Growth plate (Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 
[[Image:Growth plate closer.jpg|right|thumb|100px|<small><center>Growth plate magnified(Image sourced from Bristol Biomed Image Archive with permission)</center></small>]]
 
 
 
*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
 
*Site of many '''congenital''' or '''nutritional''' bone diseases in the growing animal
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*If growth teporarily stops -> layer of bone seals the growth plate -> moves into metaphysis when growth resumes -> forms '''Harris lines'''
 
*If growth teporarily stops -> layer of bone seals the growth plate -> moves into metaphysis when growth resumes -> forms '''Harris lines'''
  
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==Test yourself with the Bone and Cartilage Pathology Flashcards==
  
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[[Bones_and_Cartilage_Flashcards_-_Pathology|Bones and Cartilage Flashcards]]
  
  
===Bone resorption===
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[[Category:Bones - Pathology|A]]
 
 
*Mediated by two hormones:
 
**'''Parathyroid hormone (PTH)'''
 
***Produced by <u>chief cells in the parathyroid glands</u> in response to <u>decreased</u> serum calcium
 
***In response, osteoclasts increase in number and resorb mineralised matrix - increase Ca in blood
 
**'''Calcitonin'''
 
***Produced by <u>C-cells in the thyroid glands</u> in response to <u>increased</u> 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
 
 
 
 
 
 
 
<big><center>[[Bones|'''BACK TO BONES''']]</center></big>
 

Latest revision as of 12:39, 3 March 2011

Changes to normal structure

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

Physis (Growth plate)

  • 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

Test yourself with the Bone and Cartilage Pathology Flashcards

Bones and Cartilage Flashcards