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Bone - Structure & Function

Bone Ultrastructure   

Bone Cells   

Organic Matrix   

Inorganic Matrix  

Remodeling   

Embryology   

Blood Supply   

Periosteum Physis

BONE ULTRASTRUCTURE

Bone is  is a specialised form of dense connective tissue. 

Bone function = 

  1. Mechanical - gives the skeleton the necessary rigidity to function as attachment and lever for muscles and supports the body against gravity.

  2. Chemical - Calcium homeostasis & metabolism

  3. Hematological

Three types of bone can be distinguished macroscopically:

  1. Trabecular bone - (also called cancellous or spongy bone) consists of delicate bars and sheets of bone, trabeculae, which branch and intersect to form a sponge like network. The ends of long bones (or epiphyses) consist mainly of spongy bone.
  2. Compact bone - does not have any spaces or hollows in the bone matrix that are visible to the eye. 
  3. Woven bone - immature, disorganised bone.


Compact Bone (Cortical)

 

 

Trabecular Bone (Cancellous)

 

Woven Bone (primary bone) 


Compact BoneCompact Bone from University of Western Australia [click on picture for higher power view]

Haversian & Volkmann CanalsHaversian & Volkmann Canals from University of Western Australia [click on picture for higher power view]


BONE CELLS

Osteoblasts 

Osteocytes

Resting surface cells (Bone Lining cells)

Osteoclasts 

Bone cells

Bone cells [OsCl = osteoclast; OC = osteocyte; OB = osteoblast; CC = calcified cartilage; B = bone]

 


ORGANIC COMPONENTS OF MATRIX

= 40% of the dry weight of bone

1. Collagen

Collagen structure

2. Proteoglycans

3. Matrix Proteins (non-collagenous)

  1. osteocalcin 
    • binds calcium
    • attracts osteoclasts
    • levels in urine incr. in Pagets, hyperparathyroidism & renal osteodystrophy
  2. osteopontin 
    • cell-binding protein
    • anchors osteoclasts to mineralised matrix
  3. osteonectin 
    • binds calcium
    • regulation of mineralisation
  4. Sialoprotein
    • important in cell attachment
  5. Thrombospondin
    • binds calcium & important in cell attachment
  6. Serum proteins
    • in same concentrations as in serum except albumin, which is increased

4. Cytokines


INORGANIC COMPONENTS OF MATRIX

60% of dry weight of bone

1. Calcium Hydroxyapatite - Ca10(PO4)6(OH)2

2. Osteocalcium Phosphate (Brushite)


BONE FUNCTIONStages of bone remodelling

Remodelling = osteoclasts resorb bone & the resulting hole is filled by osteoblasts with new bone & osteocytes. (e.g. cutting cone of compact bone)

Modelling = osteoclasts remove bone from one area while osteoblasts add bone elsewhere. Occurs in trabecular bone in response to load changes (Wolff's Law).

Remodelling Cycle:

Cortical / Compact bone:


EMBRYOLOGY

The appendicular skeleton develop from the limb buds, which are mesodermal structures covered by ectoderm. 

The first visible outline of the embryonic limb follows a condensation of mesenchymal cells which subsequently differentiate into cartilage cells, the chondrocytes

These cells secrete a matrix and so produce cartilaginous models of the future bones. Surrounding this cartilage is the perichondrium, the outer layer of which becomes a connective tissue sheath while the inner cells remain pluripotential. 

Primary, or diaphyseal ossification centre, develops at a site where the cartilage cells and matrix have begun to disintegrate. Trabecular bone is then deposited on cartilaginous remnants. 

In long bones, another secondary centre of ossification appears at the growing cartilaginous ends, the epiphyseal ossification centre. 

A transverse plate of cartilage extends across the bone separating the epiphyseal from the diaphyseal ossification centre = the physis / epiphyseal growth plate

Interstitial growth 

Appositional growth 


BONE CIRCULATION

Bone, as an organ, receives 5-10% of  cardiac output

Blood supply is from 4 sources:

  1. Nutrient artery system

    • supplies the inner 2/3 of cortex from within (endosteal)

    • High pressure system

    • nutrient artery divides after reaching the medullary cavity, sending arteriole branches in proximal and distal directions to join with metaphyseal arteries 

    • in the child, these vessels end on metaphyseal side of the physis & contribute to endochondral ossification.

  2. Periosteal system

    • forms an extensive network of vessels covers entire length of the bone shaft

    • supplies outer 1/3 of cortex

    • low pressure system

    • v. important in children, for circumferential bone growth (appositional)

  3. Metaphyseal system

    • supplies the zone of provisional calcification in the physis

    • anastomoses with the nutrient artery system

  4. Epiphyseal system

    • supplies following layers of physis by diffusion: resting, germinal, proliferating, and upper hypertrophic cell layers.

    • in femoral and radial heads which are almost entirely covered by cartilage the vessels enter in region between articular cartilage & growth-plate cartilage, and hence, the blood supply is tenuous.

Arterial supply of the cortex is centrifugal (inside to out). The direction is reversed in a displaced fracture, with disruption of the endosteal (nutrient) system.

Venous flow is centripetal with cortical capillaries draining to venous sinusoids to emissary venous system.


PERIOSTEUM


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LF - Updated on: 23 December 2004 11:10