Bone: Structure, Function And Remodelling

5.1.07

Functions of bone
• Mechanical:
-Protection.
-Support for other organs.
-System of levers.
• Metabolic function:
-Store for calcium (hypocalcaemia causes tetany).

Joint tissues
Fibrocollagenous
Tendon, ligament, capsule Resists tension Collagen I
Bone Resists compression Collagen I
Mineral
Cartilage Resists compression Collagen II
Proteoglycan
Water

Bone
• Organised in 2 distinct forms:
-Compact bone (cortical bone) - high proportion of bone with few spaces.
-Cancellous/spongy/trabecular bone - low proportion of bone and a lot of space.
• Cancellous bone composed of network of rods and plates called trabeculae.

Bone matrix
• Type I collagen, bone proteoglycan and some non-collagenous proteins: osteocalcin, osteonectin.
• Mineral hydroxyapaptite - a complex calcium phosphate salt.
• Collagen can be laid down into distinct patterns:
-WOVEN BONE, an immature form with random fibre orientation, laid down during rapid growth and fracture repair.
-LAMELLAR BONE, which is composed of successive layers of collagen fibres with distinct orientation.

Structure
• Long bones composed of cylindrical shaft (diaphysis) connected to expanded ends of bone (epiphyses).
• Shell of compact bone surrounds medullary/marrow cavity - site of production of blood cells in immature animals (red marrow), but becomes progressively replaced by inactive yellow marrow, mainly composed of adipose tissue.
• Spongy bone occupies medullary cavity at epiphyses - extends in to metaphysis.

Bone cells
• Osteoblasts:
-Principal function: bone formation.
-Form epithelioid layer on bone surface.
• Osteocytes - osteoblasts engulfed in bone matrix during apposition.
• Lining cells:
-Osteoblasts which have completed phase of synthetic activity.
-Can be reactivated.
• Osteoclasts - large multinucleate cells that "eat" bones.

Osteoid = Bone matrix that has not yet mineralised - mineralisation process still not understood.

Bone remodelling
• Resting.
• Activation.
• Formation - mesenchymal stem cells - osteoblasts.
• Resorption.
• Reversal.