Ossification

Ossification

Features of Bone

• Type one collagen fibres (Think bONE)

• Periosteum is external surface

• Endosteum lines internal surfaces

• Mineral component – hydroxyapatite which is made up of calcium and phosphate

• Rich blood supply

Intramembranous (in flat bones)

1. Mesenchyme condenses and is highly vascular

2. Mesenchymal cells differentiate into osteoblasts

3. Osteoblasts secrete osteoid

4. Osteoblasts trapped in ECM mature into osteocytes to form woven bone

5. Bone is reorganised into lamellae bone

Endochondral (at primary and secondary ossification centres)

1. Chondrocytes in the centre of the cartilage model undergo hypertrophy and secrete ECM

2. This calcifies and results in chondrocyte death

3. Osteogenic buds invade dead cartilage, blood vessels bring in osteogenic cells

4. Osteoblasts develop and secrete osteoid, which becomes mineralised

5. Woven bone replaces cartilage, perichondrium becomes periosteum, bone reorganised into lamellae bone

Tips for learning this….

• IntraMembranous: starts with Mesenchymal cells. Steps Spell MMOOB

• EndoCHONDral: CHONDrocytes. Involves Cartillage

Primary Vs Secondary ossification centresPrimary SecondaryDiaphysis EpiphysisIncreased growth of periosteal growth collar* Long Bones

Resorption of central part results in formation of medullary cavity

No perichondriumImmature woven bone replaced by lamellar bone

*The bone collar is a cuff of periosteal bone that forms around the diaphysis of the hyaline cartilage model in developing long bones

Fracture Healing – the full version• Haematoma formation: Bleeding from small vessels; periosteal & nutrient arteries

• Inflammatory reaction - macrophages invade and remove avascular dead bone around # site

• Ingrowth of granulation tissue into the haematoma

• Vascular connective tissue / cell proliferation / cytokines & growth factors

• Callus formation – bridges gap between bone ends – fibrocellular material initially

• Osteogenic cells proliferate from the periosteum

• Closer to # site cells differentiate into chondrocytes - islands of cartilage laid down which is soft & radiolucent

• Osteogenic cells further from the # site differentiate into osteoblasts & produce woven bone

• Cartilaginous components of the callus are gradually replaced by bone through endochondral ossification

• When the # callus becomes sufficiently firm that movement no longer takes place at the # site it is “clinically united”

• However much more needs to be done before the bone is restored to its original strength

• Primary callus is gradually replaced by mature lamellar bone

• Remodelling by osteoclasts and osteoblasts of the lamellar bone into an appropriate form related to function. Excessive callus resorbed & medullary cavity re-established

Fracture healing: what I would know

• Haematoma formation

• Inflammation reaction. Macrophages invade and remove dead bone

• Ingrowth of granulation tissue – vascular tissue

• Callus formation – fibrocellular material initially

• Osteogenic cells proliferate from the periosteum

• Closer to # site cells differentiate into chondrocytes. Osteogenic cells further from the # site differentiate into osteoblasts & produce woven bone

• Cartilaginous components of the callus are gradually replaced by bone through endochondral ossification. Clinically united

• Primary callus is gradually replaced by mature lamellar bone

• Remodelling and resorption

1. Haematoma2. Granulation tissue3. Callus4. Woven bone5. Lamellar bone6. Remodelling

Learning Outcomes

Explain how bone develops via intramembranous and endochondral ossificationExplain the process of bone growthExplain how bones repair following fracture