7.2 Bone, Fracture Repair, Intramembraneous Ossification

Question 1

Describe the structure of cancellous and then compact bone.

Answer 1

1) Cancellous/trabecular
- network of fine bony columns
- combine strength with lightness
- spaces filled with bone marrow
- no haversian or volksmanns canals

2) compact
- external surface of bones
- as it says, compact! (See notes for diagram)
- concentric lamellae
- h and v canals

Question 2

Difference between immature and mature bone

Answer 2

Immature - Osteocytes and collagen randomly arranged - woven bone
Mature - Osteocytes arranged in concentric lamellae of osteons, resorption canal runs parallel with osteons long axes, collagen arranged parallel to long axis of bone

Question 3

Difference between intramembraneous ossification and endochondrial?

Answer 3

Intramembraneous - thickening - forms compact bone

Endochondrial - replacement of pre-existing hyaline cartilage template by bone

Question 4

Process of intramembraneous ossification?

Answer 4

1. Mesenchymal cells cluster to form nidus
2. Become osteoprogenitor cells
3. Become osteoblasts
4. Release EM containing osteoid (type 1 coll)
5. Osteoid mineralises forming rudimentary bone tissue spicules which are surrounded by osteoblasts and osteocytes
6. Spicules join and form trabeculae
7. Trabecular merge and form woven bone
8. Replaced by lamellae of mature compact bone

Question 5

What is an osteoblast?

Answer 5

Deposit osteoid (collagen, Ca, phosphate)
Refill resorption cavities

Question 6

What is an osteocyte?

Answer 6

Osteoblasts embedded in lacunae
Cytoplasmic processes attached which reach out to adjacent osteocytes via canaliculi
Canaliculi connect to h canals so nutrients can be passed

Question 7

What is an osteoclast?

Answer 7

Multi-nucleated cells
Absorb bone
Releases H+ and lysosomal enzymes dissolving mineral content

Question 8

Process of bone remodelling and why it happens.

Answer 8

1. Osteoclasts bores tunnel forming a cutting cone
   - does this via H+ and lysosomal enzyme to resorb bone
2. Osteoblasts lay down new osteoid at back of growth cone.

This happens in response to signals from osteocytes (if lots of pressure on bone)

Question 9

How is a fracture repaired at the site of breakage?

Answer 9

1. Haematoma formation
   - haematoma forms as blood vessels break
   - swelling and inflammation occur
   - bone cells at fracture edge die
   - osteoclasts and phagocytes clean up whilst macrophage removes clot
2. Fibrocartilageneous callus formation
   - new blood vessels
   - procallus forms (tissue rich in capillaries and fibroblasts)
   - fibroblasts secrete collagen fibres, chondroblasts - hyaline cartilage
   - osteoblasts from nearby perichondrium/periosteum and multipotent cells from bone marrow begin bone reconstruction
3. Bony callus formation
   - new trabeculae appear as soft callus ==> hard callus by endochondrial and intramembraneous ossification
4. Bone remodelling
   - cancellous bone remodelled to compac
   - final shape as original

Takes several months!

Question 10

What is osteoporosis?

Answer 10

Metabolic bone disease
Decreased mass of mineralised bone
No longer provides support

Osteoclasts resorption > osteoblasts depositing

Question 11

Difference between type 1 and type 2 primary osteoporosis?

Answer 11

Type 1 - post menopausal women
Decrease in oestrogen
Increase of osteoclast no.

Type 2 - senile - elderly
Both sexes
Decrease of osteoblast no.

Question 12

Risk factors for osteoporosis?

Answer 12

Female, age, genetic, calcium intake, vit D absorption, exercise, cigarette smoking