Bone Growth & Adaptation

Question 1

Specify the cellular and molecular composition of bone tissue.

Answer 1

Periosteum:
- covers surface of bone
- 2 layers
- protective
- osteogenic
- site of sensory nerves
- blood/lymphatic vessels
Endosteum:
- lines medullary cavity
- 1 layer
- osteogenic/osteolytic cells

Lamellar bone:
- regular concentric arrangement
- strong
- slow production during growth/remodelling
Woven bone:
- irregular
- weak
- small trabeculae
- produced rapidly (eg: fracture healing / intramembranous ossification)

Question 2

Describe the process of bone formation in the developing animal.

Answer 2

Intramembranous ossification - mesenchymal (stem) cells become osteoblasts which secrete osteoid.
Osteoblasts trapped in mineralised matrix differentiate and become osteocytes.

Long bone formation:
1. Perichondrium lays down bone on surface of cartilage in diaphysis region (intramembranous ossification) creating the bone collar
2. Cartilage tissue becomes bone tissue (endochondral ossification) - cartilage degenerates -> vascularisation -> calcium salts influx matrix -> cellular changes
3. Hyaline cartilage is left as small strips by the meta- and epiphysis (epiphyseal growth plate)

Girth:
- New layers of bone are created on the exterior of the bone to increase the girth - increase strength for better support
- Bone is removed from the interior of the diaphysis to reduce bone mass (lighter) to expend less energy
- More lamellae/osteons created as bone widens
1. bone formation at the surface of the bone produces ridges that parallel a blood vessel on the periosteum
2. the ridges enlarge and create a deep pocket
3. the ridges meet and fuse, trapping the vessel inside the bone

Question 3

Describe the process of bone growth in the young animal.

Answer 3

• Epiphyseal growth plate - cartilage template that allows bone length to increase in the metaphyseal region - remains open until skeletal maturity is reached
• Reserve chondrocytes - form a basis for cartilage tissue and makes sure the tissue is attached to the epiphysis
• Proliferating chondrocytes - undergo mitotic division to produce more chondrocytes
• (Pre)hypertrophic chondrocytes - growing process
• Endochondral chondrocytes - become calcified and die to become bone tissue

Question 4

Discuss nutritional and hormonal factors that can affect bone growth.

Answer 4

• Dietary calcium and phosphate salts determine success rate of bone production (helps mineralisation)
• Absorption depends on calcitriol hormone only made in presence of vitamin D
• Require vitamins A, B12, C and K
• Calcitonin and parathyroid hormones regulate calcium metabolism
• Insulin, growth hormone and thyroxine lead to bone growth
• Oestrogen (and testosterone) required for growth plate closure

Question 5

Describe the process of bone remodelling and explain why it occurs.

Answer 5

• Bone modelling (reshaping) - the process of bone shape change in response to the loads imposed
• Bone remodelling - a lifelong cycle process of bone removal and addition

Role of remodelling:
- maintains mechanical strength
- response to mechanical demands
- mineral homeostasis (provides calcium and phosphorus stores)

Bone remodelling cycle:
1. formation - osteoblasts synthesise organic matrix and regulate its mineralisation - osteoblasts entombed in matrix become osteocytes
2. Resorption - osteoclasts secrete enzymes which break down bone (osteoclast apoptosis)
3. Reversal - area is populated by osteoblasts (same amount of bone that is resorbed is formed)

• Osteoporosis - when too much bone is resorbed in comparison to being formed (becomes brittle)
• Osteopetrosis - when too much bone is formed in comparison to being resorbed (becomes too dense)