Bone - bio. properties and remodelling

Question 1

Why do we have bones?

Answer 1

• Calcium regulation
• Mechanical support and locomotion
• Protection of vital organs

Question 2

How do we regulate calcium?

Answer 2

• Calcitonin - decreases bone resorption
• PTH - increases bone resorption, increases 1a-hydroxylase (increases active VitD), increases renal calcium reabsorption, increases phosphate clearance.
• Vit D - increases intestinal calcium absorption
• Oestrogen - decreases bone resorption

Question 3

What is the composition of bone?

Answer 3

• Protein- 25% (matrix largely composed of type 1 collagen, with glycoproteins, proteoglycans, and other proteins making up the rest)
• Mineral (75%) - Calcium phosphate
• Cells (osteoblasts/clasts/cytes, bone marrow cells)

Question 4

What is osteogenesis imperfecta?

Answer 4

Brittle bone disease - lack of type 1 collagen in the bone’s protein matrix

Question 5

Cortical vs Trabecular bone

Answer 5

Cortical has a higher density, lower surface area, lower remodelling rate, Haversian systems (rings)
- Trabecullar bone uses struts and plates and is metabolically active

Question 6

What do osteoblasts do?

Answer 6

Synthesise matrix proteins, formation of bone mineral, derived from mesenchymal stem cells

Question 7

What do osteoclasts do?

Answer 7

• production of acid - dissolution of mineral
• production of proteolytic enymes - digestion of matrix (cathepsin K, metalloproteinase)
• transcellular removal of calcium, phosphate and matrix
• derived from macrophage type cells

Question 8

What is rickets?

Answer 8

A disease where there is demineralisation of bone - gives bow-leg as the bones bend under load

Question 9

What is the bone remodelling cycle?

Answer 9

• Quiescence
• Resorption of bone by osteoclasts
• formation of new bone by osteoblasts
• mineralisation
• quiesence

Question 10

How does trabecular bone remodel?

Answer 10

• Osteoclasts cause resorption pits, osteoblasts fill in the pit with unmineralised proteins, which they then mineralise
• Move along the trabeculae

Question 11

What changes in bone mass are there with age?

Answer 11

• men have much more bone mass on average
• both plateau at 30-40 and then decline gradually
• women have a much lower bone mass in later life - more risk of fractures
• there is accelerated bone loss in menopause (oestrogen usually inhibits osteoclasts)

Question 12

What is Paget’s disease?

Answer 12

Excessive breakdown and formation of bone in a localised area - leads to disorganised bone.

Question 13

What is osteoperosis?

Answer 13

Osteoclasts are over active - you have more resorption than bone formation
- Generalised reduction in bone

Question 14

What hormones regulate bone density?

Answer 14

Increase: Oestrogens/androgens (suppress osteoclast bone resorption). Growth hormone (IgF). Calcitonin.

Decrease: thyroxine, glucocorticoids (too much = cushing’s), PTH.

Question 15

How do fractures occur?

Answer 15

• When force exceeds bone’s strength

- bone has good tensile and compressional strength, but weaker torsional strength.

Question 16

What are the 3 stages of fracture healing?

Answer 16

1: macrophages remove debris. granulation tissue forms. fibrous tissue - vascularised
2: Soft callus formed by osteoblasts. Hard callus then formed when the bone mineralises
3: lamellar bone fromation and remodelling