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Flashcards in Bone Metabolism Deck (40)
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1

What is lamellar bone?

It's mature bone, either of the cortical or trabecular form.

2

What is woven bone?

Immature bone that is formed in response to damage or may be pathological

3

What percentage of total bone mass is made up of cancellous/trabecular bone?

20%

4

What percentage of total bone mass is made up of cortical/compact bone?

80%

5

How does the collagen organisation differ between lamellar and woven bone?

In woven bone there is random collagen organisation which makes it mechanically weak in comparison to the ordered structure of lamellar bone.

6

Describe the composition of the extracellular matrix of bone

• 35-40% organic
o Type 1 collagen for tensile strength
o Proteoglycans for compressive strength
o Growth factors, cytokines and osteoid
• 60% inorganic
o Contains a large amount of calcium hydroxyapatite on top of the collagen fibres
• 5% consists solely of water.

7

What type of collagen is ordinarily present in bone?

Type 1

8

What are the main organic components of bone matrix?

Type 1 collagen, proteoglycans, growth factors, cytokines and osteoid

9

What is the main inorganic component of bone matrix?

Calcium hydroxyapatite (on top of collagen fibres)

10

Which cells secrete sclerostin?

Osteocytes

11

What is the function of sclerostin?

Inhibits bone formation by osteoblasts

12

What is the function of osteocytes?

To sense mechanical stress on bone by detecting death and a need for remodelling; in response to stress stop secreting sclerostin (due to action of PTH)

13

How does calcitonin affect sclerostin production by osteocytes?

Upregulates secretion and therefore inhibits bone formation by osteoblasts

14

How does PTH affect sclerostin production by osteocytes?

Inhibits sclerostin production in response to mechanical stress; this signals for osteoblasts to deposit bone

15

What are osteoblasts?

A form of mesenchymal cell which replace (deposit) more bone matrix (osteoid)

16

What are osteoclasts?

Macrophage-like cells from red bone marrow that work to dissolve and digest the bone matrix

17

What are osteoprogenitor cells?

Bone marrow stem cells.

18

What are canaliculi?

These allow chemical communication between osteocytes to decide whether remodelling is required in response to stress

19

What are osteocyte lacunae?

The 'pit' or 'lake' in which individual osteocytes sit

20

Outline the process of bone remodelling

1. Osteoclast attached to bone to form a leak-proof seal
2. Protein-digesting enzymes break down the collagen fibres
3. Acid secreted dissolves the bone minerals
4. Bone proteins and minerals (mainly Ca2+) to cross the osteoclast and exitinto the interstitial fluid
5. Osteoblasts then fill the lacuna with osteoid, and the osteoid is mineralised approximately a week later

21

What does serum calcium depend upon?

Intestinal absorption, renal excretion and skeletal mobilisation

22

What is normal serum calcium?

2.2-2.6mmol/L (8.5-10.5mg/dL)

23

What is the recommended daily intake of calcium?

500-1,300mg

24

How is calcium excreted?

Calcium is excreted by kidneys (~200mg/day) and gut (~600-800mg/day in stools)

25

What are normal serum phosphate levels?

0.8-1.5mmol/L

26

Which hormones are involved in 'bone maintenance'?

Vitamin D, oestrogen, progesterone

27

Which hormones are involved with increased bone degradation?

Calcitonin, PTH, glucocorticoids (steroids)

28

Outline the effect of PTH on bone metabolism

Low serum calcium stimulates PTH production which then promotes calcium reabsorption from kidney (and phosphate excretion), calcium resorption (osteoblasts PTH receptor --> RANKL --> RANK receptor on osteoclasts) and increased 1,25 dihydroxyvitamin D production (increases calcium absorption from gut)

29

Outline the effect of calcitonin on bone metabolism

Released when serum calcium is just within the normal range. Causes inhibition of osteoclasts, increases calcium excretion from kidney and inhibits calcium absorption in the gut.

30

Outline the effect of vitamin D on bone metabolism

Increases calcium absorption in the gut, and increases bone resorption (activates RANKL --> more osteoclast differentiation)