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LOCOMOTOR 2 > Bone Metabolism > Flashcards

Bone Metabolism Flashcards

1
Q

What is lamellar bone?

A

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

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2
Q

What is woven bone?

A

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

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3
Q

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

A

20%

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4
Q

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

A

80%

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5
Q

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

A

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

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6
Q

Describe the composition of the extracellular matrix of bone

A 
•	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.
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7
Q

What type of collagen is ordinarily present in bone?

A

Type 1

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8
Q

What are the main organic components of bone matrix?

A

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

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9
Q

What is the main inorganic component of bone matrix?

A

Calcium hydroxyapatite (on top of collagen fibres)

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10
Q

Which cells secrete sclerostin?

A

Osteocytes

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11
Q

What is the function of sclerostin?

A

Inhibits bone formation by osteoblasts

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12
Q

What is the function of osteocytes?

A

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)

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13
Q

How does calcitonin affect sclerostin production by osteocytes?

A

Upregulates secretion and therefore inhibits bone formation by osteoblasts

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14
Q

How does PTH affect sclerostin production by osteocytes?

A

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

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15
Q

What are osteoblasts?

A

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

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16
Q

What are osteoclasts?

A

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

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17
Q

What are osteoprogenitor cells?

A

Bone marrow stem cells.

18
Q

What are canaliculi?

A

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

19
Q

What are osteocyte lacunae?

A

The ‘pit’ or ‘lake’ in which individual osteocytes sit

20
Q

Outline the process of bone remodelling

A
  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
Q

What does serum calcium depend upon?

A

Intestinal absorption, renal excretion and skeletal mobilisation

22
Q

What is normal serum calcium?

A

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

23
Q

What is the recommended daily intake of calcium?

A

500-1,300mg

24
Q

How is calcium excreted?

A

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

25
Q

What are normal serum phosphate levels?

A

0.8-1.5mmol/L

26
Q

Which hormones are involved in ‘bone maintenance’?

A

Vitamin D, oestrogen, progesterone

27
Q

Which hormones are involved with increased bone degradation?

A

Calcitonin, PTH, glucocorticoids (steroids)

28
Q

Outline the effect of PTH on bone metabolism

A

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
Q

Outline the effect of calcitonin on bone metabolism

A

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
Q

Outline the effect of vitamin D on bone metabolism

A

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

31
Q

What is the active form of vitamin D?

A

1,25 dihydroxyvitamin D

32
Q

Outline the effect of oestrogen on bone metabolism?

A

Increases calcium absorption from the gut (increases production of 1,25 dihydroxyvitamin D) and inhibits osteoclasts

33
Q

Outline the effect of glucocorticoids on bone metabolism

A

Decreases calcium absorption in the gut and increases bone resorption and decreases bone formation (prolonged steroid use can therefore lead to osteoporosis)

34
Q

Outline the effect of progesterone on bone metabolism

A

Adding progesterone cream in osteoporosis therapy has shown to increase bone density (mechanism unknown)

35
Q

What is the role of calcitonin on bone metabolism?

A

Prevents bone formation by osteoblasts; it up regulates the production of sclerostin by osteocytes

36
Q

What stimulates the release of PTH?

A

Low serum calcium levels

37
Q

What stimulates the secretion of calcitonin?

A

Serum calcium levels just within the normal range (2.25mmol/L)

38
Q

What is the normal daily requirement of vitamin D?

A

400IU/day

39
Q

Where is 1,25 dihydroxyvitamin D formed?

A

Kidney, and it’s the active form of vitamin D.

40
Q

How may vitamin D aid in the production of mineralised bone matrix?

A

By producing pro-collagen type 1 anad alkaline phosphatase vesicles

LOCOMOTOR 2 (28 decks)

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