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Medicine 2.1 > Bone Remodeling and Osteoporosis > Flashcards

Bone Remodeling and Osteoporosis Flashcards

1
Q

What is cortical bone?

A

Compact bone found at diaphysis of long bones

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2
Q

What is trabecular bone?

A

Spongy bone that makes up vertebral bodies and end of long bones

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3
Q

Is cortical or trabecular bone remodelled quicker?

A

Trabecular

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4
Q

What are the bone cell types and their function?

A
  • Osteocytes: Mature osteoblast, sensors of mechanical strain on skeleton embedded in bone matrix
  • Osteoblasts: bone forming
  • Osteoclasts: bone removing
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5
Q

What phases can the remodelling of bone be put into?

A
  • Quiescence
  • Resorption
  • Reversal
  • Formation
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6
Q

How long can the remodelling cycle last?

A

around 6 months

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7
Q

What cells are osteoclasts and osteoblasts derived from?

A
  • Osteoclasts = cells of monocyte/macrophage lineage

- Osteoblasts = derived from mesenchymal stem cells

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8
Q

What are osteoclasts rich in?

A

TRAP enzymes

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9
Q

What type of cell are osteoclasts?

A

Multinucleated phagocytes

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10
Q

What are molecules important in regulation of osteoclast differentiation and activity?

A
  • Transcription factor PU-1
  • M-Csf
  • C-fos
  • Nuclear factor kappa B
  • RANK and RANK ligand
  • OPG
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11
Q

What does PU-1 do?

A

Needed for differentiation of early osteoclast precursors from haematopoetic stem cells.

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12
Q

What does RANK and RANK ligand do?

A
  • Stimulate activity of osteoclasts
  • Influence later stages of osteoclastogenesis by promoting differentiation oof comitted precursors to mature osteoclasts
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13
Q

What does OPG do?

A

Inhibit osteoclast formation and activity by binding to RANK ligand.

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14
Q

What expresses OPG?

A

Osteoblasts, marrow stromal cells and others.

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15
Q

How does resorption occur?

A
  • Occurs through HCL and proteolytic enzyme secretion.

- Acid dissolves hydroyapatite, allowing enzymes to degrade collagen and other matrix proteins.

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16
Q

What are some important molecules in bone resorption?

A
  • Carbonic anhydrase II needed for acid generation

- Cathepsin K a proteolitic enyme needed for resorption

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17
Q

What is cbfa 1?

A

The key trigger for osteoblasts.
-Is a trnascription factor that expresses genes characteristic of osteoblast phenotype eg. osteocalcin, type 1 collagen, alkaline phosphatase

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18
Q

What are some molecules important in controlling osteoblast proliferation and differentiation?

A

-PTH, glucocorticoids, Wnt, IGF, FGF

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19
Q

Define osteoperosis?

A

A skeletal disorder characterised by compromised bone strength predisposing a person to an increased risk of fatigue.

20
Q

What does bone strength reflect?

A

The integration of bone density and bone quality.

21
Q

Why and how is OPG produced?

A

Produced naturally to neutralise RANK effects, to defend against bone loss.

22
Q

How is RANK expressed?

A

Several factors (PTH, TNF, IL 1) stimulate expression by cells of osteoblast lineage (activated T cell).

23
Q

Do osteotropic growth factors, cytokines and hormones up or down regulate RANK expression?

A

Upregulate.

24
Q

In premenopausal state what regulates RANK ligand expression?

A

Estrogen

25
Q

How is bone fully mineralised?

A

After basic structural matrix is formed a secondary mineralisation process starts which is much slower.

26
Q

How do bones and bone mineral acquisition change during puberty?

A
  • Mineral aquisition increases
  • Spine increases in size and trabecular thickness
  • Long bones increase in length and diameter
27
Q

When do we attain peak bone mass?

A

Around 30-40

28
Q

What are some determinants of peak bone mass?

A
  • Lifestyle
  • Genetics
  • Nutrition
  • Hormones
29
Q

How does bone mass change after peak in men and women?

A 
Men = steady decline
-Women = accelerated decrease post menopause, can often drop below fracture threshold
30
Q

Why is estrogen important?

A

-Needed for normal bone maturation and mineral acquisition
-Can modulate expression of factors that stimulate osteoclast formation.
Deficiency can negtively influence OPG.

31
Q

What are some risk factors for Osteopoorotic fractures?

A
  • Age
  • Hypogonadism
  • Smoking
  • Excessive alcohol
  • Menopause
  • Malabsorption syndrome
  • Rheumatoid
  • Family history
32
Q

What are determinats of fracture risk?

A
  • Bone strength

- Extraskeletal conditions: propensity to fall and fall conditions

33
Q

What is the DXA system used fro?

A

Bone strenght measutremnt by estimateing BMD.

34
Q

What can T scores indicate?

A
  • Normal is -1 or above
  • Osteopenia is -1 to -2.5
  • Osteoporosis is less or equal to than -2.5
  • Established osteoporosis is less than oor equal to -2.5 with prescnece of 1 or more fractures
35
Q

What can the FRAX tool do?

A

Predict future fracture risk.

36
Q

What are non pharmaceutical ways to decrease fracture risk?

A
  • Lifestyle changes
  • Reduce fall risks
  • Hip protectors
37
Q

When are pharmaceutical ways to decrease fracture risk often used?

A

In post menopause osteoporosis patients for whom non pharmaceutical methods have not worked.

38
Q

What are pharmaceutical ways to decrease fracture risk?

A
  • Ca and Vit D supplementation
  • HRT
  • SERM’s
  • Romosozumab
  • Strontium ranalate
39
Q

How do we know if our treatment is working for this?

A
  • Fewer fractures
  • Bone density (DXA) measurement
  • Use of bone markers
40
Q

What are the initial effects of mechanical stress/micro crack?

A

-Before crack, quiescent surface with embedded osteocytes secreting sclerostin to inhibit Wnt.
Pre osteoclasts circulating in blood vessel
-Sudden stress causes microcrack
-Causes osteocytes near crack to apoptose

41
Q

What happens after osteocytes apoptose in micro crack?

A
  • Stromal cells released from sclerostin inhibition and/or exposed to other factors eg. IL 1.
  • They generate pre osteoblasts
  • Also secrete M-Csf to help generate pre osteo clasts
  • Lining cells pull away from bone matrix to form a canopy that merges with blood vessels.
42
Q

What happens after canopy forms after micro crack?

A

-Pre osteoblasts start to express RANK ligand on their surfaces.
-Pre osteoclasts have receptors on their surfaces
-Pre osteoblasts proliferate and secrete more factors eg. Wnt
-Pre osteoclasts enlarge and fuse into mature osteoclasts
-Osteoclasts bind to bone matrix with integrins.
They resorb bone, takes around 2 weeks. Bone derived growth factors are released eg. IGF, TGF B.

43
Q

What happens after bone resorption after micro crack?

A
  • Eventually osteoclast apoptoses, lifespan regulated by estrogen and other factors.
  • Pre osteoblasts mature and stop making RANK ligand and secrete OPG. Blocking activation of pre osteoclasts.
  • Pre osteoblasts proliferate linng cavity. They secrete osteoid then mineralise it, filling cavity takes 3-4 months.
44
Q

What happens after osteoblasts proliferate and line cavity after micro crack?

A
  • Some osteoblasts apoptose, others turn into osteocytes, others become lining cells.
  • Meanwhile, osteocytes have been re-establishing network with each other and lining cells.
  • Microdamage repaired
45
Q

How long will new matrix continue to accumulate mineral and increase in density for?

A

3 years

46
Q

What other proteins and growth factors are in the matrix?

A
  • IGF

- TGF B

Medicine 2.1 (29 decks)

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