Bone Homeostasis

Question 1

How long does
1. Resorption
2. Formation
take?

Answer 1

1. ~3 weeks to dig resorption pit

2. ~3-4 months for osteoid to fill in pit, mineralization takes years

Question 2

2 kinds of remodelling in cortical bone

Answer 2

Haversian remodelling

Periosteal apposition

Question 3

Haversian remodelling

Answer 3

Within cortical bone
Very slow (concentric)
Osteoclasts in the cutting cone, blasts in the closing cone
Leaves a Haversian canal (clear trace)
Mineralization increases slowly

Question 4

Periosteal apposition

Answer 4

Cortical surface
No initial bone resorption
Triggered by mechanical strains (exercise)
Associated with endosteal resorption
Continues into old age

Question 5

Sclerostin

Answer 5

Secreted by osteocytes

Decreases bone formation

Question 6

When bone remodeling is increases does bone loss or formation dominate?

Answer 6

Bone loss

Because it is faster

Question 7

Calcitonin

Answer 7

Inhibits osteoclasts

Decreases resorption and increases formation

Question 8

Estrogen, progesterone and testosterone effects on bone

Answer 8

Estrogen: decreases resorption in women and men
Progesterone: increase formation in women
Testosterone: decreases resorption and increases formation in men
An acute decrease in estrogen and testosterone result in increased resorption though

Question 9

Effects of higher than physiological levels of cortisol

Answer 9

Increase resorption

Decrease formation

Question 10

PTH effect on bone

Answer 10

Hourly or continuous exposure (like in hyperparathyroidism): increases resorption
Intermittent dosing (as a therapy): increases formation