Bone-Turnover and Cells

Question 1

Why does bone turnover/ remodelling happen?

Answer 1

Constant process even in the adult
Healthy bone needs a balance between bone removal & formation

Question 2

What are the steps of bone turnover / remodelling?

Answer 2

-Osteoclasts break down old bone
-Osteoblasts build new bone

Question 3

What are osteoclasts, where are they found and what do they do?

Answer 3

Large multinucleate cells derived from haematopoietic stem cells (like macrophages)

Found on bone surface

Function: resorb bone matrix

Question 4

Characteristics of osteoclasts?

Answer 4

‘Ruffled border’ releases acid to dissolve bone matrix (demineralizes-removes calcium)
Howship’s lacunae – resorption pits

Question 5

What are osteoblasts and what is their function?

Answer 5

Immature bone cells created from osteopregenitor cells in periosteum & endosteum

Function: produce bone matrix (unmineralised matrix = osteoid) & initiate calcification

Become osteocytes once surrounded by matrix

Question 6

What are osteocytes and what is their function?

Answer 6

Mature bone cells found inside lacunae
Numerous processes lying in canaliculi
Canaliculi connect lacunae to each other  allows communication between cells
Function: not well understood, may maintain bone matrix & sense mechanical forces

Question 7

Characteristics of ECM in bone?

Answer 7

Mainly minerals, protein (collagen) & water
Modified Type I collagen fibres
Strongly cross-linked & large ‘gaps’ within fibres
Gaps provide space for hydroxyapatite crystals
Collagen fibre orientation linked to mechanical forces

Question 8

Disorders of remodelling?

Answer 8

Osteoporosis- resorption > formation
Pagets Disease -excessive resorption and formation
Osteopetrosis- bone becomes very dense and brittle as less bone resorption

Question 9

What controls bone mass?

Answer 9

Bone mass is controlled by both genes and environment (e.g. smoking).

Age- bone mass increases during childhood
-really increases in puberty due to long bone growth
-men have longer growing period and accumulate higher bone mass for longer
-at menopause, estrogen inhibits work of osteoclasts

Question 10

What other factors influence bone mass?

Answer 10

Weightlessness (astronauts) or bed rest causes significant bone loss

Higher bone mineral density (BMD) in athletes

Resistance & high impact sports particularly effective in maintaining or increasing BMD (e.g. running more effective than cycling)