Bone (re)modelling dynamics

Question 1

what are the 4 types of bone cells

Answer 1

osteoclasts
osteoblasts
osteocytes
bone lining cell

Question 2

what is the function of osteoclasts

Answer 2

reabsorb/break down bone

Question 3

what are osteoclast cells

Answer 3

multinucleated cells
formed by fusion of monocytes originated in bone marrow

Question 4

what is the function of osteoblasts

Answer 4

form bone
(blasts are builders)

Question 5

what are osteoblast cells

Answer 5

mononuclear cells that produce osteoid
formed from mesenchymal stem cells from the bone marrow

Question 6

what is osteoid

Answer 6

organic component of bone tissue

Question 7

what cell are osteoclasts closely related to

Answer 7

macrophages - migrate through all tissues and remove debris adn pathological material

Question 8

what cell are osteoblasts closely related to

Answer 8

fibroblasts - produce structural molecules in other tissue

Question 9

how fast do osteoclasts work

Answer 9

move at 10s of microns per day

Question 10

what is the process that osteoclasts use to reabsorb bone

Answer 10

1. demineralise bone with acids
2. dissolve collagen with enzymes
   (2 step process for 2 main parts of bone)

Question 11

how fast do osteoblasts work

Answer 11

move at ~1 micron per day
(10x slower than clasts)

Question 12

what are bone lining cells

Answer 12

osteoblasts that escape from osteoid and live on the surface of the bone

Question 13

what is the function of bone lining cells

Answer 13

doesn’t do much
- believed to sense loading

Question 14

what are osteocytes

Answer 14

osteoblasts that have become buried in bone (can also be considered a bone forming cell)
- very large SA in bone

Question 15

where are osteocytes found

Answer 15

sit in lacunae (~15000 lacunae per mm3)
communicate with each other via dendritic processes called canaliculi

Question 16

what is the function of osteocytes

Answer 16

mechanosensitive cells (respond to stress and strain)
primarily responsible for planning / orchestrating bone remodelling and adaptation

Question 17

what is bone modelling

Answer 17

independent action of osteoclasts and osteoblasts on different surface (not acting in unison together)

Question 18

what is the function of bone modelling

Answer 18

produce large scale changes in bone size and shape

Question 19

when is bone modelling the most active

Answer 19

during growth and development
- never goes away but easier to do large scale changes during development so declines with age

Question 20

what is bone remodelling

Answer 20

coupled seqeuntial action of clasts and blasts
(removes a small portion of bone and replaces with newly formed bone)

Question 21

what is the function of bone remodelling

Answer 21

to have small local changes in bone
(doesn’t influence bone size and shape)
- removes boen damage and mechanically “fine tunes” the skeleton to best resist laod

Question 22

when does bone remodelling occur

Answer 22

occurs throughout life but decreases after growth

Question 23

where does bone remodelling occur

Answer 23

can only happen on a surface

Question 24

how does remodelling occur in trabecular bone

Answer 24

clasts make divet in bone surface and blasts fill it in
(way more common and faster than cortical)

Question 25

how much bone remodelling occurs in trabecular bone

Answer 25

about 25% new per year

Question 26

how does remodelling occur in cortical bone

Answer 26

clasts make a cave in teh bone and blasts fill in behind them - requires a blood vessel - much slower than trabecular bone

Question 27

what is a BMU

Answer 27

basic multicellular unit - anytime clasts and blasts are working together

Question 28

how fast is remodelling accomplished by BMUs

Answer 28

about 40 microns per day

Question 29

what are the three steps in the ARF seqeunce of BMU remodelling

Answer 29

1. activation 2. reabsorption 3. formation

Question 30

explain the activation stage of BMU remodelling (process and time)

Answer 30

- differentiation of precursor cells to produce osteoclasts - 3 days

Question 31

explain the reabsorption stage of BMU remodelling (process and time)

Answer 31

- osteoclasts start to reabsorb bone (abotu 40 microns per day) - 30 days

Question 32

explain the formation stage of BMU remodelling (process and time)

Answer 32

- osteoblasts appear and begin to refill (leave vascular loop to support metabolism of the BMY and carry nutrients) - 3 months

Question 33

how long does the whole process of BMU remodelling take

Answer 33

10-12 months total (can take another 6 months after formation stage to be fully mineralised)

Question 34

why does BMU activation rise around 55yo

Answer 34

loss of estorgen leads to no inhibiton of osteoclasts and reabsorption