Bone (re)modelling dynamics Flashcards
what are the 4 types of bone cells
osteoclasts
osteoblasts
osteocytes
bone lining cell
what is the function of osteoclasts
reabsorb/break down bone
what are osteoclast cells
multinucleated cells
formed by fusion of monocytes originated in bone marrow
what is the function of osteoblasts
form bone
(blasts are builders)
what are osteoblast cells
mononuclear cells that produce osteoid
formed from mesenchymal stem cells from the bone marrow
what is osteoid
organic component of bone tissue
what cell are osteoclasts closely related to
macrophages - migrate through all tissues and remove debris adn pathological material
what cell are osteoblasts closely related to
fibroblasts - produce structural molecules in other tissue
how fast do osteoclasts work
move at 10s of microns per day
what is the process that osteoclasts use to reabsorb bone
- demineralise bone with acids
- dissolve collagen with enzymes
(2 step process for 2 main parts of bone)
how fast do osteoblasts work
move at ~1 micron per day
(10x slower than clasts)
what are bone lining cells
osteoblasts that escape from osteoid and live on the surface of the bone
what is the function of bone lining cells
doesn’t do much
- believed to sense loading
what are osteocytes
osteoblasts that have become buried in bone (can also be considered a bone forming cell)
- very large SA in bone
where are osteocytes found
sit in lacunae (~15000 lacunae per mm3)
communicate with each other via dendritic processes called canaliculi
what is the function of osteocytes
mechanosensitive cells (respond to stress and strain)
primarily responsible for planning / orchestrating bone remodelling and adaptation
what is bone modelling
independent action of osteoclasts and osteoblasts on different surface (not acting in unison together)
what is the function of bone modelling
produce large scale changes in bone size and shape
when is bone modelling the most active
during growth and development
- never goes away but easier to do large scale changes during development so declines with age
what is bone remodelling
coupled seqeuntial action of clasts and blasts
(removes a small portion of bone and replaces with newly formed bone)
what is the function of bone remodelling
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
when does bone remodelling occur
occurs throughout life but decreases after growth
where does bone remodelling occur
can only happen on a surface
how does remodelling occur in trabecular bone
clasts make divet in bone surface and blasts fill it in
(way more common and faster than cortical)
how much bone remodelling occurs in trabecular bone
about 25% new per year
how does remodelling occur in cortical bone
clasts make a cave in teh bone and blasts fill in behind them
- requires a blood vessel
- much slower than trabecular bone
what is a BMU
basic multicellular unit
- anytime clasts and blasts are working together
how fast is remodelling accomplished by BMUs
about 40 microns per day
what are the three steps in the ARF seqeunce of BMU remodelling
- activation
- reabsorption
- formation
explain the activation stage of BMU remodelling (process and time)
- differentiation of precursor cells to produce osteoclasts
- 3 days
explain the reabsorption stage of BMU remodelling (process and time)
- osteoclasts start to reabsorb bone (abotu 40 microns per day)
- 30 days
explain the formation stage of BMU remodelling (process and time)
- osteoblasts appear and begin to refill (leave vascular loop to support metabolism of the BMY and carry nutrients)
- 3 months
how long does the whole process of BMU remodelling take
10-12 months total
(can take another 6 months after formation stage to be fully mineralised)
why does BMU activation rise around 55yo
loss of estorgen leads to no inhibiton of osteoclasts and reabsorption
