Bone growth, modelling and remodelling Flashcards
how is bone continuously being formed and turned over?
by modelling and remodelling
where does bone modelling mainly take place?
principally in children - in the growing skeleton, where new bone is formed at different sites to those resorbed - net bone formation
is there much osteoclastic resorption in bone modelling?
no - little resorption, except change in the shape of the skeleton e.g. shape of long bones, size of vertebrae
where is most bone remodelling?
main process in adults
is there any change in bone mass in bone remodelling?
no net change, balanced and steady state - bone formation only occurs where bone resorption has previously occurred
is there coupling between bone modelling and remodelling
yes - they don’t occur at random
why does bone remodelling take place?
- -calcium homeostasis
- -skeletal homeostasis (bone mass)
- normal shaping of bones, increasing dimensions of specific bones, fracture repair
explain how remodelling maintains calcium homeostasis?
Ca2+ can be released from bone by osteoclasts, to restore lowered plasma Ca2+ levels, and vice versa
- set by interaction of the three target organs (intestine, kidney and bone) and is controlled mainly by systemic calcium regulating hormones
explain how remodelling maintains skeletal homeostasis?
steady state needs to be maintained : resorption = formation
- if resorption is smaller than formation then increase in bone mass if other way round decrease
what is a disease where increased bone mass occurs?
osteopetrosis
what is a disease where decreased bone mass occurs?
osteoporosis
rate of remodelling - trabecular bone
- 20% of the skeletal mass
- yet accounts for 80% of turnover
rate of remodelling - cortical bone
- 80% of bone
- only 20% of turnover
what is the basic unit of bone turnover?
basic multicellular unit (BMU)
- found in both cortical and trabecular bone
remodelling mechanism
Activation- resorption- formation (ARF) sequence
- recruitment / activation of osteoclastic bone resorption
- followed by reversal: recruitment/ activation of osteoblastic bone formation
what is the transition between resorption and making of bone referred to as? what is it marked by?
zone of reversal, marked by reversal or cement lines
what is the function of the growth plate
important site of bone remodelling - during endochondral bone development where there is an ARF sequence
what can altered growth plate development result in?
affect the subsequent bone remodelling in an endochondral bone formation sequence
what regulates resorption?
- systemic hormones
- growth factors/ cytokines
- local factors
- nuclear transcription factors (only affects individual cells)
how do systemic hormones regulate resorption?
remodelling rate is increased by PTH (parathyroid hormone), T3 (tri-iodothyronine), GH (growth hormone) and VIT D3 and is decreased by CT (calcitonin), oestrogen and glucocorticoids
what growth factors/ cytokines regulate resorption
bone morphogenetic proteins
haematopoietic cytokines
what growth local factors regulate resorption
prostaglandins
mechanical forces in regulation - how does bone accommodate loads that are imposed on them?
undergo alterations in mass, external geometry and microarchitecture
mechanical forces in regulation - what do microfractures do?
stimulate remodelling - bone is laid down along lines of maximal compressive or tensile stress (Wolff’s law)
- osteocytes
which cells are best suited to respond to forces?
osteocytes
- they are the mechanosensors of the bone -drive the modelling processes
how are osteons formed?
- Woven bone is destroyed by osteoclasts in the Howships Lacunae
- Blood vessels then penetrate the gaps as concentric lamellae are formed by osteoblasts which lay down osteoid
- Interstitial Lamellae are remnants of former osteons that have been altered by bone remodelling
other bone diseases
Paget'd disease Periodontal Chondrodysplasia Fracture healing bone tumours metastases - factors are secreted that turn on uncontrollable osteoclasts
