the ageing skeleton W10 Flashcards
types of bone cells and their features?
osteocytes - maintain bone tissue
osteoblasts - produce bone matrix
osteoclasts - resorb bone
osteogenic cells - precursor cells that differentiate depending on mechanical environment
how are osteoclasts formed?
osteoclasts are formed by fusion of multiple cells from macrophage/monophage lineage, ending up as multinucleated osteoclast precursors.
what membrane protein is present on all osteoclasts (and their progenitor cells)?
RANK is the membrane protein present on all osteoclasts
what is the signalling molecule for RANK?
RANK-ligand (RANK-L)
what is RANK-L released by? what is this release regulated by?
RANK-L is released by osteocytes and osteoblasts on the bone surface. its release is regulated by local and systemic factors like local mechanical environment or systemic release of PTH.
what do osteoblasts secrete in addition to RANK-L? what is its action?
osteoprotegrin (OPG)
decoy receptor. irreversibly binds to free RANK-L preventing it from binding to an osteoclast precursor and removing its ability to stimulate bone resorption.
what does OPG inhibit
OPG inhibits bone resorption
why do bones remodel?
repair micro- and macro-damaged bone
allow bone to respond to changing loads
act as a warehouse for storage and release of calcium
steps in the bone remodelling cycle?
quiescence
activation
resorption
reversal
formation
mineralisation
key things to remember from calcium homeostasis?
PTH triggers increase in RANK-L secretion by osteoblasts leading to more bone resorption and therefore increase in serum calcium
calcitonin from thyroid inhibits osteoclast activity, results in less bone resorption, less release of calcium, and therefore lowered serum calcium
when is peak bone mass typically achieved?
between 25 and 30
after what age does bone mass begin to decrease? what is this mainly due to?
40
mainly due to reducing estrogen levels
(in males this starts with decreased testosterone release, but testosterone is broken down to estrogen)
what is the fracture threshold?
rough value representing upper limit of bone density before osteoporotic insufficiency fractures begin to occur with normal or low energy sources
common osteoporotic insufficiency fractures? (in females around age of 60)
what are these usually caused by?
distal radius fracture
proximal humerus fracture
often caused by simple falls
how does oestrogen have a protective effect on bone?
prior to menopause, oestrogen acts directly on osteoblasts to limit amount of RANK-L that the osteoblasts release
during menopause when oestrogen levels drop sharply, this inhibitory action is lost and osteoblasts secrete increased amounts of RANK-L therefore recruiting more osteoclasts. net results is bone lost quicker than it is created.
