13. Endocrine and metabolic bone disorders Flashcards
what are the 2 components of bone?
- osteoid (organic component making up 35% bone mass - unmineralised bone made up of type 1 collagen)
- calcium hydroxyapatite crystals (inorganic component making up 65% bone mass - fills the space between collagen fibrils)
what are the 2 types of bone cells?
- osteoblasts - synthesise osteoid and participate in mineralisation/calcification of osteoid (bone formation)
- osteoclasts - release lysosomal enzymes which break down bone (bone resorption)
What is the action of PTH in bone?
PTH inhibits various activities of osteoblasts
PTH stimulates osteoblasts to produce various osteoclast activating factors (OAFs) e.g. RANKL which move to the osteoclasts and bind to the RANK receptor to stimulate the breakdown of bone matrix to release Ca
what does RANKL mean and what is its role?
RANK ligand
it is expressed on the osteoblast surface and binds to a RANK receptor expressed on the osteoclast surface to stimulate osteoclast formation and activity
what do osteoblasts express receptors for?
PTH
calcitriol (1,25 (OH)2 vitamin D)
what are the different types of bone?
- hard, CORTICAL bone around the outside
- spongy, TRABECULAR bone on the inside
what pattern is bone formed in and what does this mean?
lamellar pattern - collagen fibrils are laid down in alternating orientations giving as much mechanical strength as possible
what is woven bone?
an immature type of bone that is much weaker because it doesn’t have the same organisation of collagen fibrils (fibrils are laid randomly)
what is vitamin D deficiency?
inadequate mineralisation of newly formed bone matrix (osteoid)
what does vit D deficiency present as in children?
RICKETS
- affects cartilage of epiphysial growth plates and bones
- skeletal abnormalities (tibia bowing) and pain, growth retardation, increased fracture risk
what does vit D deficiency present as in adults?
OSTEOMALACIA
- affects bone after epiphyseal closure
- skeletal pain, increased fracture risk, proximal myopathy
where are fracture sites typically found?
in places where there is a lot of bone loading
what 2 things start to happen when the kidneys fail?
- calcitriol cannot be made so calcium is not absorbed from the gut very well - hypocalcaemia
- the kidneys excrete less phosphate resulting in raised serum phosphate which binds to calcium, further decreasing the bioavailability of serum calcium - hypocalcaemia
what does hypocalcaemia result in?
inadequate bone mineralisation and increased PTH release which increases bone resorption
both also result in osteitis fibrosa cystica
what can high serum phosphate contribute to?
vascular calcification - there is a high rate of microvascular disease in kidney failure