Bone Metabolism Flashcards
What are bone lamellae?
Bone layers
What are the 2 types of bone and that is their proportion?
- cortical/compact (80%)
- cancellous/trabecular/spongy (20%)
What are the properties of compact/cortical bone?
- found on edges of bone
- compressive strength
- from osteons laying on top of each other in circular pattern
What are the properties of cancellous bone?
- inside diaphysis and heads of bone
- mesh like
- relatively strong but bendable for movement
What is the medullary cavity?
- sits inside shaft
- bone marrow
- lightens skeleton and houses haematopoietic cells
What is bone marrow like at birth?
- virtually all bone marrow is red bone marrow
- yellow bone marrow forms peripherally and moves towards central axial skeleton
What is bone marrow like in adults?
- little red bone marrow as don’t need many haematopoietic stem cells
- found in spongy bones of vertebrae, ribs, sternum, cranium and epiphyses of long bones
What is woven bone?
- less resistant to stress
- can be put down quickly
- more disorganised vs. lamellar
- random collagen I organisation
- immature/mechanically weak, healing
What are the 2 categories of bone composition?
- organic (35-40%)
- inorganic (60%)
What are the components of organic bone material?
- collagen type 1
- proteoglycans
- growth factors/cytokines/osteoid
What are the features of collagen type I?
- 90% tensile strength
- rope-like collagen forming long fibres
What are the features of proteoglycans?
- compressive strength as hold water
What produces organic bone material?
- cells = osteoblasts, osteocytes, osteoclasts
What is the main inorganic bone component?
- calcium hydroxyapatite
What are the features of calcium hydroxyapatite?
- precipitates onto surface of collagen fibres
- doesn’t cover it properly otherwise bone would be brittle and unbendable
- forms intermittent crystal precipitate leaving little gaps at regular intervals allowing bendability
Why is bone not picked up on an MRI?
- quite dry -> 5% water
What are the main cells found in bone?
- osteoprogenitor cells
- osteoblasts
- osteocytes
- osteoclasts
What are the features of osteoprogenitor cells?
- CT cells
- from mesenchyme during embryonic development
- sits on either side of periosteum and become osteoblasts if needed
What are the features of osteoblasts?
- secrete mineralised organic component
- become surrounded by it to become an osteocyte
What are the features of osteocytes?
- remain in bone structure and secrete collagen locally
- have canaliculi
- death signals remodelling
What are canaliculi?
- processes from osteocytes
- go into canals through bone matrix
- sense how much bone is bending/compressing/crushed
- to sense if new bone is required
What are the features of osteoclasts?
- from haematopoietic stem cells within bone marrow
- macrophage like cells
What are haversian canals?
- contain blood vessels
Why does bone remodelling occur?
- renews bone deterioration
- redistribution of bone matrix along mechanical stress lines
What is sclerostin?
- glycoprotein
- secreted by osteocytes when healthy
- inhibits osteoblasts
- switched off when osteocyte dies stimulating bone growth
How do osteoclasts and osteoblasts signal?
- bidirectional communication
- via ligands and receptors to inhibit osteoclasts increasing bone production
What is the function of EphrinB2?
- ligand
- on surface of osteoclasts
What is the function of EphB4?
- receptor
- expressed by osteoblasts
What increases ephrinB2 expression?
PTH
What is the difference between trabecular and cortical bone?
- trabecular bone 3-10 times quicker than cortical bone as larger SA and responds to stresses on bone quicker
What do osteoclasts do in bone production?
- signalled by osteocytes (amoeboid movement)
- attach to area and form leak proof seal
- inferior ruffled border secretes vesicle with HCl and protein digesting enzymes
- HCl dissolves calcium hydroxyapatite and bone mineral part
- protein digesting enzymes break down collagen
- end up with mixture of digested collagen, calcium and phosphate salt taken up by osteoclast
- packed into a vesicle and transported across cytoplasm
- release calcium salts into interstitial fluid
What do osteoblasts do in bone production?
- move in and secrete new osteoid which is at first not mineralised
- osteoid mineralised 1 week later
How is bone remodelling done using tetracycline?
- antibiotic taken up by mineralising bone and fluoresces
- check if bone being mineralised
- 2 doses 14 days apart (2 lines indicate mineralisation)
Where is calcium in bone?
- 99% in skeleton
- 1% in cells/blood/body fluids
What is the function of calcium in bone metabolism?
- signalling molecule
- regulates muscle contraction, neuronal signalling, heart beats
What does maintenance of calcium depend on?
- intestinal absorption
- renal absorption
- skeletal mobilisation
What is the function of phosphate in bone production?
- contain in hydroxyapatite
- uses PTH to modulate absorption and excretion
- not as closely regulated as calcium
How is calcium regulated?
- low plasma Ca2+ = PTH secretion from parathyroid gland on back of neck
- high plasma Ca2+ = calcitonin release
What does PTH promote?
- immediately Ca2+ reabsorption from kidney and PO4- excretion
- LT Ca2+ reabsorption from bone increases number and activity of osteoclasts
What does osteoclast formation require?
- RANKL
- M-CSF
(produced by osteoblasts and stromal cells)
What is RANK-L?
- osteoblasts have receptor for PTH
- stimulate osteoblasts to produce RANKL -> RANK (receptor) stimulated on osteoclasts
- causes osteoclast precursor to proliferate and differentiate
What inducers RANK-L?
- vitamin D
What inhibits RANK-L?
- estrogen
What is M-CSF?
Macrophage colony-stimulating factor
What is OPG?
- decoy receptor for RANKL
- modulates signalling on RANK by soaking up RANKL to stop signalling to osteoclast precursor
- induces synthesis of active vitamin D increases calcium absorption from gut
What is the effect of calcitonin?
- released when high calcium
- inhibits osteoclast differentiation and activity
- increases calcium excretion from kidney
- inhibits calcium absorption from intestines
