Cellular structure of bone Flashcards
What are the functions of bone?
- Support and movement:attachment site for muscles
- Protection for internal organs
- Provides home for bone marrow
- Act as a mineral reservoir
- Endocrine function as it is a source of some non-classical hormones
What is the composition of bone?
- Protein:organic osteoid matrix(25%)
- Minerals(75%)
- Cells
What type of collagen does the organic protein matrix have and what does that give?
Mainly type 1 collagen
-Gives both flexibility and tensile strength
What is bone mineral mainly made up of?
Mainly hydroxyapatite
What is bone minerals composed of?
Composed of both calcium and phosphate
What qualities does bone mineral have and what does this give?
Its rigid and brittle:
-Gives high compressive strength
What cells are present in the bone?
- Osteoblasts
- Osteoclasts
- Osteocytes
What cells are present in the bone marrow?
- Mesenchymal stem cells
- Haematopoietic cells
What are osteoblasts ?
They are bone forming cells
What are osteoblasts derived from?
Derived from mesenchymal stem cells
What do osteoblasts secrete?
Secrete osteoid collagen matrix of bone
What do osteoblasts promote?
Promote mineralization of osteoid
What are osteoclasts?
They are bone reabsorbing cells
What are osteoclasts derived from?
Derived from haematopoietic stem cells
Structure of osteoclasts?
Large and multinucleate
What do osteoclasts secrete?
Secrete acid to dissolve bone mineral enzymes to digest organic matrix
What is the life cycle of the osteoclasts controlled by?
Controlled by apoptosis
Where do osteocytes come from?
Come from terminally differentiated osteoblasts
What are osteocytes encased in?
Encased in bone mineral matrix
What do osteocytes extend?
Extend multiple dendrites via minute canals in bone matrix
What does the lacuno canalicular system maintain?
Maintains communication with bone surface and blood vessels
How does bone remodelling occur in cortical bone?
- There is a longitudinal system of canals and spaces
- There will be a leading edge, where cells differentiate into osteoclasts and start to digest the bone
- Osteoblasts differentiate and lay down new bone here
How does bone remodeling occur in trabecular bone?
- Osteoclasts will eat away at the bone and osteoblasts will form new bone
- The lining cells on the surface of the bone detach underneath it and form a basic multicellular unit
What are the stages in bone remodelling?
- Activation
- Promotion of the differentiation of new osteoclasts - Resorption
- Duration of the osteoclast activity
- Removing old bone and creating the pits - Reversal
- Osteoclast apoptosis, terminating osteoclast activity and overlapping with osteoblast differentiation - Formation
- Osteoblasts line the surface, leading to . the formation of osteoid which eventually become mineralised
What are the controls of remodelling?
-Load bearing exercise
-Cytokines and other local signals
-Endocrine
-Oestrogen inhibits osteocyte apoptosis and promotes
osteoclast apoptosis
What is osteoporosis known as?
A loss of bone mass beyond a certain threshold
What is RANK?
Surface receptors on pre-osteoclasts, stimulates osteoclast differentiation
What is RANK LIGAND?
Produced by pre-osteoblasts and osteocytes
What is OPG?
Is a decoy receptor produced by osteocytes and bind to RANK-L, preventing action of RANK
What is the Wnt signalling pathway required for?
Required for osteoblast differentiation
What is Wnt signalling pathway negatively regulated by?
Negatively regulated by DKK and sclerostin
What are the steps in the Wnt signalling pathway?
1.Wnt protein has G coupled receptors called Frizzled
2.Frizzled requires a co-receptor LRP5/6 which both need to interact in order to activate Wnt signalling pathway
3.Beta catenin is inactivated until Wnt binds which causes beta-catenin to be free
-This then accumulates and then translocates to the
nucleus where it initiates transcription
What is osteoporosis pseudoglioma caused by?
By the inactivation of LRP5 with no co-receptor
What is sclerosteosis and van buchem disease caused by?
By the mutation of the SOST gene, inactivating sclerostin protein
What is osteoporosis caused by?
By a mutation which inactivates RANK-L protein
What does osteoporosis cause?
Causes:
- Loss of bone density
- Increased fracture risk
- Increase in bone resorption over formation
