Bone structure Flashcards
What are the functions of the skeletal system?
Blood cell production
Support
Protection of organs
Movement (leverage)
Storage of minerals and lipids
Endocrine signalling - skeleton in control of other tissue
What are the different bone shapes?
Long - humerus
flat - parietal
short - carpal bones
irregular - vertebra
What is the basic composition of bone?
composite material:
Organic component (Collagen I 90%)
Mineral reinforcement (hydroxyapatite) - 50% of bone volume but 75% of bone weight - excellant strength to weight ratio
What is the basic structure of bone?
two types of bone:
Cancellous bone - spongy and trabecullar
Cortical bone - dense and compact
What are the main constituents of bone?
collagen (wt 20%), Calcium phosphate (69%), water (9%)
Describe the structure of collagen?
structural protein - type 1 gives bone flexibility and strength
Formed as chains - twist into triple helices (strings) - helices form from anaoscale bundles of protein
Firbrils arranged in layers, mineral crystals deposit between layers
Describe the structure of mineral in bone
Essential for hardness and rigidity that enable
skeleton to resist loading
Main comp of bone = hydroxyapatite
50% of bone volume, 75% of dry bone mass
Minerals crystals deposited along bone collagen fibres of osteoid (newly formed bone) soon after deposition
Describe the structure of the osteon in cortical bone
consists of central canal surrounded by concentric rings of lamellae
Haversian canals contain blood vessels that connect with vessels on surface of bone
Transport system for nutrients
Each canal contains one/two cappilaries and nerve fibres
What are interstitial lamella in cortical bone?
Packets of bone between osteons
What are canaliculi?
canals connecting lacunae to each other and to haversian canals
What are the rates of bone formation and their effects?
Woven bone: fast
irregular collagen fibres & many osteocytes
Lamellar bone: Slow
Regular collagen fibres & few osteocytes
Describe the structure of cancellous bone
Highly porous bone
20% of skeletal mass but 80% of bone surface
less dense, more elastic and higher turnover rate than cortical
Consists of microscopiic framework of plates and rods (trabeculae)
Large pores - contain marrow and cells
Canaliculi receive nutrient supply from bone marrow
Highly reisistant to compressive loads
Trabeculae organised to provide max strength - along lines of stress trajectories
What is trabeculae
few lamellae and cells
cell nutrient supply by canaliculli
What is the inside and outside of bone called?
inside - endosteum
outside - periosteum
What are osteoprogenitor cells?
Stem cell whose division produce osteoblasts
What are osteoblasts?
Immature bone cell that secretes organic components of the matrix
What are osteocytes?
Mature bone cell that maintains the bone matrix
What are osteoclasts?
Multinucleate cells that secretes acids and enzymes to dissolve bone matrix
What do osteoblasts do?
Make proteins that form bone matrix
and control mineralization.
Become entrapped in matrix and
differentiate into osteocytes
Others remain on new bone and
differentiate into lining cells
describe lining cells
osteoblasts remain on new bone and differentiate into lining cells:
Cuboidal in shape
On bone surface
Single nucleus
10 um in diameter
Make approx 1um matrix/ day
What do osteocytes do?
most abundant cell type - 90-95% of cells
Reside within evenly distributed lacunae
Cell processes connect enabling communication between osteocytes and bone lining cells
Channels (canaliculi) radiate from lacunae to osteonic (haversian) canals to provide nutrients
Secrete growth factors to activate
lining cells or stimulate osteoblasts
and osteoclasts
60 processes (dendritics)
One nucleus
10um in diameter
Full differentiated
Attached to bone via tethering elements
direct bone remodelling to accomodate mech strain and repair fatigue damage
What do osteoclasts do?
Large multinucleated cells
Mature osteoclasts are formed from fusion of
precursors
Receptors on the osteoclast precursors
are activated by factors secreted by
osteoblasts and osteocytes
Osteoclasts resorb bone and then undergo apoptosis
Originate from monocytes
Approx 10 nuclei
100um in diameter
Form sealed compartments with bone and secrete acids and enzymes from their ruttled border that resorbs bone
Describe the process of endochondral ossification
During embryonic development of long bones, cartilaginous tissue develops into bone
Chondrocyte proliferation and hypertrophy
Mineralisation
Describe the process of bone growth
- Perichondrium becomes the periosteum. Contains osteoprogenitors which differentiate into osteoblasts.
- Formation of bone collar: Osteoblasts secrete osteoid which stabilises the cartilage template.
- Calcification: Chondrocytes in the centre stop secreting collagen and start secreting an enzyme called alkaline phosphatase which enhances mineralisation. Chondrocytes swell and undergo apoptosis
- Invasion of periosteal bud: hyperthrophic chondrocytes secrete VEGF which induces blood vessel formation. Blood vessels bring HSCs and MSCs which make up bone marrow
- Formation of trabeculae: MSCs differentiate into osteoblasts which make bone inside cavity
Describe the process of intramembranous ossification
Osteoprogenitor cells differentiate into osteoblasts
Osteoblasts lay down matrix (osteoid)
Mineralization
Describe the process of bone modelling
After ossification, bone differentiation continues within the tissue.
Osteoclasts and osteoblasts build the characteristic microstructure of bone.
Describe the process of bone remodelling
After tissue has matured, osteoclast resorption and osteoblast formation
continually maintain bone - Remove microcracks and damage
What is the BMUs?
Collab of osteoclasts and osteoblasts
Describe bone remodelling in cortical bone
BMU forms a cylindrical canal, burrowing at a speed of 20-40μm/day
Cutting cone ~ 10 osteoclasts which dig a circular tunnel
Closing cone: Thousands of osteoblasts that fill the tunnel to produce an osteon of renewed bone
Tunnel: 250-350 um in diameter
Describe the process of bone remodelling in cancellous bone
Same sequence as cortical bone - bone laid in pancake like packets
What is osteoporosis?
debilitating bone disease - occurs through imbalance in remodelling process
What affects remodelling dynamics?
birthrate of BMUs (ac.f) - higher = higher number of secondary osteons (large canals, more lamellae, considered weaker
Speed of BMU - sigma period (orc)
What does the sigma period do?
dictates longevity of an osteon
days taken to remodel a fixed 2D slice of bone
120 days for complete BMU to pass a plane
- 20 days osteoclasts increasing diameter
- 10 days of quiescence
- 90 days of deposition by osteoblasts
What are the bone anabolic responses?
Long bones are thick walled tubes
Can carry loads but light weight
Periosteal bone formation increases second moment of area
New bone most effective if at regions of high stress
Increase in second moment of area
Increase in resistance to bending and torsional loading
what is wolffs law?
Trabecular trajectories roughly align with the direction of max principal
stress
What are the 3 rules that govern bone adaptation?
Rule 1: Dynamic stimulus
Dynamic strains induce bone formation, not static
Loading frequency
Rule 2: Case of diminishing returns
extended duration of loading does not yield proportional increases in bone mass
Cells become desensitised
Rule 3: Bone cells accomodate to routine loading
Bone cells have memory of previous mechanical environment
Cytoskeletal and ECM remodelling
Greatest affect on bone formation is the intial loading stimulus
How is bone lost
disuse results in low stress
Decreased formation and increased bone turnover/resorption
