Bone structure

Question 1

What are the functions of the skeletal system?

Answer 1

Blood cell production

Support

Protection of organs

Movement (leverage)

Storage of minerals and lipids

Endocrine signalling - skeleton in control of other tissue

Question 2

What are the different bone shapes?

Answer 2

Long - humerus
flat - parietal
short - carpal bones
irregular - vertebra

Question 3

What is the basic composition of bone?

Answer 3

composite material:
Organic component (Collagen I 90%)
Mineral reinforcement (hydroxyapatite) - 50% of bone volume but 75% of bone weight - excellant strength to weight ratio

Question 4

What is the basic structure of bone?

Answer 4

two types of bone:
Cancellous bone - spongy and trabecullar
Cortical bone - dense and compact

Question 5

What are the main constituents of bone?

Answer 5

collagen (wt 20%), Calcium phosphate (69%), water (9%)

Question 6

Describe the structure of collagen?

Answer 6

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

Question 7

Describe the structure of mineral in bone

Answer 7

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

Question 8

Describe the structure of the osteon in cortical bone

Answer 8

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

Question 9

What are interstitial lamella in cortical bone?

Answer 9

Packets of bone between osteons

Question 10

What are canaliculi?

Answer 10

canals connecting lacunae to each other and to haversian canals

Question 11

What are the rates of bone formation and their effects?

Answer 11

Woven bone: fast
irregular collagen fibres & many osteocytes
Lamellar bone: Slow
Regular collagen fibres & few osteocytes

Question 12

Describe the structure of cancellous bone

Answer 12

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

Question 13

What is trabeculae

Answer 13

few lamellae and cells
cell nutrient supply by canaliculli

Question 14

What is the inside and outside of bone called?

Answer 14

inside - endosteum
outside - periosteum

Question 15

What are osteoprogenitor cells?

Answer 15

Stem cell whose division produce osteoblasts

Question 16

What are osteoblasts?

Answer 16

Immature bone cell that secretes organic components of the matrix

Question 17

What are osteocytes?

Answer 17

Mature bone cell that maintains the bone matrix

Question 18

What are osteoclasts?

Answer 18

Multinucleate cells that secretes acids and enzymes to dissolve bone matrix

Question 19

What do osteoblasts do?

Answer 19

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

Question 20

describe lining cells

Answer 20

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

Question 21

What do osteocytes do?

Answer 21

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

Question 22

What do osteoclasts do?

Answer 22

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

Question 23

Describe the process of endochondral ossification

Answer 23

During embryonic development of long bones, cartilaginous tissue develops into bone
Chondrocyte proliferation and hypertrophy
Mineralisation

Question 24

Describe the process of bone growth

Answer 24

1. Perichondrium becomes the periosteum. Contains osteoprogenitors which differentiate into osteoblasts.
2. Formation of bone collar: Osteoblasts secrete osteoid which stabilises the cartilage template.
3. Calcification: Chondrocytes in the centre stop secreting collagen and start secreting an enzyme called alkaline phosphatase which enhances mineralisation. Chondrocytes swell and undergo apoptosis
4. Invasion of periosteal bud: hyperthrophic chondrocytes secrete VEGF which induces blood vessel formation. Blood vessels bring HSCs and MSCs which make up bone marrow
5. Formation of trabeculae: MSCs differentiate into osteoblasts which make bone inside cavity

Question 25

Describe the process of intramembranous ossification

Answer 25

Osteoprogenitor cells differentiate into osteoblasts
Osteoblasts lay down matrix (osteoid)
Mineralization

Question 26

Describe the process of bone modelling

Answer 26

After ossification, bone differentiation continues within the tissue.
Osteoclasts and osteoblasts build the characteristic microstructure of bone.

Question 27

Describe the process of bone remodelling

Answer 27

After tissue has matured, osteoclast resorption and osteoblast formation
continually maintain bone - Remove microcracks and damage

Question 28

What is the BMUs?

Answer 28

Collab of osteoclasts and osteoblasts

Question 29

Describe bone remodelling in cortical bone

Answer 29

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

Question 30

Describe the process of bone remodelling in cancellous bone

Answer 30

Same sequence as cortical bone - bone laid in pancake like packets

Question 31

What is osteoporosis?

Answer 31

debilitating bone disease - occurs through imbalance in remodelling process

Question 32

What affects remodelling dynamics?

Answer 32

birthrate of BMUs (ac.f) - higher = higher number of secondary osteons (large canals, more lamellae, considered weaker
Speed of BMU - sigma period (orc)

Question 33

What does the sigma period do?

Answer 33

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

Question 34

What are the bone anabolic responses?

Answer 34

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

Question 35

what is wolffs law?

Answer 35

Trabecular trajectories roughly align with the direction of max principal
stress

Question 36

What are the 3 rules that govern bone adaptation?

Answer 36

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

Question 37

How is bone lost

Answer 37

disuse results in low stress
Decreased formation and increased bone turnover/resorption