12. Physiology of bone repair

Question 1

Bone physiology basics

Answer 1

Healthy Bone Physiology is a balance
between:
Bone resorption
Bone formation

imbalance on bone resorption side leads to osteopetrosis (osteopenia, rickets)

imbalance on bone formation side -> osteoperosis

Question 2

Classifications of bone structure

Answer 2

• Long bone
	• Flat bone
Macroscopic:
	• Cortical bone
	• Cancellous (spongy)
		○ Spicules, trabeculae (bits and pieces)
Microscopic
	• Lamellar
		○ Osteons
	• Woven
		○ Immature
		○ Disorganised

Question 3

Composition of bone

Answer 3

living cells and acellular matrix
3 principle cell types:
osteoclasts
osteoblasts
osteocytes
Mostly extracellular matrix

Question 4

Osteoblasts

Answer 4

on surface bone, produce protein component acellular matrix – regulate bone growth and degradation

Question 5

Osteocytes

Answer 5

quiescent mature cells embedded in bone matrix.

Maintain bone.

Question 6

Osteoclasts

Answer 6

responsible for bone degradation and remodelling

Question 7

Organic vs inorganic components of bone

Answer 7

Organic – cells and proteins
Inorganic – minerals, eg Ca2+ & PO4- (hydroxyapatite)

bone dominated by extracellular matrix - few cells

Question 8

Haversian system in lamellar bone

Answer 8

As opposed to woven bone

Communication system between cells immobilised in bone matrix

Osteons with Lamellae surrounding Haversian canals in centre to allow fluid movement

Osteocytes embedded in canaliculi in osteon

Ground substance between cells

Haversian system runs parallel to bone and along long axis of bone

Question 9

Where do osteocytes come from?

Answer 9

From mesenchyme
From precursor cells in bone marrow stroma
Osteoblasts are post-mitotic
Most osteoblasts will undergo apoptosis
Number of osteoblasts decrease with age
A low % of osteoblasts will become osteocytes locked in lacuna

Question 10

Osteoclasts

Answer 10

Function: Resorption
Multinucleate
40-100 micrometer in diameter. 
15-20 closely packed oval-shaped nuclei.
Can proliferate

Same precursor as monocytes (haematopoietic stem)
- Phagocytose (bone matrix & crystals)
- Secrete Acids
- Secrete proteolytic enzymes from lysosomes
Ruffled border = where bone resorption occurs

Question 11

Bone constituents

Answer 11

Extracellular matrix is 70% minerals
Plus abundant proteins and sparse cells
High compressive strength and tensile strength

Question 12

Acellular elements of bone

Answer 12

collagen fibres – protein, flexible but strong

hydroxyapatite – mineral, provides rigidity

calcium/phosphate crystals > 50 %

Question 13

What are glycosaminoglycans

Answer 13

long polysaccharides
Highly negative
Attract Water
Repel each other
Resists compression

Abundant in Cartilage

These are another set of organic molecules in bone ground matrix

Question 14

Growth factors in bone ECM

Answer 14

Growth factors are suspended in matrix

They are revealed by osteoclast action

Which leads to proliferation & mineralisation

bone remodelling = bone turnover = the activation-resorption-formation sequence

Question 15

Bone cells remodel bone

Answer 15

1. Osteoclasts resorb bone to make Howship’s lacuna, then migrates away
2. Osteoblasts deposit bone onto pre-existing bone

osteoblasts lay down bone but also encourage osteoclasts

bone remodelling is very active

Question 16

Bone formation

Answer 16

bone forms either as compact or cancellous and by either intramembranous or endochondral bone formation

Question 17

Endochondral ossification

Answer 17

bone formation based on a cartilage model.
• Chondrocytes proliferate and secrete extracellular matrix and proteoglycans.
• Osteoblasts (derived from osteoprogenitor cells) arrive and then osteoid is laid down and mineralisation begins.
• Precise modelling of the final bone is done by osteoclasts.

Question 18

Intramembraneous ossification

Answer 18

bone formation without a cartilage model.
• Osteoblasts (derived from osteoprogenitor cells) lay down osteoid and begin mineralisation, forming tiny bony spicules.
• Nearby spicules join together into trabeculae (woven bone).

Question 19

Factors governing remodelling

Answer 19

two major factors
Recurrent mechanical stress

calcium homeostasis
Plasma calcium is essential in maintaining structural integrity of skeleton

Question 20

Effect of mechanical stress on bone

Answer 20

strengthens bone

• inhibits bone resorption
  promotes deposition
• Surface osteoblasts & osteocyte network detect stresses
  -skeleton reflects forces acting on it

Without weight bearing
bone rapidly weakens
e.g. bed rest, or lack of gravity e.g. astronauts

Question 21

Bisphosphonates

Answer 21

For osteoporosis
E.g. Alendronate

inhibit osteoclast-mediated bone-resorption

related to inorganic pyrophosphate

• the endogenous regulator of bone turnover
• Accumulate on bone & ingested by osteoclasts
• Interfere with osteoclasts metabolism

Question 22

Teripatide

Answer 22

For osteoperosis
Encourages osteoblast formation of bone
portion of human parathyroid hormone (PTH)
Intermittent application activates osteoblasts more than osteoclasts

Question 23

Denosumab

Answer 23

For osteoperosis
Prevent osteoclast maturation
Monoclonal antibody targetting RANKL

Question 24

Molecular mechanism of osteopetrosis (autosomal recessive)

Answer 24

Molecular Mechanism
Osteoclasts cannot remodel bone
due to:
- Defective Vacuolar proton pump or
- Defective Chloride channel

Question 25

Results of osteperosis

Answer 25

Excess bone growth Bone growths at foramina press on nerves Brittle (dense) bones Blindness Deafness Severe anaemia

Question 26

Phases of fracture healing stages

Answer 26

1. Reactive phase: haematoma and inflammation 2. Soft callus formation 3. Hard callus formation 4. Remodelling Duration: upper body 2-3 weeks, lower body > 4 weeks

Question 27

Hormones of calcium regulation

Answer 27

PTH – parathryoid hormone, parathormone - Parathyroid chief cells - Increases plasma Ca2+ Vitamin D: 1,25-di-OH cholecalciferol (calcitriol) - Made in stages: Skin -> Liver -> Kidney - Increases plasma Ca2+ Calcitonin - Made by thyroid C cells - “tones down” blood calcium - Calcium goes into bone - Used as a treatment for osteoporosis

Question 28

Vitmin D: production and activation

Answer 28

Cholecalciferol (Vitamin D3) -> 25-OH cholecalciferol -> 1, 25-di-OH cholecalciferol (calcitriol) -> increased Calbindin in gut enterocytes -> increased Intestinal absorption of Ca2+ and increased Ca2+ reabsorption in kidneys -> increased plasma Ca2+

Question 29

How does PTH stimulate resorption via osteoblasts?

Answer 29

PTH binds PTH-R on osteoblast, causing RANKL to bind RANK on osteoclast precursor osteoclast precursos differentiates and fuses and forms activated osteoclast activated osteoclast carries out bone resorption

Question 30

Vitamin D

Answer 30

Increases intestinal Ca2+ absorption -> Increases calbindin Stimulates kidneys to reabsorb calcium stimulates osteoclasts indirectly - > via osteoblasts - > This is a comparatively weak effect Vitamin D facilitates bone remodelling and thus increases serum Ca2+

Question 31

Causes of low plasma calcium

Answer 31

Loss Pregnancy Lactation Kidney dysfunction Low Intake Insufficient ingestion of Calcium Rickets (low vit D) Parathyroid dysfunction

Question 32

Vitamin D

Answer 32

Increases intestinal Ca2+ absorption - Increases calbindin Stimulates kidneys to reabsorb calcium stimulates osteoclasts indirectly - via osteoblasts - This is a comparatively weak effect Vitamin D facilitates bone remodelling and thus increases serum Ca2+

Question 33

Results of chronic hypocalcaemia

Answer 33

Skeletal deformities Increased tendency toward bone fractures Impaired growth Short stature (adults less than 5 feet tall) Dental deformities Rickets can be caused by chronic hypocalcaemia

Question 34

Acute hypocalcaemia

Answer 34

Leads to excitability C – Convulsions A – Arrhythmias T – Tetany ``` Chvostek’s Sign (Latent Tetany) Trousseau’s Sign Carpopedal spasm (Latent Tetany) DiGeorge Syndrome ```

Question 35

How does low plasma calcium cause excitability?

Answer 35

Effect seems paradoxical i.e. counter-intuitive Hypocalcaemia makes membranes “more excitable” and “less stable” Sodium is more able to leak through it Explains latent tetany and its signs Hypercalcaemia paradoxically reduces excitability By making membranes more stable

Question 36

Signs and symptoms of decreased excitability

Answer 36

Can be asymptomatic Reduced excitability Esp. Constipation Depression + other psychiatric Abnormal heart rhythms Short QT interval, ST segment gone Widened T wave Severe hypercalcemia Coma Cardiac arrest