Physiology of Bone

Question 1

What are the functions of the skeleton?

Answer 1

• Enables movement by providing a site for muscle attachment.
• Protects the vital organs.
• Provides structural support.
• Storage of minerals (Ca²⁺, PO₄^3-, Mg).
• Haematopoiesis - blood cell formation in the bone marrow.

Question 2

Describe the structure of bone.

Answer 2

• Collagen fibre framweork in a mucopolysaccharide-rich semisolid ‘ground substance’.
  
  • This gives bone tensile strength.
• Hardened by precipitation of calcium phosphate (hydroxyapatite) crystals within matrix.
  
  • This gives bone compressional strength.
  • Cartilage is similar to bone but is not calcified.
• Structural strength near reinforced concrete but lighter.
• Made of osteoblasts, osteoclasts and osteocytes.
• Supplied by blood vessels and nerves.
• Contains bone marrow.

Question 3

Describe the structure of a long bone.

Answer 3

• Epiphysis (head)
  
  • Flared bone at the end
  • Articular surface
  • Covered by hyaline cartilage
• Diaphysis (shaft)
  
  • Hollow cylinder
  • Contains bone marrow in marrow cavity
    
    • RBC formation
  • Nurtient foramen for main blood supply to the bone
• Periosteum
  
  • Fibrous connective tissue sheath covering external surfaces. Cells include:
    
    • Fibroblasts - synthesise collagen.
    • Mesenchymal cells - can differentiate into osteoblasts and chondroblasts.
    • Osteoclasts

Question 4

What two things must be in balance for bone mineral density to be maintained?

Answer 4

Osteoblast activity (building bone) and osteoclast activity (destroying bone).

Question 5

Describe the 2 main types of bone.

Answer 5

• Trabecular / cancellous bone
  
  • ​Spongy and porous
  • Gives supporting strength to the ends of the weight bearing bone.
• Cortical bone
  
  • ​Solid
  • Bone on the outside forms the shaft of the long bone.
  • Provides stiffness and strength

Question 6

Describe how trabeculae are laid down and what their function is.

Answer 6

• Trabeculae are laid down along lines of stress.
• They allow distribution of stresses on the bone.

Question 7

Why are horizontal trabeculae important?

Answer 7

They are important for strength.

Question 8

Describe the blood supply to and from long bones.

Answer 8

• Haversian canals carry blood along the long axis of the bone.
• Volkmann’s canals carry blood perpendicularly.
• Majority of cells in very close contact to blood vessels (micrometers away), but, not all cells are in direct contact with blood supply (osteocytes).

Question 9

What is the functional unit of cortical bone?

Answer 9

An osteon

Question 10

Describe the structure of compact bone in detail.

Answer 10

• Haversian canals carry blood along the bone.
• Cells are arranged in concentric circles (like onions).
• Inside each layer are collagen fibres.
• Fibres in each layer can be orientated differently - very flexible.
• Structure requires calcium hydroxyapatite to add strength and harden it.
• These components make bone neither too brittle nor too flexible.

Question 11

What is the functional unit of trabecular bone?

Answer 11

A trabecula

Question 12

Describe the structure of trabecular bone in detail.

Answer 12

• Fewer lamellar layers in the ‘spongy’ trabecular region.
• Less coordinated (weaker and more flexible) than compact bone.
• More open, less dense, site of haemopoiesis in bone marrow.
• Bone surface area of trabecular bone is far more accessable.
• These are sites of easy access (easy exchange).

Question 13

Describe the composition of bone matrix.

Answer 13

• Organic matrix:
  
  • Mostly protein fibres - collagen
    
    • Collagen is highly organised in parallel arrangement (in cotrtical and trabecular bone).
  • Crystallised mineral salts
    
    • Hydroxyapatite Ca₁₀(PO₄)₆(OH)₂\
  • Water

Question 14

Describe some common disorders of bone matrix.

Answer 14

• Rickets
  
  • Caused by vitamin D deficiency
  • This results in failure of Ca²⁺ absorption as vitamin D enables calcium absorption
• Scurvy
  
  • Caused by vitamin C deficiency
  • This leads to a lack of collagen

Question 15

What is an osteoblast?

Describe its structure.

Answer 15

• Bone forming cell
• Covers the surface of bone, forming an osseous matrix in which it becomes enclosed as an osteocyte.
• Mononucleate cells, derived from osteoprogenitor cells that line the surface of bone.
• When stimulated to form bone, it will deposit organic matrix (collagen) then hydroxyapatite.
• Some become entombed during this process = maturation to osteocytes.

Question 16

What is an osteoclast?

Describe its structure.

Answer 16

• A large, multinucleated cell derived from haematopoietic cells, in response to mechanical stresses and physiological demands they resorb bone matrix by demineralisation.
• Multinucleate - several cells fuse together.
• They form a ‘sealing zone’ on bone.
• They release H⁺ and hydrolytic enzymes to dissolve the mineral, liberate calcium and break down the extracellular matrix.
• Regulated by hormones (eg. estrogens) and osteoblasts.

Question 17

What is an osteocyte?

Describe its structure.

Answer 17

• A bone cell.
• Trapped ‘retired’ osteoblasts.
• Mature bone cells embedded in lacunae, relatively inactive.
• Maintain bone matrix through cell-to-cell communication (via projections in canaliculi) and influence bone remodelling.
• Mechanosensing cells

Question 18

What are osteoprogenitor cells?

Answer 18

Stem cell population which gives rise to osteoblasts, among other cells.

Question 19

What controls the equilibrium between osteoblast and osteoclast activity?

Answer 19

• Signalling between the different cells in bone and the action of hormones:
  
  • Calcitonin decreases the activity of osteoclasts
    
    • This causes decreased blood calcium levels.
  • Parathyroid hormone (PTH) increases the activity of osteoclasts.
    
    • This causes release of calcium.

Question 20

Describe the trabecular bone remodelling cycle.

Answer 20

• Can be either to deal with microdamage, or scheduled repair or turnover.
• Has a role in regulating calcium.

Question 21

Describe cortical bone remodelling.

Answer 21

• Osteoclasts reabsorb bone.
• Osteoblasts lay down new matrix, eventually get surrounded and become…
• … OSTEOCYTES

Question 22

Describe how bone is remodelled throughout life.

Answer 22

• Bone mass and density can increase:
  
  • Caused by excessive mechanical stimulation.
• Bone mass and density can decrease:
  
  • Non-weight bearing (immobilisation)
  • Sex-hormone deficiency (eg. menopause)
  • Endocrine / nutritional disorders
• Wolff’s law
  
  • Bone adapts to the load under which it is placed.
  • Weight-bearing exercise, orthodontic braces

Question 23

What happens to bone during menopause?

Answer 23

• Bone dissolution is greater than bone formation in menopause so bone density decreases.
• Decrease in bone density can result in fractures.

Question 24

What happens to trabecular bone during osteoporosis?

Answer 24

Question 25

Describe the age-related changes in bone mass in both males and females.

Answer 25

Question 26

What are the modifiable risk factors for osteoporosis?

Answer 26

• Lifestyle
  
  • Increased weight-bearing exercise
  • Adequate calcium intake
  • Avoidance of excess alcohol
  • Avoidance of smoking
  • Fall prevention

Question 27

What are the non-modifiable risk factors for osteoporosis?

Answer 27

• Biological sex
• Age (but not everyone gets osteoporosis)
• Family history
• Race
• Previous fractures give high risk of another

Question 28

How does bone develop from the fetus to the adult?

Answer 28

• First, a cartilage model is laid down.
  
  • Formed by chondrobasts.
  • Reshaped by chondrocytes.
• Replaced by bone - this is ossification.
  
  • Endochondral ossification in long bones.
  • Intramembraneous ossification in flat bones.
• Bone growth begins in the shaft during fetal life.

Question 29

Describe fetal endochondral ossification.

Answer 29

• Ossification begins in the diaphysis.
• Primary ossification centre is active before birth.

Question 30

Describe the endochondral ossification which occurs after birth.

Answer 30

• A secondary ossification centre develops in the epiphysis.
• After birth bone begins to develop in the ends of the bone.

Question 31

Describe bone growth at the epiphyseal plate.

Answer 31

• Bone is laid down in the shaft and in the head.
• The bony parts are separated by a plate of cartilage.
• For as long as the plate of cartilage is present and active, the bone will increase in length.

Question 32

Describe the components of the epiphyseal plate.

Answer 32

• Ossification zone
  
  • ​Disorganised bone from calcification zone becomes remodelled / reorganised.
• Calcification zone
  
  • ​Mineral layer when swelling of chondrocytes causes spontaneous mineralisation.
• Hypertrophic zone
  
  • ​Chondroblasts become entombed and differentiate into chondrocytes.
  • Chondrocytes swell by increasing cytoplasm.
• Growth / proliferating zone
  
  • ​Chondroblasts undergo mitosis and increase in number.
• Resting zone
  
  • ​Chondroblasts putting down the cartilage model at epiphyseal plate.

Question 33

When does fusion of epiphyseal plates occur?

Answer 33

Occurs by the end of puberty

Question 34

Describe what happens in response to a fracture.

Answer 34

• Depends on cells in the local periosteum.
• Takes 2-4 weeks for healing
  
  • This depends on the severity, position of the fracture and the age of the patient.
• Inflammation and additional blood flow lead to callus formation.
  
  • ​Osteoblasts quickly form woven bone, to bridge the gap.
  • Woven bone is weak as the collagen fibres are irregular.
• Lamellar bone is laid down.
  
  • Collagen is organised in regular sheets to give strength and resilience.