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Flashcards in Physiology of Bone Deck (34)
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1

What are the functions of the skeleton?

  • Enables movement by providing a site for muscle attachment. 
  • Protects the vital organs. 
  • Provides structural support. 
  • Storage of minerals (Ca2+, PO43-, Mg).
  • Haematopoiesis - blood cell formation in the bone marrow. 

2

Describe the structure of bone.

  • 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. 

3

Describe the structure of a long bone.

  • 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

4

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

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

5

Describe the 2 main types of bone.

  • 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

6

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

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

7

Why are horizontal trabeculae important?

They are important for strength.

8

Describe the blood supply to and from long bones.

  • 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). 

9

What is the functional unit of cortical bone?

An osteon

10

Describe the structure of compact bone in detail.

  • 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. 

11

What is the functional unit of trabecular bone?

A trabecula

12

Describe the structure of trabecular bone in detail.

  • 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).

13

Describe the composition of bone matrix.

  • 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

14

Describe some common disorders of bone matrix.

  • 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

15

What is an osteoblast?

Describe its structure.

  • 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. 

16

What is an osteoclast?

Describe its structure.

  • 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.

17

What is an osteocyte?

Describe its structure.

  • 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

18

What are osteoprogenitor cells?

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

19

What controls the equilibrium between osteoblast and osteoclast activity?

  • 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. 

20

Describe the trabecular bone remodelling cycle.

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

21

Describe cortical bone remodelling.

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

22

Describe how bone is remodelled throughout life.

  • 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

23

What happens to bone during menopause?

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

24

What happens to trabecular bone during osteoporosis?

25

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

26

What are the modifiable risk factors for osteoporosis?

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

27

What are the non-modifiable risk factors for osteoporosis?

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

28

How does bone develop from the fetus to the adult?

  • 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. 

29

Describe fetal endochondral ossification.

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

30

Describe the endochondral ossification which occurs after birth.

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