Physiology of Bone Flashcards

1
Q

What are the functions of the skeleton?

A
  • 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.
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2
Q

Describe the structure of bone.

A
  • 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.
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3
Q

Describe the structure of a long bone.

A
  • 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
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4
Q

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

A

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

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5
Q

Describe the 2 main types of bone.

A
  • 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
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6
Q

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

A
  • Trabeculae are laid down along lines of stress.
  • They allow distribution of stresses on the bone.
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7
Q

Why are horizontal trabeculae important?

A

They are important for strength.

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8
Q

Describe the blood supply to and from long bones.

A
  • 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).
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9
Q

What is the functional unit of cortical bone?

A

An osteon

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10
Q

Describe the structure of compact bone in detail.

A
  • 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.
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11
Q

What is the functional unit of trabecular bone?

A

A trabecula

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12
Q

Describe the structure of trabecular bone in detail.

A
  • 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).
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13
Q

Describe the composition of bone matrix.

A
  • 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
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14
Q

Describe some common disorders of bone matrix.

A
  • 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
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15
Q

What is an osteoblast?

Describe its structure.

A
  • 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.
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16
Q

What is an osteoclast?

Describe its structure.

A
  • 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
Q

What is an osteocyte?

Describe its structure.

A
  • 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
Q

What are osteoprogenitor cells?

A

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

19
Q

What controls the equilibrium between osteoblast and osteoclast activity?

A
  • 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
Q

Describe the trabecular bone remodelling cycle.

A
  • Can be either to deal with microdamage, or scheduled repair or turnover.
  • Has a role in regulating calcium.
21
Q

Describe cortical bone remodelling.

A
  • Osteoclasts reabsorb bone.
  • Osteoblasts lay down new matrix, eventually get surrounded and become…
  • … OSTEOCYTES
22
Q

Describe how bone is remodelled throughout life.

A
  • 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
Q

What happens to bone during menopause?

A
  • Bone dissolution is greater than bone formation in menopause so bone density decreases.
  • Decrease in bone density can result in fractures.
24
Q

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.
31
Describe bone growth at the epiphyseal plate.
* 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.
32
Describe the components of the epiphyseal plate.
* **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.
33
When does fusion of epiphyseal plates occur?
Occurs by the end of puberty
34
Describe what happens in response to a fracture.
* 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.