Lecture 22 - Bone Structure And Remodelling

Question 1

Osteon (haversian system)

Answer 1

osteon is the structural unit of compact bone.

consists of an elongated cylinder that runs parallel to the long axis of bone, acting as a tiny weight- bearing pillar.

An osteon cylinder consists of several rings of bone matrix called lamellae

Question 2

Lamellae

Answer 2

Lamellae contain collagen fibres that run in different directions in adjacent rings, to withstand stress and resist twisting.

Question 3

central (Haversian) canal

Answer 3

runs through core of the osteon and contains blood vessels and nerve fibres.

Question 4

Perforating (Volkmann’s) canals

Answer 4

lined with endosteum and occur at right angles to the central canal.

They connect blood vessels and nerves of periosteum, medullary cavity, and central canal.

Question 5

Lacunae

Answer 5

are small cavities that contain osteocytes.

Question 6

Canaliculi

Answer 6

are hairlike canals that connect the
Lacunae to each other and to the central canal.

form when matrix hardens and cells are trapped.

These canals allow communication between the osteocytes and permit nutrients and wastes to flow to and from the central canal.

Question 7

Interstitial lamellae

Answer 7

lamellae that are not part of osteon.
Some fill gaps between forming osteons; others are remnants of osteons cut by bone remodelling.

Question 8

Circumferential lamellae

Answer 8

are layers of lamellae that extend around entire surface of diaphysis.
They are located just deep to periosteum, but superficial to endosteum. They help long bones
to resist twisting.

Question 9

Spongy bone

Answer 9

poorly organized but is actually organized along lines of stress to help bone resist any stress.

No Osteons but the trabeculae contain irregularly arranged lamellae and osteocytes interconnected by canaliculi.
Capillaries in the endosteum supply nutrients

Question 10

Trabeculae

Answer 10

like cables on a suspension bridge, confer strength to bone.

Question 11

Osteoid

Answer 11

makes up one-third of organic bone matrix, is secreted by osteoblasts

consists of ground substance and collagen fibres, which contribute to the high tensile strength and flexibility of bone.

Question 12

Resilience of bone is due to…

Answer 12

Sacrificial bonds in or between collagen molecules that stretch and break to dissipate energy and prevent fractures.

If no additional trauma occurs, the bonds can re-form.

Question 13

Hydroxyapatite’s (mineral salt)

Answer 13

makeup 65% of bone by mass.

consist mainly of tiny calcium phosphate crystals in and around collagen fibres and are responsible for hardness and resistance to compression.

Question 14

Bone vs steel

Answer 14

Bone is half as strong as steel in resisting compression and as strong as steel in resisting tension.

Question 15

Bone remodelling

Answer 15

Bone remodelling consists of both bone deposit and bone resorption and occurs at the surfaces of both the periosteum and the endosteum.

Remodelling units are packets of adjacent osteoblasts and osteoclasts coordinate the
remodelling process

5-7% of bone mass is recycled each week.

Spongy bone is replaced approximately every 3-4 years and compact bone is replaced approximately every 10 years.

Question 16

Bone deposit

Answer 16

New bone matrix is deposited by osteoblasts

Question 17

Calcification front

Answer 17

a transition zone between the osteoid seam and the older mineralized bone.

Question 18

Triggers for bone deposit

Answer 18

may include mechanical signals, increased concentrations of calcium and phosphate ions and matrix proteins or enzymes to bind calcium and phosphate ions.

Question 19

Bone resorption

Answer 19

function of osteoclasts.
They dig grooves as they break down matrix by secreting lysosomal enzymes and protons (H+). The acidity converts calcium salts to soluble forms.

Osteoclasts phagocytize demineralized matrix and dead osteocytes.

Digested products are transcytosed across the cell, released into interstitial fluid and then into blood. Once complete, osteoclasts undergo apoptosis.

Question 20

Parathyroid hormone

Answer 20

stimulates osteoclasts to resorb bone, releasing calcium into the blood.

Question 21

Calcitonin

Answer 21

produced by parafollicular cells of thyroid gland in response to high levels of blood calcium levels. Effects are less strong than PTH but it can lower blood calcium levels temporarily.

Question 22

Wolffs law

Answer 22

Wolff’s law states that bones grow or remodel in response to demands placed on them.

Stress is usually off-centre, so bones tend to bend. Bending compresses one side and stretches the other side.

The diaphysis is thickest where
bending stresses are greatest
Bone can be hollow because
compression and tension cancel
each other out in the middle of the bone.

ALSO EXPLAINS…

Being right- or left-handed results in thicker and stronger bones of the corresponding arm.

Bones are thickest where most likely to bend.

Trabeculae form trusses along lines of stress.

Large, bony projections occur where heavy, active muscles attach. Weightlifters have thickenings at muscle attachment sites of most used muscles.

Bones of fetus and bedridden people are featureless because of the lack of stress on bones.

Question 23

Mechanical stress

Answer 23

causes remodeling by producing electrical signals when bone is deformed.

Determines where remodelling occurs

Compressed and stretched regions are oppositely charged.

Question 24

What does compression and tension change in bone?

Answer 24

fluid flows within canaliculi

may stimulate remodeling.

Question 25

Hormonal controls

Answer 25

determine whether and when remodeling occurs in response to changing blood calcium levels