Lecture 22 - Bone Structure And Remodelling Flashcards
Osteon (haversian system)
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
Lamellae
Lamellae contain collagen fibres that run in different directions in adjacent rings, to withstand stress and resist twisting.
central (Haversian) canal
runs through core of the osteon and contains blood vessels and nerve fibres.
Perforating (Volkmann’s) canals
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.
Lacunae
are small cavities that contain osteocytes.
Canaliculi
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.
Interstitial lamellae
lamellae that are not part of osteon.
Some fill gaps between forming osteons; others are remnants of osteons cut by bone remodelling.
Circumferential lamellae
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.
Spongy bone
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
Trabeculae
like cables on a suspension bridge, confer strength to bone.
Osteoid
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.
Resilience of bone is due to…
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.
Hydroxyapatite’s (mineral salt)
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.
Bone vs steel
Bone is half as strong as steel in resisting compression and as strong as steel in resisting tension.
Bone remodelling
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.
Bone deposit
New bone matrix is deposited by osteoblasts
Calcification front
a transition zone between the osteoid seam and the older mineralized bone.
Triggers for bone deposit
may include mechanical signals, increased concentrations of calcium and phosphate ions and matrix proteins or enzymes to bind calcium and phosphate ions.
Bone resorption
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.
Parathyroid hormone
stimulates osteoclasts to resorb bone, releasing calcium into the blood.
Calcitonin
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.
Wolffs law
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.
Mechanical stress
causes remodeling by producing electrical signals when bone is deformed.
Determines where remodelling occurs
Compressed and stretched regions are oppositely charged.
What does compression and tension change in bone?
fluid flows within canaliculi
may stimulate remodeling.
Hormonal controls
determine whether and when remodeling occurs in response to changing blood calcium levels
