Flashcards in Bone Morphology Deck (56):
What does the organic component of bone contain?
Collagen type I (90%), non-collagen structural proteins (8%), growth factors, bone cells (2%)
What does the inorganic component of bone contain?
Calcium hydroxyapatite (calcium phosphate crystal)
Mg2+, Na+, K+, F-
What percentage of bone do the organic and the inorganic components make up each?
30% and 70%
(40/60% by weight)
What is the function of type I collagen in bone?
Tensile strength and flexibility
What is the function of non-collagen structural proteins in bone?
What is the function of hydroxyapatite is bone?
Hardness and rigidity
What percentage of the skeleton mass does compact bone make up?
What is the function of compact/cortical bone?
High resistance to bending, mechanical strength
Forms the basis of muscle attachment, locomotion
What is the relative turnover rate of compact bone?
Slow, unless in severe prolonged mineral deficits
What is the percentage of the surface area covered by trabecular/cancellous bone?
What is the turnover rate of spongy bone?
What is the outer layer of the membrane surrounding bone?
Describe the organisation of compact/cortical bone.
Highly organised parallel collagen fibrils that are mineralised and form lamellae around a central canal
What is the function of trabecular/cancellous bone?
In areas of high stress
Mechanical support, contributes to metabolic processes by serving as a reservoir of calcium and phosphate, elastic
Describe the two layers of periosteum.
Outer fibrous connective tissue layer, contain fibroblasts.
Inner osteogenic layer, with osteoblast progenitor cells.
What is the endosteum?
Vascular membrane of connective tissue covering internal bone surfaces; contains osteogenic cells.
On which part of the bone does periosteum not occur?
Joints, where cartilage is present
Which cytokines contribute to osteoclast maturation?
M-CSF, RANKL, IL-1
What strength does the parallel arrangement of lamellae to the axis achieve?
Strength resisting load parallel to the axis.
What strength does alternate arrangement of collagen fibres between lamellae achieve?
Resistance to twisting forces.
What are interstitial lamellae?
Ruminants of old osteon that fill gaps between osteons.
What are circumferential lamellae?
Below periosteum, extend the circumference of the diaphysis to resist twisting forces
What are the two types of vascular canals in compact bone?
Perforating/Volkmanns - connect Haversian canals to periosteum
What are canaliculi?
Canals that connect lacunae to each other and to the Haversian canal, allowing osteocyte communication.
When does endochondral ossification of long bones begin in development?
Second half of gestation.
At which point in development do secondary ossification centres form?
How does bone grow in length?
New bone is deposited onto the diaphyseal side of the epiphyseal growth plate
When do epiphyseal growth plates close?
At 18-25 years
Stimuli: reduction in GH and IGF-1
Chondrocytes stop dividing, the epiphyseal plates thin, and bone replaces cartilage.
How does appositional growth of bone occur?
Bone is resorbed at the endosteal surface and is added to the external surface. This is increased with weight-bearing.
Up to what state in life is bone remodelling positive and up to which is bone mass maintained?
Positive until the third decade, mass is maintained until 50 years old.
What is bone remodelling carried out by?
Basic multicellular units made of osteoclasts and osteoblasts.
What are the four stages of bone repair?
Fibrocartilagenous callus formation (in few days) - fibroblasts, chondroblasts and osteoblasts
Bony callus formation (weeks-months)
Bone remodelling (weeks-months)
What is the mechanism of action of osteoblasts?
Use alkaline phosphatase to remove phosphate from molecules, promoting mineralisation
How do osteoclasts self-regulate?
Secrete TSG-6 to bind RANKL
Which regulators of bone remodelling to osteoblasts secrete?
RANKL and OPG
Which molecules decrease osteoclast activity?
OPG and calcitonin
Which molecule increases osteoclast activity?
What is the mechanism of action of osteoclasts?
The central region of osteoclasts secretes acids, H+ ions via H+ATPase and Cl- via chloride channels to form HCl to lower the pH to 4.5, which mobilises bone minerals.
Osteoclasts secrete lysozymes to digest the matrix.
Mobilised materials are phagocytosed and released into the ECF to increase blood ion concentration
Where is Ca2+ stored in the body?
99% in bone, 1% in serum
Which hormones are involves in plasma Ca2+ balance?
Parathyroid hormone, vitamin D/calcitriol and calcitonin (minor)
What is the function of PTH in Ca2+ homeostasis?
Stimulated by fall in plasma Ca2+ to indirectly increase osteoclast activity, stimulate OPG and RANKL, enhance renal Ca2+ absorption and intestinal absorption (via vitD)
What is the function of vitD in Ca2+ homeostasis?
Increase Ca2+ uptake from the intestine and Ca2+ kidney resorption, increases mineralisation of bone.
What is the function of calcitonin in Ca2+ homeostasis?
Decrease bone resorption and increase renal Ca2+ excretion when Ca2+ levels are high (opposite to PTH)
Explain Wolff's law.
Bone grows or remodels in response to the demands place on it.
Explain how bone is modified based on demand.
Long bones are thickest when bending stresses are greatest
Curved bones are thickest where they are most likely to buckle
Trabeculae of spongy bone forms trusses/struts along lines of compression
Bone projections exist when heavy, active muscles attach
Lack of demand reduces bone mass.
What is osteoporosis?
Bone resorption > bone deposition
Bone mass is lost but the matrix/mineral ratio is unchanged.
Which bone is most affected by osteoporosis?
Spongy bone of the spine. Trabeculae number and strength decreases, causing micro-fracture and vertebral collapse. Over time, shortened stature and dowager's hump develop due to compression if untreated.
How does oestrogen protect from osteoporosis?
Restrains osteoclasts and promotes bone deposition.
What are contributors to osteoporosis?
Low oestrogen, low dietary vitD, Ca2+ and protein, small body form, insufficient exercise, home conditions, e.g. Cushing's syndrome.
What is Paget's diseasE?
Accelerated bone turnover as osteoclasts are abnormally activated and produce irregular patterns of bone resorption; followed by intense osteoblast response and irregular bone formation.
Abnormally high spongy/compact bone ration.
Late in disease, bone deposition > bone resorption.
Which bones does Paget's disease affect most?
Spine, pelvis, femur and skull.
What are the symptoms of untreated Paget's disease?
Bow legs, increased skull size, curved spine.
What are Ricket's and osteomalacia?
Inadequate mineralisation of bone matrix, causing soft and weak bones. Often due to insufficient Ca2+ or vitD.
Which is more severe - Ricket's or osteomalacia?
Rickets - affects growing bones, epiphyseal plates cannot calcify and widen ends of long bones and lengthen bones. Gross skeletal changes are caused by demineralisation of bones and lack of rigidity.
What are the outcomes of osteomalacia?
Increased fracture risk, especially in vertebral bodies and femoral necks.