Transcript 06_bone_remodeling

Bone Remodeling
• Bone remodeling = combination of bone deposition
and bone resorption.
• Deposition = taking minerals (Ca2+, Mg2+, Mn2+,
phosphate) from the bloodstream and using it to build
hard bone tissue.
• Resorption (re + absorption) = breaking down hard
bone tissue into the above materials and adding them
back to the blood.
• In order for the a bone to maintain its size and
strength: rate of resorption = rate of deposition
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Developmental Aspects of Bones
• By age 25, nearly all bones are completely ossified
• In old age, bone resorption predominates
• A single gene that codes for vitamin D binding to
cells determines both the tendency to accumulate
bone mass early in life, and the risk for osteoporosis
later in life
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Long Bone Growth and Remodeling
• Growth in length –
cartilage
continually grows
and is replaced by
bone as shown
• Remodeling – bone
is resorbed and
added by
appositional
growth as shown
Figure 6.10
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Hormonal Regulation of Bone Growth During
Youth
• During infancy and childhood, epiphyseal plate
activity is stimulated by growth hormone
• During puberty, by testosterone and estrogens
• Initially promote adolescent growth spurts
• Later induce epiphyseal plate closure, ending
longitudinal bone growth
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Remodeling
• Remodeling units – adjacent osteoblasts and
osteoclasts deposit and resorb bone at periosteal and
endosteal surfaces
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Deposition
• Accomplished by osteoblasts (“bone builders”)
• Occurs where bone is injured or added strength is
needed
• Requires a diet rich in protein, vitamins C, D, and A,
calcium, phosphorus, magnesium, and manganese
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Bone Resorption
• Accomplished by osteoclasts (“bone crackers”)
• Resorption involves osteoclast secretion of:
• Lysosomal enzymes that digest organic matrix
• Acids that convert calcium salts into soluble forms
• Dissolved matrix is secreted into the interstitial fluid
(fluid between cells) and then into the blood
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Importance of Ionic Calcium in the Body
• Calcium is necessary for:
• Transmission of nerve impulses
• Muscle contraction
• Blood coagulation
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Control of Remodeling
• Two control loops regulate bone remodeling
• Hormonal mechanism that maintains calcium
homeostasis in the blood (PTH and calcitonin)
• Mechanical and gravitational forces acting to the
skeleton (“stresses”)
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Hormonal Mechanism
• Rising blood
Ca2+ levels
trigger the
thyroid to release
calcitonin
• Calcitonin
stimulates
calcium salt
deposit in bone
Figure 6.11
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Hormonal Mechanism
• Falling blood
Ca2+ levels signal
the parathyroid
glands to release
PTH
• PTH signals
osteoclasts to
degrade bone
matrix and
release Ca2+ into
the blood
Figure 6.11
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Response to Mechanical Stress
• Wolff’s law – a bone
grows or remodels in
response to the forces
or demands placed
upon it
Figure 6.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Response to Mechanical Stress
• Long bones are
thickest midway
along the shaft
(where bending
stress is greatest)
• Curved bones are
thickest where they
are most likely to
buckle
Figure 6.12
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Response to Mechanical Stress
• Trabeculae (bone
fibers) form along
lines of stress
• Large, bony
projections occur
where heavy, active
muscles attach
Figure 6.12
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings