7 Bone Physiology I
BONE STRUCTURE AND THERMAL REGIME
Köhler article
BONE SCALING
BONE HOMEOSTASIS
Bone stress
Remodeling pathways
Donohue article
ENDOSKELETON vs DERMAL SKELETON
SKELETAL TISSUE USE AND FUNCTION
Stress vs strain
Stress is measured in units of
force / area (Gpa)
Strain is measured as change in
length relative to the original length
(dimensionless)
Q: Which skeletal tissues will strain (deform) most? Least?
Q: Which skeletal tissue will break most easily? Least?
Q: How does bone used for different purposes vary in composition?
a
b
c
a
b
c
Q: what is the relationship
between the inorganic content
and the modulus of elasticity?
Glossophaga soricina
Swartz and Middleton 2007
Q: How might you explain the differences in mineral content among the
various arm bones of a bat? http://youtu.be/rMSIauO-MJc?list=PL011EB0DA5D2CC028
Q: Why aren’t large animals just “bigger models” of small animals?
5X
Support is dependent on bone x-sec, NOT bone volume.
Q: Will the x-sec of legs increase less fast than, equally as fast as, or
faster than mass (≈ volume)?
Q: Which animal was larger in life?
Q: If linear dimensions increase by 5 …..by how much does leg cross
section and volume (mass) increase if proportions are maintained?
5X
Q: How might we predict the factor by which the diameter of the leg
bones of the small horse ”should” increase to support the larger horse?
BONE STRUCTURE AND THERMAL REGIME
The classical dichotomy:
Ectotherms
Few growth plates or epiphyses
Slow growth
Indeterminate growth LAGs - lines of arrested growth
Haversian (compact) bone rare
Endotherms
Diaphyses, growth plates, and epiphyses
Fast growth
Determinate growth Few or no LAGs
Haversian (compact) bone common
Dueling interpretations of dinosaurs:
Köhler et al. 2012. Nature 487: 358
tropical
tropical
arid
arid
continental
temperate, Med.
polar
temperate dry/wet
Q: How does the metabolism of endotherms vary? How does bone
growth reflect that variability?
BONE FUNCTION
1. STIFFNESS: Resisting deformation
Protection
Transmission of muscular power (= movement)
Oppose gravity
Q: Other than imparting stiffness, what other functions do bones provide?
12
BONE FUNCTION
2. HEMATOPOESIS: Produce blood cells (later)
3. MINERAL (Ca++) HOMEOSTASIS (next lecture)
Storage and Mobilization
bone remodeling
90% Ca
Osteoblasts
(deposition)
Osteoclasts
(resorption)
13
BONE HOMEOSTASIS (remaining stiff and storing/mobilizing Ca++)
BONE HOMEOSTASIS (remaining stiff and storing/mobilizing Ca++)
Q: Is the same amount of bone always desirable? How much bone should
there be?
BONE HOMEOSTASIS (remaining stiff and storing/mobilizing Ca++)
Q: What factors might enhance or constrain osteoblast activity or cell life
span? osteoclast activity or cell life span?
osteoclast activity or life span
osteoblast activity or lifespan
Harada and Rodin 2003
Review of bone remodeling
Rik Huiskes, Ronald Ruimerman, G. Harry van Lenthe and Jan D. Janssen
Nature 405, 704-706(8 June 2000)
Q: Predict what happens to bone when it is subject to increased stress.
Q: How does loading (i.e., stress) produce a remodeling response?
Rik Huiskes, Ronald Ruimerman, G. Harry van Lenthe and Jan D. Janssen
Nature 405, 704-706(8 June 2000)
MICROCRACKS vs. MECHANOTRANSDUCTION
microcrack stimulus
mechanotransduction
stimulus
MECHANOTRANSDUCTION
Bone stress
radius
before
ulna
after
stress = force
x-sec
Lanyon et al. 1982
From Lanyon, 1982
strain = dL
L
Q: Disuse osteopenia / osteoporosis. What are some situations /
conditions when bone is subject to decreased stress?
Q: How might we be able to measure relative levels of osteoblast and
osteoclast activity directly?
collagen synthesis
pro-α-chain
procollagen
collagen
collagen breakdown
carboxy-terminal
propeptides
(PICP)
Carboxy-terminal
telopeptide
fragments (ICTP)
Bone metabolism in hibernating bears
Donahue et al. 2006. J. Biomechanics 39: 1480-1488.
Q: Why expect disuse osteoporosis to occur in hibernating bears?
Q: How did Donahue et al. assess bone metabolism in hibernating
bears?
PICP = collagen synthesis
marker
Donahue et al. 2006. J. Biomechanics
39: 1480-1488.
Q: What possible mechanism(s)
for avoiding disuse osteoporosis
do serum levels of PICP and
ICTP suggest?
ICTP = collagen degradation
marker