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Lecture notes on bone remodeling /hormones
What are the two things that happen during bone remodeling? Osteoclast eat the outside of the
bone
Why would PTH be activated? Because calcium in blood is too low. What detects low blood
calcium levels? The chemo receptor which is in the hypothalamus.
Chemo receptor releases PTH from where when it detects low blood calcium? The parathyroid
gland.
Where is the parathyroid gland located?
Will all osteoclast cells act at the same time? No it’s going to be specific to the periosteal layer
on the surface of compact bone in long bones usually.
What percentage of bone mass is changed every week? 5 to 7 %
Why is bone mass remodeled weekly?
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Calcium imbalance is the major reason
Weight change from muscle mass or from gaining or losing weight
Increase stress (under the mechanical gravitational forces acting)
If you look at someone’s weight it’s their mass in kilograms or grams times the force of gravity
acting on them. So if you have more mass your weight goes up.
If I work out more I build muscle mass. I weigh more and have to build more bone to carry extra
weight.
What two things are involved with bone remodeling? Bone deposition (osteoblast) and bone
resorption(osteoclast) daily.
True or false: in a healthy person the rate of resorption of bone should equal rate of bone
depositon. True.
Bone disorders like osteoporosis, osteomalacia , rickets are the result of what? An imbalance
between the rate of deposition vs. the rate of resorption.
What type of bone would you have more of if you are depositing too much bone? Spongy bone
in the form of woven bone or trabeculae or diploid (flat bone).
Sodium channels in muscles are opened with what? Calcium.
Without calcium you would never have the ability to get the vesicle that came off the Golgi
apparatus. The vesicle contain the product.
In the neuromuscular junction neuron, it’s vesicles contain ACh acetylcholine receptors (p285).
Without the ACh , a sodium channel could never be opened on a muscle
All neurons need what for nerve impulse transmission? Calcium
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You have to have calcium for neuromuscular junction in muscle contraction
You have to have calcium for blood clotting
What are the three mechanisms that help monitor if blood calcium is too low?
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Kidneys constantly monitoring calcium levels because kidneys have the ability to
determine if a solute is not concentrated enough in the blood. This would reabsorb or
secrete calcium depending on what the calcium level is in the blood.
Skin produces vitamin D and the Large intestine is able to absorb calcium because of
vitamin D.
Bone
If calcium gets too high in the blood what cells in what location would you want to target?
Osteoblast in the endosteum.
PTH and Calcitonin target osteoblast and osteoclast in a bone break in both the endosteum and
periosteum locations.
If you are remodeling and blood calcium levels are low you would want to target osteoclast on
the periosteum layer (the outer surface of the bone)
If you targeted an osteoclast in endosteum would you get a whole lot of calcium? No because
that’s where spongy bone is found.
If you want to raise calcium levels what is the best way to do that? To target the osteoclast in the
compact bone layer because that is where the calcium is (the outer surface of compact bone).
What is responsible for the growth of a long bone during adolescence? Growth hormone
What is responsible for the growth spurt reached, at the end of puberty, during adolescence? Sex
hormones, either testosterone or estrogen.
If I target an osteoclast during the activity of PTH function, what I’m I going to need to inhibit at
the same time? Osteoblast because I don’t want to be depositing bone as I’m breaking up that
bone because all that calcium’s gona go right back into the new bone instead of the blood.
Bone remodeling is determined by mechanical and gravitational forces because of a change in
muscle and bone mass but more readily it’s occurring because of calcium variation and that’s
usually appositional growth that we’re targeting (either thinning of the compact bone layer or
thickening of it from the inside out).
Heavy bone depositing is called what? Hypertrophy (accelerated bone growth)
What would be experienced because of bone disuse? Bone atrophy, breakdown ( resorption) of
bone because muscle mass would be decreasing over time.
What cells are involved with bone deposition? Osteoblast cells
Where is osteoblast targeted? Osteoblast is targeted in the Endosteum, the inner lining of bone
because that’s where spongy bone is deposited.
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Osteoblast secrete what? Osteoid
How does Osteoid get mineralized? By using hydroxyapatite
What is hydroxyapatite? The mineral salts of calcium phosphate which mineralize Osteoid
(hardens bones).
Hormonal control is from Calcitonin release which is produced by C cells (parafollicular cells)
chromaffin cells in the thyroid gland area.
Thyroid gland produces thyroid hormone from their ducts. In between each duct (in a triangular
area) is where the parafollicular cells or C cells( chromaffin cells) are located.
Calcitonin is released by these C cells in the thyroid gland when blood calcium levels are high.
When Calcitonin is released what cell is targeted? Osteoblast cells are targeted to deposit new
bone which brings down blood calcium.
What cells are inhibited when Calcitonin is released? Osteoclast cells are inhibited therefore
inhibiting bone resorption, which enhances calcium deposits in the bone matrix, and thickness
and hardness is added to compact bone layer
Bone thickening is not just the result of high calcium it’ also the result of increased stress or
mechanical gravitational forces acting on the bone makes calcium too high.
If you work out all the time and don’t take in enough calcium, calcium will be taken from bone
and from other areas of the body that are vital and you will harden the bone anyway and then you
will result in a low blood calcium level.
Bone resorption involves osteoclasts that have the ability to secrete hydrochloric acid which
softens the bone (solublizes) enzymes follow by speeding up process and digesting the matrix
and release calcium into the blood.
Where are the 3 locations where osteoblast and osteoclast do this? Endosteum, periosteum, and
in the growth plates.
If you are past puberty only endosteum and periosteum would be active with osteoblast and
osteoclast.
Targeting of osteoblast activity in adulthood what are some things you know from that
statement?
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It would not include the epiphyseal plate lengthening so longitudinal growth is out
Only be one of the three types of appositional growths (widening of the bone )
Endosteum layer would be targeted because that’s the layer that Calcitonin is related
to.
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The idea behind bone resorption is that minerals are freed from bone (using the acids and
enzymes in osteoclast your freeing the calcium ions by dissociating it and dissolving the matrix)
and this is under the control of the chemo receptors in the hypothalamus.
In the hypothalamus you would target the pituitary gland and the pituitary would target the
parathyroid gland which are found behind the thyroid gland. Inside the parathyroid gland you
have chief cells that release PTH. PTH is produced by this gland because of low blood calcium
levels.
What is included in the stimulation of bone resorption? Osteoclast in periosteum to target
breakdown of compact bone layers.
What is inhibited during bone resorption? Osteoblast is inhibited during bone resorption because
if it was not inhibited the calcium would go right back into the trabeculae bringing blood calcium
levels down.
What would stop bone resorption? Negative feedback would shut down the original stimulus
because the chemo receptor that started it would detect that things are now balanced.
If it went too far and created a surplus of calcium, too much blood calcium, Calcitonin would
have to regulate it.