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- [Voiceover] As the bones are growing,

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there's various different hormones

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that are gonna help
control the bone growth.

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In preadolescence, you're
starting off with growth hormone,

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causing stimulation of bone lengthening.

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The growth hormone is produced
by the pituitary gland

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up near the brain,

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but as you continue on
into early adolescence,

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estrogen and testosterone are taking over.

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Estrogen, female; testosterone, male.

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They're to stimulate the
bone lengthening again,

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but as you continue into
the late adolescence,

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these same two hormones,
estrogen and testosterone,

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instead of causing more bone lengthening,

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they now are causing the cartilage,

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found in the growth plates,
to be replaced with bone.

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Once the cartilage in the
growth plates is gone,

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that's it, the bone cannot lengthen.

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The same hormones that
can cause lengthening

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in early adolescence will
then cause the cessation,

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or stopping, of bone
lengthening in late adolescence.

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So if you look at these
chondroblast cells,

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we mentioned chondroblast before,

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the ones that make cartilage.

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Well, the chondoblasts make the cartilage,

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but eventually the
osteoblasts will come in

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replacing the chondoblasts.

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As the osteoblast cells start to grow,

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they're gonna start to form bone.

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Once they mature, they
kind of stay in place,

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they get stuck; they're called osteocytes.

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These are the mature bone
cells, the ones that maintain,

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the ones that monitor, the ones
that keep the bone healthy.

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So blast is immature,
it's making something.

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C-Y-T-E cyte is mature, it's
just maintaining something

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and then we have these osteoclast cells.

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The osteoclast cells are the ones

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that will break down the bone.

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They're gonna take the calcium
out, take the minerals out,

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basically they're just
dissolving the bone.

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So you can take a look table 5.1,

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get a little more
information on these cells,

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and also have (mumbles) animation.

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So check back in learning module,

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take a peek at that animation.

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But as you age, even after
those growth plates are closing,

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the bone can still change shape.

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It can't get longer,
but it can get thicker,

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it can get wider, it could add
strength, can lose strength.

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These are all dependent
on numerous factors.

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What's your dietary intake?

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How much exercise do you have?

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What age are you?

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Even gender, male versus
female, comes into play

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and all this regulation
throughout the rest of life

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after the growth plates are closed,

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it's all regulated by two hormones,

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or primarily regulated by two hormones,

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parathyroid hormone,
or PTH, and calcitonin.

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Parathyroid hormone is meant

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to remove calcium from the bone.

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Well if you're thinking, "Why
would you want to do that?"

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Well, you might want to remove calcium

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because your bones might
be too dense or too heavy.

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You don't need that much density.

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Or maybe you need calcium
someplace else in your body,

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so you pull it out of the bones
to send it someplace else.

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The flip side, or the
antagonist, is calcitonin.

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It's going to actually
add calcium to the bone,

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so it cause calcium to
be deposit in the bones,

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making the bones denser,
stronger, thicker.

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So parathyroid hormone and calcitonin.

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But sometimes a simple
maintenance isn't good enough.

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Sometimes the bones become damaged.

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So bone repair usually starts

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with the formation of a hematoma.

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A hematoma is going to be a
large, kind of swollen area

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filled up with fluid.

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Now, odds are the fluid's
gonna be blood, in this case,

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but the hematoma's gonna
serve several purposes.

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One function, it's gonna allow
for the callus formation.

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A callus is going to be initial stages

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of recreating the bone to go
back into the damaged location.

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So what you have here is
gonna have a callus forming

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for more of a kind of cartilage material

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that will slowly get stronger and stronger

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until it turns into a solid bone material.

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So here's an example.

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This is the femur, for instance.

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If you take compressive force

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and push down on the top of the femur,

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all right, and you see how the bone's

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kind of bent a little
bit in the middle there.

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So it has more pressure to one side

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as the bone tries to buckle a little bit.

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So what happens is, you're
body's going to say,

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"Okay fine, no problem, I'll
take bone off the one side,

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"put it on the other side."

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The result is you end up with

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a relatively straight bone coming down.

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That bone now matches the force.

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Both sides have about the same amount

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of force being exerted.

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So your body is constantly
trying to match up

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and moderate the force on your bones.