00:01
Bone and the skeletal system.
00:05
The structure and function
of bone is very important in
understanding how the process it
will be to both store calcium and
phosphate and also provide
structure so the muscles can pull
on these bony levers so you
can ambulate or walk around.
00:23
Let’s take a long bone as an example, talk
through some of these salient features,
and then we will start to look
at the different types of bone.
00:32
When we look at the long bone
structure, it’s important to
think about a couple of items,
the first is cortical bone.
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Cortical bone is the top layer of hard bone
that’s across the shaft of the long bone.
00:50
Trabecular bone is the spongy
kind of lattice network of bone
that’s usually located in the
upper portions of the long bone.
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Then you have marrow, which is located
throughout the shaft of the bone.
01:07
Why do you have these two
different types of bone?
I think the best way to
think of this is that
there is an engineering
feat being done here.
01:17
What you need is the hardest
bone you can have to have structural
integrity for the body to
provide support to pull on, but it
needs to be as light as possible.
01:32
If this was just
solid cortical bone,
it would be so heavy, it would
be hard to move around.
01:39
The trabecular bone, although it’s
not as strong, is nice because
it provides an avenue in which
there is less weight involved.
01:49
So you’re always playing a game between
putting down more calcium and phosphate
versus taking some of
it away so that you can
optimize this strength
versus weight component.
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How do you form
bone to start with?
The first thing that
you need is collagen.
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Collagen is a type of connective tissue
and you can run collagen up and down,
add ground substance to it.
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Think of ground substance
as more like the cement
around the collagen, which
are kind of act like rebar.
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Rebar is what provides structural
support for a building.
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That’s that iron bars that
are placed so that they can
bend a little bit, but still
provide structural support.
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Then what do you need?
You need crystals to form.
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It’s hard to get crystals to
form in ground substance.
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So what you do is you supersaturate
it with the calcium and phosphate.
02:57
Eventually, they form
crystals and it’s them
crystals that will be
hard enough to form bone.
03:05
Then you need to have it
mature, and over time,
the bone will mature and
calcify to a greater degree.
03:12
It takes a while for bone to
get into this mature process,
and therefore, have enough
calcium and phosphate in it.
03:23
So what really is bone?
Well, about 30% of it
is organic material.
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And what do we mean
by organic material?
Two percent of it is cells, and
it’s interesting to think about a
bone weighing a certain amount and
only 2% of that is actual cells.
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Collagen is about 93%
and ground substance
is another 5%.
03:50
So that forms the organic
component of bone.
03:55
The inorganic component of bone is
about 70% of the bone by weight.
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What component is that?
That is the calcium and phosphate
crystals that form the hardness of bone.
04:10
So we have both organic
and inorganic components,
only a small amount being cells and the rest
of it being collagen, ground substance,
and then its inorganic partners, the
calcium and phosphate crystals.
04:29
So what are these two types of bone and how
are they different in their components?
Well, if we think about trabecular bone,
which is also known as spongy bone,
this is more of a
lattice network.
04:43
So there are gaps in
between in this lattice.
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If we would break any
particular lattice or cut it,
you can see these
circular-like structures.
04:52
There are osteocytes there, there
are lacunae, there are canaliculi.
04:58
These are all structures
of this trabecular bone
but only in the parts that
you can transverse or cut.
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The in-between areas,
this lattice area,
that is where blood vessels
and other things can be.
05:14
If we contrast trabecular
bone to its hard cousin,
which is cortical bone, we get
more of this type of a network.
05:23
So for cortical bone, this is the
outermost layer that is the strongest.
05:29
So if you want to think about why they
only have this outer layer that’s strong,
maybe think of your bicycle.
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Your bicycle has a strong outer
tube but it’s hollow on the inside.
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Cortical bone works
somewhat like that.
05:45
That is the hard outside that’s maybe made
up of aluminum or steel in your bicycle.
05:50
Now, let’s go through some of
its individual components.
05:54
If we look at its individual components,
there are a lot of smaller structures.
06:00
These smaller structures will
have blood vessels in them.
06:03
They will have nerves.
06:05
But they form these osteons,
which are the circles,
and what we like to
call Haversian systems.
06:14
The Haversian systems are the lat
network of those circular structures.
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It provides a very
solid component.
06:26
You see that there are no
lattice-like structures.
06:30
These are all placed in
tight proximities together.
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So the lamellae are the circles,
the osteon is the whole thing
as well as sometimes we refer
to that as a Haversian system.
06:44
How does a bone grow?
A bone will grow in certain
spots and these spots are
usually known right where
the growth plates are.
06:54
The first thing that
needs to happen is you
have to change the
chondrocytes in those areas.
07:00
You have to lay
down this matrix.
07:04
At this process, we
have mitosis occurring.
07:07
Mitosis will proliferate the
number of cells involved,
and then, we add a number
of substances to them,
things like lipids, some
glycogen, some phosphatases.
07:18
This matrix then
starts to calcify.
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And interestingly, as this
calcification occurs,
you start to get
maturation and hypertrophy
and then eventually,
these cells will die.
07:32
But these form then
this calcified matrix.
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If these were to further progress into
spongy bone, you will get these zones of
ossification, and then, you’ll have blood
vessels starting to transverse through those.
07:47
This is the process of
growth and maturation.