00:01
Bone remodeling.
00:03
This is probably our most important
clinical aspect of bone.
00:09
Bone will remodel to
three primary things.
00:13
One is bone microdamage, which
we’ll use in your example.
00:17
Stress on the bone will
also cause bone remodeling.
00:22
And what do I mean by stress?
It simply has to have the
amount of torque on the bone.
00:28
The other one that can
do bone remodeling are
hormones: parathyroid
hormone and calcitonin.
00:35
Let’s now move to our example of
bone microdamage causing remodeling.
00:42
The first thing that happens is this bone
microdamage is sensed by osteocytes.
00:49
Osteocytes are located deeper
within the bone and these
structures or cells will sense
the microdamage has occurred.
00:59
They then signal another cell
known as bone lining cells.
01:05
It is this bone lining cells that get
the process started of remodeling.
01:12
They will recruit osteoclasts,
and osteoclasts are
the type of bone cell that
break down current bone.
01:23
We then get bone
resorption occurring
and recruit osteoblasts
to that particular area.
01:32
Osteoblasts lay down new bone.
01:36
Once that new bone is deposited or laid
down, what happens to those osteoblasts?
They can either be trapped within
the bone and become osteocytes
or they turn into
bone lining cells.
01:52
So this is the process that
we’re going to step through.
01:56
Now let’s take each one of these processes
in greater detail so we understand
the mechanisms behind bone remodeling
which is so important for us clinically.
02:09
The first step that we’ll review here
is what happens to the initial signal.
02:15
Remember, osteocytes need to
signal bone lining cells.
02:20
How does it do that?
Well, it senses that there might have
been microdamage that has occurred.
02:26
This microdamage oftentimes sense
these osteocytes into apoptosis,
which is a programmed cell
death of the osteocyte.
02:37
The bone lining cells sense
that apoptosis occurring.
02:44
These bone lining cells
then do a couple of things.
02:47
The first thing they do is
send out chemoattractants.
02:52
These chemoattractants will
bring other cells into
closer proximity to where this
locale is of microdamage.
03:02
The second thing it does
is the bone lining cells
start to disassociate
from the bone surface.
03:10
This creates a little
bit of a pocket.
03:14
Those chemoattractants will
bring other cells to it,
and the cells that it’s trying
to attract are osteoclasts.
03:24
Osteoclasts will be brought to this particular
area by those attractant molecules.
03:32
They then go with inside the pocket that has
been created by those bone lining cells.
03:40
It is also attracted, at that same
time, osteoblast precursor cells.
03:46
So we have osteoclast precursors
then osteoblast precursors.
03:50
The osteoclast precursors have a specialized
receptor known as a RANK receptor
and the osteoblast precursors
have a rank ligand or RANKL.
04:03
These two things fit together
and these cells then
form big groups of cells
by merging together.
04:13
They all do this under the cover
of a bone lining kind of pocket.
04:20
This protects these cells and gives a local
environment for them to do their job,
but keeps them separate now from the
other portions of the circulation.
04:35
These large cells that
have come together
and form these large
multinucleated osteoclasts
are going to be the primary breaker down
of bone, and they do that with enzymes,
and they will start to liberate
both calcium and phosphate.
04:58
They will simply dig a hole in
the bone by dissolving it away.
05:06
Once they have done their job
in dissolving that bone,
they will be terminated and
osteoblasts will be recruited
to the area to start
filling in that new bone.
05:20
Osteoblasts do this bone
remodeling portion.
05:25
Interestingly, some osteoblasts
will be trapped within the matrix.
05:31
These trapped osteoblasts will
eventually become osteocytes.
05:37
The osteoblasts continue to
make new bone until they have
filled in the area that was
dug out by the osteoclasts.
05:48
Then these new cells, these
osteoblasts, can turn into
new bone lining cells and
this completes the process.
05:59
And you might think to yourself
why undergo bone remodeling?
That is a lot of work, right?
You are undergoing bone remodeling
right now, all the time.
06:11
In fact, they say as a
young adult you might
be remodeling about 20%
of your bone per year,
an older adult maybe 2% to 4%
is constantly being remodeled.
06:23
So this process occurs throughout
the body in many different bones.
06:29
This microdamage though
will occur specifically in
certain bones that have
undergone some sort of trauma.
06:36
So you can have this in
trauma bones or even in
regular bones that are
undergoing normal stress.
06:42
And that is one of the
reasons why you get larger
bone mineral densities when
you’re younger as you’re
remodeling more than when
you’re older and you
start to lose some of that
bone mineral density.
06:56
Now, I said that there
were three main components
or factors that
recreated remodeling to
occur: microdamage,
which we covered;
stress on the bone; and the
third were the hormones.
07:10
So now let’s go through the
hormonal aspects of controlling
the amount of blood levels
of calcium and phosphate.