- [Instructor] Okay, so now now let's talk
about the cellular structure of bone.
You can think of bone as being mostly made up
of the bone matrix and then the cells
that actually help to form this bony matrix.
Now the majority of bone
is made up of the bone matrix,
which consists of two principle building blocks.
The first of which is called osteoid
and this forms the organic portion of the matrix,
and then you have something called hydroxyapatite
which forms the inorganic portion of the bone matrix.
So first lets talk about the organic portion
of the bone matrix.
The organic portion of the bone matrix
is made up of, like I said, osteoid,
which consists of a soft but highly ordered structure
of proteins and then collagen, specifically Type I collagen.
So together, the orientation of these highly organized
collagen fibers along with the proteins
helps to give bone it's tensile strength.
So what does tensile strength really mean?
If you think about a really hard piece of rubber,
like a rubber eraser for example,
I kind of think of tensile strength
as being similar to the way that piece of rubber
gives ever so slightly if you were to push on it
but it still holds its shape.
And so that's what you can think of
as bone's tensile strength.
Now for the inorganic portion of bone.
The inorganic portion of bone is made up of hydroxyapatite.
And I'll draw it's chemical formula here.
It is calcium along with phosphate and water.
So basically all hydroxyapatite is
is calcium phosphate crystals.
And these calcium phosphate crystals
are the mineral portion of bone
that gives bone it's rigid strength and density.
So now you have a better understanding
of the osteoid or organic portion of the bone matrix
and hydroxyapatite which make up
the inorganic portion of the bone matrix.
So now lets talk about the cells
that are actually responsible
for making up this bony matrix.
There are four different types of cells
to be familiar with in bone.
And the first group are the osteoprogenitor cells.
Next we have what are called osteoblasts.
Then there are the osteocytes,
and finally we have the osteoclasts.
And so you can see they all share the same root,
osteo, meaning bone.
So lets make a little bit more room here
and talk about each of these cells one by one
and what they do.
So first up we have the osteoprogenitor cells.
These cells are basically
just the precursor to osteoblasts.
So basically they're just an immature version
of osteoblasts and they differentiate into osteoblasts
under the influence of various growth factors.
So that brings us up to osteoblasts.
Now osteoblasts are the cells that are responsible
for synthesizing both collagen and proteins,
specifically a couple of proteins
called osteocalcin and osteopontin,
and together these make up osteoid.
Osteoblasts are also responsible for producing
alkaline phosphatase,
which is the enzyme that is responsible
for forming hydroxyapatite,
or the mineral portion of bone.
Once osteoblasts have synthesized
enough collagen and proteins and alkaline phosphatase
to form the organic and inorganic portion
of the bony matrix around them
and they're finally surrounded by all of this
they mature into the osteocyte.
So the mature version of an osteoblast
once it's all done synthesizing the bony matrix,
becomes an osteocyte.
So that brings us to osteocytes.
Now the spaces that osteocytes occupy within bone
are called lacunae.
And the term lacunae really just refers
to an empty space within bone.
So under a microscope, these spaces look, to me,
like little lakes.
And so that's how I remember what lacunae are,
they are little lakes, or absences,
empty spaces within bone.
And these osteocytes have little arms or branches
that reach out to communicate with other osteocytes
or osteoblasts, which kind of gives them
this star-like appearance.
And these little branches are like sensors
that pick up new information, send out signals,
basically allowing osteocytes to communicate
with other cells that help to maintain bone.
And then the final cell type is the osteoclast.
Now osteoclast are derived
from a cell line called monocytes.
And they are responsible for bone resorption.
So basically they break bone back down again
and they do this with an enzyme called
tartrate resistant acid phosphatase.
So osteoblasts help build up bone
with an enzyme alkaline phosphatase,
and then osteoclasts help to break that bone back down again
with an enzyme tartrate resistant acid phosphatase.
So bone is basically constantly being remodeled.
Built up by osteoblasts
and broken down again by osteoclasts.
And an easy way to keep this straight
is to think that osteoblasts,
the b in blasts is for building bone
and then osteoclast, the c in osteoclasts,
is for crashing bone down.
So osteoblasts build up bone,
osteoclast break it back down again.
Now as osteoclasts are resorbing bone,
they start to form little empty spaces in bone.
And wait, what are empty spaces in bone called?
They're called lacunae.
And so just like osteocytes occupy lacunae,
osteoclasts occupy special kind of lacunae
called Howship's lacunae,
which are just the little resorption pits
formed by osteoclasts as they break down
the bone surrounding them.