and the tissues of the skeleton.

The learning objectives

that we'll cover are here.

We'll cover a little bit

about what bone's made of --

cells and matrix.

We'll talk about bone growth

and remodeling.

And quite bit

about calcium homeostasis

which involves our bones.

The functions of bones --

support and protection are

probably pretty obvious

and moving our muscles around.

But also, don't forget

that new blood cells are formed

in our bone marrow.

Also, the bones are going

to play an important role

in calcium homeostasis

in maintaining our extracellular

calcium -- serving

as a calcium reservoir.

In the first learning objective,

we'll cover here is

to describe the cells, matrix,

and organization of bone

and associated tissues.

So we'll mention a little bit

here about cartilage

and ligaments.

But mainly we'll focus on bone

and bone marrow.

So if we look at bone --

so we've learned so far

as the osteocyte.

The extracellular matrix

around those cells is mainly

collagen protein as well

as the minerals calcium

and phosphate.

And so that's what gives bone

its sort of hardened

extracellular matrix is the

calcium phosphate and mineral.

If we look at tendons

and ligaments,

the cell we're interested

in is the fibroblast

which makes this dense

connective tissue of collagen

and elastin.

These proteins make

up these fibers, collagen

and elastic fibers.

A cell I don't think we've

learned yet is the chondrocyte.

Chondrocytes help maintain a

special connective tissue

called cartilage.

Cartilage is mainly collagen

and elastin proteins as well.

So cartilage is kind of famous

for making up things

like your ears and nose

and being on the ends of bones

such as our ribs.

So if you look at each of these,

the sort of thing they share is

that they're made

of extracellular matrix

of collagen built

by certain cells.

Okay, I just wanted

to mention ligaments.

Ligaments attach bone to bone.

And they stabilize our skeleton.

Tendons attach muscles to bone,

and they allow movement

of our bones.

Okay, ligaments

and tendons are basically just

dense connective tissue.

Cartilage, cartilage is

well-known for being at the ends

of our bones.

So wherever we form a joint

so that those bones don't clash

against each other,

it's protected by cartilage

and also our ears and nose

and things like that.

So to summarize, keep in mind

that when we talk

about the skeleton,

we're talking

about the different types

of cells that make

up the different type

of skeletal tissues.

And each of them shares collagen

as an important protein

and then slightly

different cells.

If we look at this picture

of bone here, and it's cut open

to show you

that inside the core,

deep inside bone,

are these marrow spaces

and it includes red bone marrow

and yellow bone marrow.

There's usually an outer sort

of shell of compact bone

and then an inner core

of trabecular bone

for these marrow spaces.

And what's interesting

about bone marrow is

in adults we have yellow bone

marrow in certain places filling

our bones with adipocytes --

kind of space filler,

packaging so energy.

The interesting stuff though is

the red marrow found

in certain parts of our skeleton

which gives rise

to all our new blood cells

whether they're leucocytes

for defense

or erythrocytes carry oxygen

around the body attached

to hemoglobin the protein inside

those erythrocytes.

And also, the bone marrow also

makes those little platelets

which help in blood clotting.

So red bone marrow gives rise

using these hematopoietic stem

cells to all the cells

of our blood.

If we look at real bone,

what we see is this outer shell

of compact dense bone covered

by a fleshy periosteum

which is sort

of like a connective tissue

cover of our bones.

And then deep

to the compact bone is this

trabecular bone.

Trabecular bone has these spaces

or sort of grooves

and little nooks and crannies

where we have our bone marrow.

And there's lots and lots

of living cells in our bone,

and so we need a rich blood

supply that feeds our bone

and helps keep it nourished.

We also have a nervous supply

as well so that's why

if you've ever broken a bone you

know it hurts so badly.

So sometimes your blood supply

can be interrupted to your bone.

And in that case,

we get a condition called

osteonecrosis

or avascular osteonecrosis

which can cause death

to your bone.

Again, we have this fleshy outer

cover of bone called

the periosteum.

It's mainly dense connective

tissue of collagen built

by fibroblast.

But there's a couple of layers

of fibroblast

and osteogenic stem cells

in there which will become

important when we talk

about fracture repair

in that periosteum.

Then there's compact bone just

deep to that.

The compact bone has mostly bone

matrix with some osteocytes

in there that maintain the

bone matrix.

And so again,

that bone matrix is collagen

protein with some calcium

phosphate mineral.

Then deeper

to that is the trabecular bone.

The trabecular bone you can see

has little spaces

that are filled

with bone marrow,

either red bone marrow

or yellow bone marrow filling

the trabecular bone.

Trabecular bone is hardened.

Some textbooks call it spongy

bone, which I don't really

like because that makes me think

of it as soft.

But it's really all hard.

I think the reason they

like spongy was

because there were spaces in it.

So we'll call it compact bone

and trabecular bone.

If we zoom in on compact bone,

what we see is very little

space, open space.

It's instead these compact

columns of bone matrix called

an osteon.

And you can see several osteons

in this little picture here.

The osteons have these little

rings of osteocytes which help

to build the osteon.

And then they help maintain it.

Each osteon has a little central

canal, a little tunnel,

that allows things --

soft things like blood vessels

and nerves to run

through the hardened bone.

So again, the bone matrix is

collagen and mineral.

And so we see several osteons

making up this compact

bone section.

If you look at trabecular bone,

you don't see any osteons.

Instead, you see marrow spaces.

And then in the hardened bone

matrix it's a little more

irregularly shaped.

It's still pretty much the same

stuff, osteocytes embedded

in a bone matrix of collagen

and mineral.

And then the other thing you'll

notice is that you see these

lining cells that cling

to the edge of the bone

in the marrow spaces.

This is called endosteum.

So endosteum is the cellular

lining in the interior spaces

of bone.

It includes cells

like the osteoclast,

the osteoblast

and osteogenic cells,

which we'll learn more about.

Again, if you look

at the marrow spaces

of trabecular bone,

you see all these little cells

clinging to the edge

of the bone.

And so we call those cells

endosteum as a group.

Okay, the fleshy outer cover

of bone we call periosteum.

And that's connective tissue,

dense connective tissue somewhat

similar to maybe the dermis

of the skin.

And it has cells

in there as well.

If we zoom in again

to that same picture,

you can pause it

and check this out,

but we can see the osteon.

Compare that to trabecular bone

where we see sort

of this irregularly shaped

bridges of hardened bone.

What I wanted you

to notice were the cells

in there.

You can see the osteoblast

clinging to the edges

of the bone.

But the bones embedded

in the matrix of the osteocytes.

You even see some osteogenic

stem cells clinging

in there near the osteoblast

and some really large osteoclast

that eat away the bone.

Stem cells called the osteogenic

stem cells,

create new osteoblasts

in our bone.