0:00

Okay, let us take a look crudely

0:03

with a cartoon diagram of what the hyaline cartilage at birth looks like

0:08

and then we'll talk about some of the development.

0:10

We have a nice blank screen here.

0:11

I'm going to use a blue color just so you have an idea of what cartilage is.

0:17

And typically, I think of cartilages as being blue.

0:20

Let's just draw a nice big area here.

0:22

So think of this as the end of the long bone.

0:25

This is the epiphysis.

0:26

The metaphysis would be somewhere over here

0:28

and the diaphysis would be somewhere down here.

0:30

So right now we're concentrating on this part, which is the epiphysis.

0:36

I'm going to just call it EPI. Okay?

0:38

So when you're first born,

0:40

it's basically just a bunch of cells, bunch of cartilage cells.

0:49

They're very tightly packed.

0:51

And there are actually three components to this epiphysis.

0:55

There's what's called the articular

0:57

cartilage, which is at the surface. We're going to call that A for articular.

1:00

There is the majority, which is going to be

1:05

sort of in the center here, and that's called the epiphyseal cartilage.

1:11

The whole thing is the epiphysis and the majority is called epiphyseal cartilage.

1:15

And there's an area close to here,

1:18

the metaphysis, that's going to be the physeal cartilage.

1:23

And there's different composition of each section.

1:25

And actually, it depends on what the chemical and cellular

1:31

composition is to call this the articular, epiphyseal and physeal cartilage.

1:36

Let's talk now a little bit about the articular cartilage.

1:39

The articular cartilage, as I said, is the outermost layer of the epiphysis.

1:44

It contains three major things.

1:48

One, it's called aggrecan,

1:49

that's not very important.

1:51

Two, it has type 2 collagen.

1:53

And three, the most important thing, is it has a lot, a lot, a lot of water.

1:58

It's about 70% water by volume.

2:01

That's why on T2-weighted images and STIR images, the articular cartilage

2:06

looks very, very bright because of all the water that's unbound.

2:09

Okay? So, major thing to realize is it has a lot of H2O in the articular cartilage.

2:17

The articular cartilage also has several zones from the outside to the inside.

2:21

We're not going to worry so much about that.

2:23

The really important thing is the very

2:25

inner zone, which is calcified, and that often looks a little dark in MRI.

2:29

And I'll show you that as we go through these images.

2:31

The second thing I want to concentrate

2:33

on is the epiphyseal cartilage, which is this big area over here.

2:38

This is all epiphyseal cartilage.

2:40

In that epiphyseal cartilage, you've got chondrocytes, collagen,

2:43

glycosaminoglycans, and also has lots of water.

2:47

But the water in here, H2O in here,

2:52

are often bound to molecules, very tightly bound to molecules.

2:58

Because the water is tightly bound,

3:00

it's going to have a different signal characteristic

3:02

on MRI. It's often going to look a little darker, actually a lot darker

3:06

on the fluid sensitive sequences such as T2 or STIR.

3:09

Okay?

3:10

And you've got vessels coursing through there that actually we're going to talk

3:14

about how that contributes to

3:15

contribution to forming a bone later on.

3:19

And finally, the physeal cartilage,

3:21

which is this area over here next to the physis.

3:24

It's about 75% volume of cells.

3:27

So you think because there's so many cells there, it's going to be not so much water.

3:31

But the cells here are actually very big

3:34

and the intracellular component has a lot of water.

3:39

So that also an MRI is going to look very, very bright.

3:43

As a review, articular cartilage is going to be

3:45

bright, physeal cartilage is going to be bright,

3:49

and the epiphyseal cartilage, which is the majority here, is going to be very dark.