Intramembranous and Endochondral Ossification

So, in this video, we're going to talk a little bit about ossification,

which is the term that we use for formation of bones.

If you look at this picture over here, we've got a fetus that is forming.

And this process of ossification, or the formation of bones, actually begins

in utero and it starts at about 8 to 9 weeks of pregnancy.

So, it's actually ossification.

It's actually the formation of bone that causes us to go from

calling a developing baby an embryo to actually calling them a fetus.

So, the fetal stage begins

when we start to see bones being produced

or when we start to see ossification happening.

There are two different types of ossification and you're going to get

much more familiar with them through the activities

that you do in this particular folder.

But I want to give you just kind of a general introduction

to them now, and talk about them just a little bit.

So, the first type of ossification that I want to mention is,

Intramembranous ossification,

and the name of this particular type of ossification

gives you some information about what's actually happening here.

So “intra” is within basically, right?

And membranous

refers to a membrane.

So, this is a type of ossification that's happening within a membrane.

This is the primary way that flat bones in the body are formed.

So, there's a couple of good examples of flat bones in the

body that I want to use, in talking about this.

The collar bones.

So, the clavicles are an example of flat bones that are primarily formed

through intramembranous ossification.

In addition, the skull bones.

So, if you look at the skull on the fetus here,

they are considered to be flat bones as well.

And they form primarily again through this intramembranous ossification.

If you look at this picture over here, and you

actually look at the fetus and the skull that's forming,

you'll notice that we have these different areas that at one point

were just a fibrous membrane.

And now they're kind of starting to harden into something that's more solid,

which is bony tissue starting to form.

You'll also notice that as part of the skull,

we have big areas where it's still really just a fibrous membrane

and that intramembranous ossification has not yet occurred.

You can't see it in this particular diagram,

but there's a big area of cartilage right here

at the top of the head, where the bones

haven't fused together and where we've still just got

that membrane in which the bone will eventually ossify.

This is the soft spot that you see on infants.

It's hugely important because first of all,

it's going to allow the baby to pass through the birth canal with its head being able

to be shaped and formed and squished a

little bit because we don't have totally solid,

hard tissue making up the skull.

But we've got lots of areas of

cartilage which are softer and which are more malleable.

The other thing is during the first two years of life,

the brain is going to grow largely.

It's going to grow much larger than it was at birth, and having these

softer areas where the bones aren't fused together where we've still just got

that membrane, is going to allow

the brain to expand within the skull.

So, baby soft spots, particularly that one right here,

there's also big ones along the side that many people aren't as familiar with.

But those soft spots usually aren't completely ossified or completely

replaced with bone until about two years of age.

Which is going to allow for the brain to continue to grow.

So that's intramembranous ossification and you're

going to have a chance through the activities

that you do in this folder to learn about it more.

The other type of ossification that we see happening

in the body is what's known as endochondral ossification.

So again, you can break down the name of this type of ossification

to find out what basically is happening. And that's my whiny german shepherd.

Endochondral means that it's happening within the cartilage. So “endo” is within,

“chondral" is a term that basically means cartilage.

So, endochondral ossification is bone formation that's

happening from a hyaline cartilage model.

If you look at the picture that we have again over here, of this fetus that's forming.

You'll notice that we've got a lot of long bones and

irregular bones, and these types of

bones primarily formed through the endochondral

ossification mechanism.

And you can see, for example, if you look down here at the lower leg bones,

and then this area of the knee and the upper leg bones,

we've got bone that's forming from a hyaline cartilage model.

And then we've got areas of that hyaline cartilage model where the

bone hasn't completely formed, where we're still just looking at cartilage.

So that's the process for endochondral ossification.

It's the primary mechanism by which the other bones,

besides flat bones, are actually formed.

And in this particular type of ossification,

bones are forming from a hyaline cartilage model as

opposed to, from a membrane in the body.

One of the things that you should know about ossification though,

Is that when we say flat bones

are formed primarily from intramembranous ossification

and long bones are formed primarily from endochondral ossification.

It's not as cut and dried, it's not as black and white as that.

So, the clavicles, for example,

are representative of this. The clavicles are flat bones.

They are formed primarily through intramembranous ossification,

but there's also some endochondral ossification that happens within them as well.