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

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

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

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

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

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So, it's actually ossification.

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

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

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So, the fetal stage begins

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when we start to see bones being produced

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or when we start to see ossification happening.

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

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much more familiar with them through the activities

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that you do in this particular folder.

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

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

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

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Intramembranous ossification,

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and the name of this particular type of ossification

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

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So “intra” is within basically, right?

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And membranous

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refers to a membrane.

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

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

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

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

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The collar bones.

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

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through intramembranous ossification.

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In addition, the skull bones.

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

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they are considered to be flat bones as well.

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And they form primarily again through this intramembranous ossification.

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If you look at this picture over here, and you

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

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

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were just a fibrous membrane.

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

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which is bony tissue starting to form.

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

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

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and that intramembranous ossification has not yet occurred.

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You can't see it in this particular diagram,

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but there's a big area of cartilage right here

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at the top of the head, where the bones

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

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that membrane in which the bone will eventually ossify.

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This is the soft spot that you see on infants.

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It's hugely important because first of all, 

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

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to be shaped and formed and squished a

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little bit because we don't have totally solid,

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hard tissue making up the skull.

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But we've got lots of areas of

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cartilage which are softer and which are more malleable.

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

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the brain is going to grow largely.

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

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

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that membrane, is going to allow

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the brain to expand within the skull.

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

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

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

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replaced with bone until about two years of age. 

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

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So that's intramembranous ossification and you're

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going to have a chance through the activities

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

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The other type of ossification that we see happening

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

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

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

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

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“chondral" is a term that basically means cartilage.

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So, endochondral ossification is bone formation that's

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happening from a hyaline cartilage model.

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

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

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irregular bones, and these types of

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bones primarily formed through the endochondral

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ossification mechanism.

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

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

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

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

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

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So that's the process for endochondral ossification.

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

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besides flat bones, are actually formed.

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And in this particular type of ossification,

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bones are forming from a hyaline cartilage model as

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opposed to, from a membrane in the body.

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

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Is that when we say flat bones

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are formed primarily from intramembranous ossification

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and long bones are formed primarily from endochondral ossification.

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

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So, the clavicles, for example, 

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are representative of this.  The clavicles are flat bones.

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They are formed primarily through intramembranous ossification,

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