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- [Voiceover] As bones start to develop,

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they actually start very
early on in fetal development.

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They start as the formation of cartilage.

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So, you have these chondroblast cells.

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Chondroblasts are cells
that will make cartilage.

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So they actually form
this outline of cartilage

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to concrete this flexible bone,

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before the true bone is formed.

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But, as you go through fetal development,

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later and later, closer
and closer to birth,

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you'll have the osteoblast cells.

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The cells that can produce bone

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replace the cartilage and
start to make new bone.

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They'll slowly replace, bit by bit,

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to create an actual, solid bone.

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But this whole process
of replacing cartilage

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doesn't really complete until after birth.

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Early in childhood, you'll have these

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ossification centers that are going

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to start to close up.

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Meaning when an infants
first born, they actually

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don't have a knee or an ankle or a wrist.

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Most of the joints are
still just cartilage.

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Those joints must turn
into bone as that very

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young child is starting to mature.

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By the time you get up into adolescence,

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now we have the bones elongating.

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As the bones elongate,
that's the growth of a child,

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as they get taller and taller.

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So, it's the elongation
of these growth plates.

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But, growth plates don't
keep growing forever.

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There comes a point,
later in a child's life,

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where the growth points just seal up,

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and once that growth plate seals up,

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the bones will no longer get longer.

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The person can no longer get taller.

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The bones can still change shape,

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things still modify, but they can never

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grow in length anymore, once
that growth plate seals.

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Here we have this formation of bone.

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Left hand side, will start with that fetus

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to the first two months pretty much.

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You can notice, that's all cartilage.

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That blue is pure cartilage.

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As you move along, in that
two to three month range,

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you start to develop
some of the periosteum.

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That's the bone around the outside.

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This is the very first
developing stages of bone.

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Keep on going into childhood now,

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this is after birth, you see there's

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a bit of bone, but you
still have quite a bit

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of blue cartilage left.

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That blue cartilage is
being converted over to bone

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as that brand new born
infant starts to mature

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and grow months and months older.

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Eventually, throughout adolescence,

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the bones get longer, because of

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the cartilage growth plate.

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It just keeps adding more and more

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bone, making the bones longer and longer,

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helping the individual grow.

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So you start off as cartilage, and you

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add bone slowly, throughout a long

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duration of time.

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The way it actually works is,

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you have this growth plate
at the end of a bone,

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now you can see in this image,

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how you have a darker
blue and a lighter blue,

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in the growth plate.

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The darker blue is where new
cartilage is being formed.

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As new cartilage is made, it's actually

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pushing the top of that bone upwards.

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But, as you make new cartilage, the older

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cartilage, which is the
lighter blue, below,

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starts being converted into bone.

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So, the body can constantly add new layers

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of cartilage on top, pushing the end up,

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and allow the bone to
replace the old cartilage.

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So, this is the bone elongation,

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how one grows in height.