- [Voiceover] As bones start to develop,
they actually start very early on in fetal development.
They start as the formation of cartilage.
So, you have these chondroblast cells.
Chondroblasts are cells that will make cartilage.
So they actually form this outline of cartilage
to concrete this flexible bone,
before the true bone is formed.
But, as you go through fetal development,
later and later, closer and closer to birth,
you'll have the osteoblast cells.
The cells that can produce bone
replace the cartilage and start to make new bone.
They'll slowly replace, bit by bit,
to create an actual, solid bone.
But this whole process of replacing cartilage
doesn't really complete until after birth.
Early in childhood, you'll have these
ossification centers that are going
to start to close up.
Meaning when an infants first born, they actually
don't have a knee or an ankle or a wrist.
Most of the joints are still just cartilage.
Those joints must turn into bone as that very
young child is starting to mature.
By the time you get up into adolescence,
now we have the bones elongating.
As the bones elongate, that's the growth of a child,
as they get taller and taller.
So, it's the elongation of these growth plates.
But, growth plates don't keep growing forever.
There comes a point, later in a child's life,
where the growth points just seal up,
and once that growth plate seals up,
the bones will no longer get longer.
The person can no longer get taller.
The bones can still change shape,
things still modify, but they can never
grow in length anymore, once that growth plate seals.
Here we have this formation of bone.
Left hand side, will start with that fetus
to the first two months pretty much.
You can notice, that's all cartilage.
That blue is pure cartilage.
As you move along, in that two to three month range,
you start to develop some of the periosteum.
That's the bone around the outside.
This is the very first developing stages of bone.
Keep on going into childhood now,
this is after birth, you see there's
a bit of bone, but you still have quite a bit
of blue cartilage left.
That blue cartilage is being converted over to bone
as that brand new born infant starts to mature
and grow months and months older.
Eventually, throughout adolescence,
the bones get longer, because of
the cartilage growth plate.
It just keeps adding more and more
bone, making the bones longer and longer,
helping the individual grow.
So you start off as cartilage, and you
add bone slowly, throughout a long
duration of time.
The way it actually works is,
you have this growth plate at the end of a bone,
now you can see in this image,
how you have a darker blue and a lighter blue,
in the growth plate.
The darker blue is where new cartilage is being formed.
As new cartilage is made, it's actually
pushing the top of that bone upwards.
But, as you make new cartilage, the older
cartilage, which is the lighter blue, below,
starts being converted into bone.
So, the body can constantly add new layers
of cartilage on top, pushing the end up,
and allow the bone to replace the old cartilage.
So, this is the bone elongation,
how one grows in height.