00:00
Speaking of interstitial growth,
this is going to occur
at the epiphyseal plate.
00:06
Recall that the epiphyseal plate
is a line of cartilage
that separates
the epiphysis from the
diaphysis.
00:14
Starting from the area farthest away
from the diaphysis
and closest to the
articular cartilage,
you have a zone of
resting cartilage.
00:25
This is followed by a zone of
proliferating cartilage,
then hypertrophic cartilage
and then calcified cartilage.
00:33
So taking a closer look at the cells
in each of the individual zones,
starting with the zone of
resting cartilage,
we have chondrocytes
which are not dividing.
00:44
Just below this zone,
we have the
proliferating cartilage.
00:49
In this zone
the chondrocytes
are actively going through
cell division
and allowing for more chondrocytes
to be made.
00:58
Just below this,
we have the zone of
hypertrophic cartilage.
01:02
In this area,
the chondrocytes are enlarging or
hypertrophying
and as well they are secreting
extracellular matrix.
01:11
They then become
engulfed or in caged
in that extracellular matrix,
which causes nutrients
to no longer be able to
get to those chondrocytes.
01:22
Because of this,
it starts to calcify,
and after calcification,
those cells are going to die.
01:29
These dead chondrocytes
are then replaced with
bone cells and bone tissue.
01:37
So we've talked
about interstitial growth
or the lengthening of bones.
01:42
But we also need to note that in
bone remodeling,
there's also a
thickening of the bone
also referred to as
appositional growth.
01:50
This occurs in four major steps.
01:53
The first step is going to be
the formation of
a periosteal ridge.
01:58
In the second step,
we're going to form a
haversian canal
that is going to house
the blood vessels of the bone.
02:06
And the third step,
this haversian canal
is going to get smaller
as the bones grow inward.
02:14
And in the fourth step,
the bone as a whole
begins to grow outward
as the periosteum creates
more bone matrix
from the outside.
02:23
Let's talk about each
one of these steps
individually.
02:26
In step one,
we create the
periosteal ridge.
02:30
The blood vessels in the periosteum
start to be engulfed or envelope
by the periosteum of the bone.
02:40
In step two,
this ridge now fuses
and forms a canal
around the blood vessel.
02:48
This now makes what used to be
the periosteum
on the outside of the bone.
02:53
Now an endosteum
that is spacing the
inside of the bone
toward the blood vessel.
03:01
And the third step,
we are now going to start
creating bone
toward the blood vessel.
03:07
So the bone is starting to grow
inward
toward the blood vessel
in the haversian canal.
03:16
And in the final step, we are also
going to be creating bone
that is going outward
from the periosteum.
03:24
Please note that when
we are doing this,
we are going to continue
to go through this process
over and over
around the different blood vessels
in the periosteum.
03:35
The layers that are made
on the outside
are referred to
a circumferential lamellae
or that translates
to basically
the circumferential layer
or the outer layers
of the bone.
03:48
During appositional growth,
we're going to be
thickening the bone
but we want to make sure
that the bone
does not become too heavy.
03:56
So as osteoblasts
are depositing bone
on the outer surface
of the bone.
04:03
Osteoclasts are going to be
working on the inside of the bone,
widening the medullary cavity
from within.
04:13
So now that we've discussed
both types of growth,
both interstitial growth
where we're growing in length,
and appositional growth
where we're growing
in thickness,
let's talk about some factors
that will influence
this bone modeling.
04:27
One of those factors is the
response to stress.
04:30
The more stress you put
on your bones,
the more that the bone will grow
in thickness.
04:37
This means that someone who
exercises a lot
or puts a lot of stress
on their bones
will have thicker bones
than those who live a
sedentary lifestyle.
04:48
Another factor is growth.
04:50
Specifically, when an individual
goes through puberty,
there is a large amount of
interstitial growth
or growth in length
of the long bones
especially the bones in the legs.
05:03
This is controlled by different
hormones.
05:06
The two main hormones that control
this growth are going to be
androgens and estrogens.
05:12
An example of an androgen is
testosterone.
05:15
Now,
while men mostly have a lot more
testosterone than women
and women have more estrogen
than men,
both men and women
have some of both.
05:29
Androgens are more responsible for
the lengthening of the bones.
05:33
and estrogens are actually going to
reverse this process
and start the process of stopping
the bone modeling.
05:41
This is the reason why
women stop growing before men
because they have more estrogen.