00:00
As I mentioned in the very start
of this lecture, there are
three types of cartilage. So let us have a
look at these three types of cartilages.
00:10
It is very important that you know how to distinguish
each of these three types of cartilage and also
it is important for you to understand the
functional role each of these cartilage has
in various locations of the body. Remember
though, that even though there are three types
of cartilage, each of these cartilages contain
cell,s chondrocytes. They each contain the glycoaminoglycans
that are so important in maintaining a lot
of water content and resisting compressive
forces. They also contain proteoglycans and
also glycoproteins that are also very important
in stabilising the extracellular matrix and
making sure the cells adhere to most components
of the extracellular matrix. By far the most
common type of cartilage is hyaline cartilage.
01:14
We will have a look at that in more detail,
but it contains basically collagen type II.
01:21
We have elastic cartilage in certain locations
of the body and as the name suggests, they
contains predominantly elastic fibres, although
there is collagen in this type of cartilage
as well, giving it some sort of support. And
the last type of cartilage is fibrocartilage.
01:40
As the name suggests, it is very very fibrous.
It has a network of strong collagen type I
fibres. It is very much like dense regular
connective tissue such as a tendon or a
ligament. And it is often difficult to distinguish
fibrocartilage from dense connective tissue
that is regular. So let us look at hyaline
cartilage first. As I said earlier, it is
the most abundant form of cartilage in the
body. Have a look at this image that you have
seen before. See if you can point out to yourself,
a chondrocyte. There are many here in this
image. Sometimes you can see the chondrocytes
appearing in pairs and those pairs we call
isogenous groups. Sometimes they are in collections
of three or four. And I will mention again
these isogenous groups later on, when I talk
to you about the growth of cartilage.
02:50
When you look at this image and you see the matrix,
you can see that it is different colour in
different locations. If you look very closely
around the chondrocyte, you can see that the
matrix has a very bluish tinge and we call
that the capsular or the pericellular matrix.
03:12
That matrix, very close to the cell. Further
out, we have territorial matrix around the
cell, which looks very bright pinky reddish
colour and then finally we have matrix further
out, away from the chondrocytes called the
interterritorial matrix. I do not think it
is really important to worry too much about
understanding the names of all these areas
of the matrix. The reason why I mention it
here is because it just reflects the concentrations
of matrix components as they are secreted
by the cell. They are secreted by the chondrocytes
and therefore they stain a lot more dense.
Then when they begin to diffuse out and rearrange
themselves further out in the matrix separated
away from the cells. So it really reflects
the different compositions of the matrix as
the cell is producing that matrix and forming
the matrix. So these staining differences,
although they are not important for you to
memorize and they are not really an important
part of the histology of cartilage, they just
merely reflect the very active chondrocytes,
particularly during developing cartilage.
04:37
These chondrocytes were busy every day making
sure that the matrix components are replaced
because some of them have rather short half lifes,
some have very long half lifes. So they are
busy busy cells. Well, let us continue looking at
the matrix of hyaline cartilage. It is clear
or glassy, hence the name hyaline. But also
understand that there are many different collagens
that are embedded in the extracellular matrix
of this cartilage. When I described collagen,
the structure of collagen in a general connective
tissue lecture, I explained that the collagen
fibres are made up of collagen fibrils, each
made up of individual collagen molecules. And
the collagen molecule itself consists of three
alpha chains of polypeptides. These polypeptides can
contain between 600 and 3000 different
amino acids. So because of that, there are
different collagens in the body. In general
connective tissues, I mentioned only collagen
type I and collagen type II and collagen type
III because they are the main collagens that
we will come across in this histology course.
But in this particular image you see, there
are a list of different collagens that as
I said earlier, are embedded in the extracellular
matrix. And what these collagens do is they
bind the extracellular matrix together.
06:34
They combine or they bind the matrix with the cells,
with other proteins, with other glycoproteins
and they create a three-dimensional lattice
in the extracellular matrix. And that is very
important for the chondrocytes because they
detect compressive forces. They detect the
stresses that are imparted on the extracellular
matrix. They have the ability to sense all
sorts of mechanical stresses. It is like you
going and lying on a mattress, a soft mattress
or a hard mattress. You can tell the difference.
Similarly, the chondrocytes also can tell
the difference and they continually adjusting
the matrix components to make sure that the
matrix is of the right components, the right
constituents, to be able to fulfill the role to
which it needs to be designed, resisting
compressive forces also being flexible and so on.
07:46
Hyaline cartilage is found at the surfaces of joints.
Their job here is to be cushions and to resist
compressive forces. They are friction free
surfaces at joints. They are lubricated by
synovial fluid. In this section, you can see
two opposing bones joined together but separated
by a very thin space, which is filled with
synovial fluid and the surface of the bone
has articular cartilage upon it. Now that
synovial space is filled with synovial fluid
secreted by a synovial membrane on the outside
of the joint. It is very difficult to see
here and the joint is supported on the outside
by ligaments. But this articular cartilage is
designed, as I said earlier, at a joint to resist
compressive forces and to act as a cushion
and a friction free surface for joint movement.
If you look at very high magnification of
the articular surface of cartilage, then it
has really four different zones. Have a look
at the arrangement of the chondrocytes at
the very peripheral part of the cartilage,
at the edge of the joint or in contact with
the synovial fluid. That synovial fluid not
only lubricates the joints, but it provides
nourishment for the cartilage cells.
09:31
Remember I said that cartilage is avascular. Well, the way
in which cartilage gets its nutrition is because
the extracellular matrix has a watery soluble
component, an aqueous component. So nutrients can
very readily move through those aqueous
components of the extracellular matrix and,
therefore, the chondrocytes can get their
nutrition. In the case of articular cartilage,
the nutrition comes to these cells through
the synovial fluid. But there are different
zones you see in the articular cartilage.
There's a superficial zone with a cartilage
cell seem to be elongated parallel to
surface. The intermediate zone where they tend
to be lining rather randomly throughout the
articular cartilage and there's a deep zone where
they tend to be lined up in columns. And there
is a further area, right outside, next to the
bone where the cartilage tends to be calcified.
Now those four zones although it is often
difficult to distinguish between each of those
zones just looking at the cartilage, it is
easy to distinguish the zones if you could
have a look at through arrangement of the
collagen. We cannot see that there, but in
special preparations, you will find that in
the superficial zone, the cartilage is arranged
parallel to the surface. In the intermediate
zone, the cartilage is arranged in a more
random criss cross networking type arrangement.
11:13
And towards the deeper zone, the collagen
fibres are arranged along the long axis of
the bone and this arrangement is ideal for
articular cartilage to be able to withstand
compressive forces coming down onto the joint
during movement and because of body weight
and gravity. Now you cannot see in this slide,
but just adjacent to the calcified zone of
cartilage, towards the synovial joint is an
area where the cartilage cells are constantly
dividing and replacing other cartilage cells
that have limited lifespan at the very surface
component of the joint. Another place in which
we find hyaline cartilage is supporting certain
structures. Here you see an image through
the trachea and also the external ear canal.
12:15
These are areas where you really need a very
strong support. You can feel the cartilage
rings in your throat yourself. If you did
not have cartilage there, your trachea or
your major windpipe going down to your lungs
would collapse. So cartilage here is very
important to maintain an open airway and we
will look at this type of cartilage again
when we look at the respiratory system later
on, similarly with the external ear canal.
12:51
One of the remaining functions of hyaline
cartilage is it acts as a fetal skeleton. And
during development of bone, that fetal skeleton
of cartilage, that template of cartilage is
replaced by bone. Now be very careful to hear
what I said a moment ago. I said that the
cartilage is replaced by bone. The cartilage
does not turn into bone and that is an important
issue that we need to remember when I will
talk to you about the growth of bone and
the formation of bone in a later lecture.
Elastic cartilage quite obviously has got
predominantly elastic fibres within it. In
a normal H&E section stain here, you can
see the elastic fibres. Remember from connective
tissue lectures in a general description of
the elastic tissue, elastic fibres are branched
fibres. And although you cannot see in detail
here, they also interact with the collagen
fibres in this type of cartilage, but the
prominent component of elastic cartilage as
the name suggests is elastic fibres.
14:18
And that is important to maintain flexibility support,
strong support, but also flexibility of the
structures where you find elastic cartilage.
And one place you find it, is in the pinna
of the ear shown here. I can twist my ear
around and then let it go and it springs back
to place because of the flexible elastic
cartilage within it. Well, the other remaining
type of cartilage is fibrocartilage. As I
said earlier, it is very similar to dense
connective tissue. It is often difficult to
tell the difference between fibrocartilage
and dense connective tissues. Here you see
it in a joint in the vertebral column.
15:15
It is important to be in the joint of the vertebral
column where we need to withstand very strong
compressive forces, particularly during movement,
but also body weight forces that are passed
down the vertebral column. And again if you
look at the section, you can see lots and
lots of pink stained collagen predominantly collagen
type I is there, which is a very strong thick
collagen, but there's also type II collagen there
that you find in hyaline cartilage. And sometimes
in the intervertebral joints that you see
on the right-hand side, if you could look
at that joint under a microscope, you would
see hyaline cartilage there as well as fibrocartilage.
16:05
And that is often difficult to see the two,
unless you look for these very strong deep
stained bands of collagen type I within the
fibrocartilage. Again look at the chondrocytes,
you can see little tiny dots of chondrocytes.
The very clear space around the chondrocytes
that you would have noticed looking at hyaline
cartilage earlier is called a lacuna.
16:31
Normally the chondrocyte would occupy the entire space.
Think back on that slide or image of hyaline
cartilage, where all you saw of the chondrocyte
was a little tiny dark dot representing
perhaps the nucleus or the debris of the cell
after all the processing has taken place for
you to review it under a microscope. But normally
the lacuna space is really occupied by the
entire cell. It is just an artifact really
due to shrinkage of the cell during processing.
17:11
The sorts of places where you find fibrocartilage
is obviously where you want to withstand very
strong compressive forces such as the intervertebral
disc, the symphysis pubis in the pelvic joint,
the temporomandibular joint to withstand
the compressive forces of chewing, and also
the menisci of the knee to again withstand
the forces or the compressive forces particularly
during movement and the dissipation of body
weight down through the body.