Let us look and see how cartilage grows. When
we look at the outside of cartilage, both
hyaline cartilage and elastic cartilage are
surrounded by a capsule. I've mentioned the term
capsule before in my general lecture on connective
tissues. Well, in the case of cartilage, both hyaline,
and both elastic cartilage, the capsule is really
called the perichondrium. Peri meaning around,
chondrium cartilage. We will see that bone
has a similar capsule called the periosteum,
around bone. Well, if you look very carefully at
this image, again try and point out to yourself,
a chondrocyte. Remember what I said about
the lacunar space, try and identify a lacunar
space in this image. Well look on the outside of
the cartilage. It is very fibrous.
On the very very edge, you can see two tiny little
round circles. They represent little blood
vessels coming into the capture of the cartilage.
That is how the cartilage gets its nutrition
from those blood vessels on the outside and
the nutrients, the oxygen diffusing through
the aqueous component of the cartilage that
I mentioned earlier when I described nutrition
to articular cartilage via the synovial fluid.
But that capsule has two layers, an outer fibrous
layer and an inner chondrogenic layer. And that
inner chondrogenic layer, if you look very
carefully this image, you can just see a region
where there are some cells with elongated
nuclei. Or on the very edge of the cartilage,
you see thinner, smaller chondrocytes and
they are packed a little bit closer together.
Well that is the chondrogenic layer.
And during development, those cells can continually
divide and form cartilage cells and deposit matrix.
And therefore the cartilage can grow in thickness.
We call that appositional growth, growth on
the outside to increase the thickness of the
cartilage. Whereas interstitial growth is growth
within the cartilage. I've mentioned earlier
isogenous groups. These represent where cartilage
cells are dividing and when they divide, they
are very close together, but then they start
to secrete matrix and they move further and
further apart, which is why you see in sections
of cartilage, some chondrocytes isolate on
their own, whereas others are lying very close
together representing the stages of appositional
growth. And once the cartilage matures, then
the chondrogenic layer on the perichondrium
disappears or at least is very minimum and
also the cartilage cells within the matrix
lose the ability to divide. So the question
is, can damaged cartilage be repaired? And
really the simple answer is no.
Why? Because of the fact that that perichondrium, particularly
the chondrogenic layer dwindles as the cartilage
matures and therefore there are very very
few cells or stem cells there that can divide
and bud off and therefore allow the cartilage
to repair itself if it is damaged.
The cells within the cartilage, the chondrocytes, lose
their ability to divide as well. And also as
I mentioned earlier, cartilage is avascular.
It does not have a direct blood supply and
therefore that impairs any ability for a tissue
to be able to repair itself. Often if cartilage
becomes vascularized through injury, it turns
into bone. So normally cartilage has very
very limited abilities to be able to repair
itself when damaged.
So, in final summary, make sure you are aware
of the structural and the functional differences
between the three types of cartilage. Hyaline,
elastic and fibrocartilage have really three
different sorts of fibre types. Make sure
you understand a little bit about the matrix
components, particularly the glycoaminoglycans
that bind the water into the cartilage and
resist compressive forces. Make sure you are
aware of the importance of the perichondrium
during growth of cartilage, during development,
the chondrogenic layer on the inside and the
more fibrous layer on the outside. This gives
rise to appositional growth of cartilage, whereas
cartilage can grow within as well because
of the ability for the cartilage cells within
the matrix components to divide. And just refresh
your memory again on what I said just a few
moments ago as to why cartilage
cannot repair itself.
So I hope you enjoy this little story about
cartilage. And in my next lecture, I am going
to again talk about another form of specialized
connective tissue and that is bone.