Sometimes it is important to
understand at this level before
we look at some of the organ systems later
on, that connective tissue elements in certain
locations often get called special names.
We know about the lamina propria and I am
going to emphasise the lamina propria a lot
because it is so important when we look at
all sorts of tissues and all sorts of organs.
But here is a section of an organ that happens
to be the testis, part of the male reproductive
system. And you can see a bright pink or red
capsule around the testis. Most organs have
a capsule full of collagen.
That collagenous capsule is also around lots and lots of
other organs, to separate the organs from each other.
So when you hear the word capsule, think of
collagen. Sometimes organs are separated by
capsules on the outside and that connective
tissue of that capsule, the collagen can invade
into the organ, in this case a gland, and divide
the gland up by having connective tissues
septa or sometimes that word trabeculae is
used. And those septa carry in blood vessels
and nerves. Sometimes we can look at a gland.
Here is a gland, some mucous secreting gland
and the connective tissue could be called stroma,
whereas we refer to the secretory components
of the gland as being the parenchyma. In this
image, it happens to be a mucous secreting
component of the gland. You can see the clear
stained cytoplasm reflecting that some of
the mucous is being lost during normal processing
and you can see the nuclei of the cells squashed
up against the base of the cell. So parenchyma
and stroma are common terms we use when we
are looking at glands. The stroma is the very
fine connective tissue wrapping around parenchymal
cells and also the very fine duct system you
see here as well.
Let's look at the extracellular constituents
of connective tissue, the fibres. Well there are
three different sorts of fibres. The collagen
fibre, I've mentioned. It is the most dominant
fibre. It is the most abdundant. It is very
very strong. Now collagen is actually the
fibres are made up of collagen molecules.
Those collagen molecules are made up of a
helix of alpha chain of polypeptides. Those
alpha chains of polypeptides levels have also embedded
in them, sugars. So collagen is actual fact
is really is a glycoprotein. Now those polypeptides
can be made up of up to 600 to even 3000 or
so different amino acids. And because of that,
there are many different sorts of collagens.
I will refer to collagen type I, type II and
type III during most of this course in histology.
Think of collagen type I as being like a very
thick rope. A rope that might tether a boat
to its mooring. Collagen type II is a thinner
rope. A rope you might use for climbing
or swinging, whereas collagen III fibres are
just like of piece of string. So these really
means that the three types of collagen are
different in their size and in their strength, and
we will see that certain tissues of the body
such as tendon and fibroid cartilage that
need to resist very compressive forces or
have very very high tensile strength have
collagen type I in them. Whereas very fine
reticular fibres have collagen type III in
them. Elastic fibres are different.
They are very branched. And in this slide, it is very
difficult to tell the difference between elastic
fibres and the collagen. In most H&E sections,
the elastic tissue appears as a translucent
fibre running through the tissue, but in certain
instances, you can use certain stains to show
up these elastic fibres. They are branched.
They form a network throughout the collagen.
In this case, it happens to be in the wall
of the blood vessel that we will talk about
later on in another lecture. The reticular
fibres are very fine fibres and they contain
a lot more sugars in them than the collagen
fibres and the elastic fibres. And so, different
stains like the periodic acid shift reagent
and also silver stains enable us to see these
very fine reticular fibres. In this image,
you see the dark stained reticular fibres and
in the background, you can see lots and lots
of little pale stained nuclei. This is in a
lymph node. Reticular fibres are dominant
in lymphoid tissues, and these fibres are secreted
by reticular cells. And these reticular cells
cover the reticular fibre and as we learn
later on, these reticular cells are very very
important in how the lymph node behaves and
challenges and reacts against a pathogen or
an invading antigen. So at the top, it is very
difficult to tell the difference between collagen,
between elastic tissue and between smooth
muscle. But thankfully histologists have come
up with different ways of distinguishing these
sorts of fibres.