So let's now move on and look at connective
tissue cells. Remember, the three components
of connective tissues are fibres, matrix and
nerve cells. We will get in more detail and
these cells are really derived from mesenchyme.
Here is a very very what looks like a complicated
diagram with some labels scattered all around.
And I put this in this lecture to give you
a challenge to make you look at it and try
and decide by looking at the left-hand flow
of cell differentiation that represents
cells that are derived from mesenchyme.
Look at that derivation of these cells as opposed
to those on the right-hand side that are derived
from blood. Remember I said early in the lecture,
that the cells come under two categories.
Those that are resident in connective tissue
and those that wander in from the blood.
So my challenge to you, is to have a look at
the names of the cells down below and then
find their location in one of the two
differential pathways, either from mesenchyme
or either from blood. And I think it is important
that you do that yourself because that helps
you understand where each of the cells actually
come from and we'll revise through this as
we go through this lecture and also as we
look at blood later on.
One of the main connective tissue cells
that we should look at, is the fibroblast.
It makes all the fibres, collagen, reticular fibres
and elastic fibres. It also makes the components
of the extracellular matrix. Up until now,
I have used the term reticular fibre, but
really reticular fibres are collagen fibres.
They are collagen type III fibres. They are very
fine fibres. They were already seen formed
the framework of lymph nodes and other hemopoietic
tissues that we will talk about later on.
The early histologist sort out different sorts of fibres,
but later on with more research they are
really just found to be normal connective
tissue collagen fibres, but of the type 3.
So when you hear the term or when you read
about the term reticular fibre, just sort
of think what they really are collagen fibres,
but very fine fibres. Well let's get back to the
fibroblast shown here. All you see is a nucleus,
sort of rounded up or triangular shaped. It is
associated with collagen fibres, you see stained
red here. And if you look on the electron micrograph
picture, these fibroblasts are full of granular
endoplasmic reticulum, that reflects the huge
protein factory inside the cell. When you see
these fibroblasts actually working, doing
their job, during development, you will see
details of the cytoplasm. It will be basophilic
reflecting the enormous amount of the protein
factory within the cell cytoplasm. Here, in
fully differentiated adult tissue, the fibroblasts
are really just sitting there maintaining
the extracellular matrix in the fibres.
They are not busy, as busy anyway, as what they
would have been during early development.
Sometimes we might refer to these fibroblasts as
being resting fibroblasts or fibrocytes. And the
top small image shows you the nuclei of one
of these fibroblasts and the very extensive
cytoplasmic processes. You don't appreciate that
when you see each of these sections as the
one you see in this main image here. Well let's
move on now and have a look at the macrophage.