Let us look at thrombopoiesis or the formation
of the platelet. Here is a huge cell.
This is the megakaryocyte. It is multilobed. It
often has many many many different components
to the nucleus. It is a huge cell up to about
70 microns or even bigger. And what happens
is that the cell membrane around that megakaryocyte
invaginates a number of times into the internal
components of the cell and those invaginations
come together and join up and therefore part
of the cytoplasm is cleaved or broken off
and released into the bone marrow and then
into the blood system, the plasma and the
red blood cells within the vascular network.
And that is a platelet or a thrombocyte. And
as I explained in the lecture on blood, these
thrombocytes have very very important functions,
particularly in helping reduce the leakage
of fluid during damaged parts of the wall
of the blood vessel.
Well now let us look granulopoiesis, the formation
of the granulocytes. Here you see another
image of bone marrow. And what we want to concentrate
on now, is the granulocyte, monocyte, progenitor
cell lineage. The lineage is going to give
rise to the granulocytes and also the monocytes.
But as I have mentioned earlier, I am only
going to look at the formation of the neutrophil.
Well the first cell that starts off in this lineage
is the myeloblast. It is a large cell again.
It is about 20 microns in diameter and it
has got a large euchromatic nucleus.
Now when a blast cell forms, when cells often revert
to being a blast type cell, then they are
very busy cells. They are manufacturing components
and we will see that as we look at cells particularly
with the immune system and other tissues later
on. Because they are very busy cells, therefore
they have a very euchromatic nucleus rather
than a more heterochromatic nucleus, which
reflects usually that they are not so active,
they are resting. Well now let us look
at the next cell type in the pathway. Let
us see where these myeloblasts then moves into
or differentiates into, what sort of cell
is it and what is it look like? Well it differentiates
into the promyelocyte and this promyelocyte
contains or starts to generate the factory
that is going to make azurophilic or primary
granules. I say the word primary here because
these granules appear in all the granulocytes,
the neutrophils, the eosinophils and the basophils.
But you will recall if you have knowledge
of these three different granulocytes that
granules are quite different in their content as
they differentiate into their respective cell
types. So initially this part of the lineage
starts to accumulate these granules and therefore
they stain with the azu dyes, but they are
yet to go through the differentiation into
acquiring granules and the contents of those
granules that are specific for the cell type.
So this is a general cell, which we have to
really determine which direction it goes into.
Does it become a neutrophil or a basophil or
an eosinophil? The next stage of this cell