One of the other cells that we
see in connective tissue is the stem cell.
On the left hand side, there are certain stem
cells in epithelial tissue. These epithelia
shown here happen to be lining the intestinal
area in the small intestine and epithelial
cells are continually lost or that need to
be replaced. So in epithelia, you have these
stem cells. While on the right hand side, you have
a stem cell called the pericyte. The pericyte
is associated with blood vessels, little blood
capillaries and they do a number of things.
They can control the growth of endothelial
cells, in other words the growth of blood vessels,
but they are relatively undifferentiated.
But they can be stimulated to become advent
tissue cells or paravascular cells to strengthen
the capillary wall. In some circumstances,
they can develop into smooth muscle cells
as the blood vessel network develops further.
And here you see some smooth muscle cells around
the very very small lumen of a blood vessel,
that perhaps could have been derived from
one of these pericytes. You know in actual
fact, the pericytes are surrounded by the
external lamina of the endothelial cells that
lines the blood vessel, so it is seperated
from the connective tissue. So really or probably
not, by definition, a connective tissue cell,
what we tend to talk about pericytes when
we talk about very small blood capillaries
lying in connective tissue spaces. Well, one of
the other connective tissue cells that I have
mentioned before, are the immune cells in the
lamina propria. Here is an image of the lamina
propria supporting an epithelial surface.
Sometimes these immune cells such as the
lymphocyte here can wander off into the
epithelium and they are really on sentry duty.
They are really wandering through taking out surveillance
activities to make sure that they can identify
antigens, they are programed to identify and
then alert the body to the invading antigens
that they detect. Well finally, let's
look at the plasma cell.
The plasma cell operates in connective tissues,
but as we learn when we deal with the lymphoid
tissues, it is derived from the lymphocyte.
On the right hand side, you can see the plasma
cell labeled, lying within connective tissue.
It is easy to identify because it has a characteristic
clock face nucleus with the heterochromatin
is arranged around the periphery of the nucleus
creating what appears to be a clock face.
These are very very busy cells. They are making
antibody. And so when you look at the cytoplasm,
sometimes it has its basophilic appearance
and this reflects the protein factory within
the cytoplasm that is making the antibody,
the protein. The little clear halo you often
see around the nucleus is the golgi complex,
the part of the cytoplasm, that packages of
these proteins, these antibodies for export.
On the left hand side, it is very difficult
to see the plasma cell because they are embedded
amongst other nuclei or other cells close
to the epithelial surface. But again, if you
look very carefully through these cells, you
can identify them by having the very characteristic
clock face nucleus.
Well, as a summary, let's go back to this
diagram and my challenge for you is to now
review the sorts of cells that I have just
described that are resident in connective
tissue and they are all derived from the mesenchyme
cell. And make sure you can also differentiate
in this diagram, cells that are originating
from blood cells because in a later lecture
on blood, which is another form of connective
tissue, I am going to discuss these cells
as well. Remember the resident cells that
are derived from mesenchymes are always present
in connective tissues whereas the wandering
cells derived from blood wander in and
out, undergoing a surveillance role, looking
for foreign proteins, foreign cells and antigens.
And we will talk about all of these cells
when we look at the lymphoid tissue. And also
before we move on to the lectures on cartilage
and bone, make sure you have a good understanding
of the sorts of fibres that make up connective
tissue. The collagen fibres, reticular fibres
and elastic fibres because those fibres are
going to be very important and dominate in
certain types of cartilage and also in bone.
And also have a general understanding of the
extracellular matrix because again that often
determines the type of properties that the
connective tissues have, that we will talk
about later on cartilage, bone and blood.
So I hope you enjoyed this lecture.
I hope you are looking forward to the subsequent
lectures on cartilage and also bone.