Let us look now in more detail at the cells
that make up the epidermis. We've already looked
at the different layers of the epidermis,
which reflect the process of keratinization
and our waterproofing role of skin, but let us
look at what cells are present and what they
do. First of all, we have actually mentioned
the keratinocyte before. It is a cell as you
see here stained with a special stain, that
shows you really the accumulation of keratin
within the cell. It is predominantly the cell
type in the epidermis. It is the important
cell layer that is in contact with the external
environment. And as I have told you in a previous
component in this lecture, its main job is
to produce the waterproofing agent keratin
and those insoluble proteins and lipids on
the surface layer of the stratum corneum.
Let us look at the melanocyte. The melanocyte
is the cell in the epidermis that produces
the pigment melanin. And melanin is very important
because it protects the layers of the stratum
basale, the cells in the stratum basale, the
nuclei in the cells and also the nuclei in
the layer above the stratum spinosum, it protects
those nuclei from excessive ultraviolet radiation.
And the process of creating melanin is a very
very rather long process, rather complicated
physiological process, ao I am not going to
describe it here. But what I want to emphasize
is that when the melanin is produced by the
melanocyte, it is donated to the surrounding
keratinocytes. The surrounding keratinocytes
actually ingest the melanin pigment by endocytosis.
And then the melanin is transported and surrounds
the nucleus. Here is an image of skin.
It is actually thin skin. So have a look now
and recall the section of the skin that I've
showed earlier. The layers are much thinner.
The keratin layer is much thinner.
So this really does describe the difference between
thick skin and thin skin that I've mentioned
before purely being really the difference
in the thickness of the epidermis. But what
I want you to concentrate on here is that
dark brown pigment you see in the basal layers
of the epidermis of this thin skin. That is
melanin and if you look very very carefully,
you will notice that the bulk of the melanin
is concentrated above the nucleus between
the exterior surface and the nucleus, again
shielding those nuclei from excessive ultraviolet
radiation. Those nuclei I remember are in
cells that are dividing constantly.
So you don't want that excessive ultraviolet radiation
damaging the DNA during that division process,
during the process of mitosis. Let us now
move on to the Langerhans cell. It is originated
from bone marrow. It moves into skin from
bone marrow. It is a dendritic cell and what
I mean by dendritic cell is that it has long
processes. You see these processes in both
these images. They are antigen-presenting
cells and they live mostly in the stratum
spinosum. And have a look at the image on the
right, the one with the pink background to it.
Have a look carefully into that pink background,
you can just make out the profiles of circular nuclei.
They belong to the nuclei of the keratinocytes
of the stratum spinosum. These cells as you
can see here not only have long processes,
but they also hold hands with each other.
These processes are joined together from one
cell to another. They are linked together.
Now I use the analogy that is like chicken wire. it
is like the wire of the fins and that is what
these Langerhans cells or dendritic cells
actually create in the epidermis, a whole
layer, if you like, of chicken like wire cell structure.
And if you are an invading pathogen, trying
to get through the epidermis to underlying
connective tissue and therefore the rest of
the body, its almost impossible to pass through
that epidermis and not come into contact with
on of these cell processes of these dendritic
cells, these antigen-presenting cells. And
when a foreign component or bacteria or an
antigen comes in contact with the cell processes,
the antigen presenting cells then ingests that
antigen, processes it and then puts it on
the surface of the cell and that cell then
leaves the epidermis. The antigen presenting
cell leaves the epidermis, and travels to a nearby
lymph node. And there it puts a little flag
up on the surface, and alerts T-cells and other
cells of the immune system that then recognizes
antigen and mount an immune response against
it to help protect their body. And we'll
learn about that in more detail in a later
Another cell type is the Merkel cell. It is
also a dendritic cell meaning it has long
processes extending throughout the cells,
throughout the layers of the epidermis. These
cells are hard to see. They are very difficult
to see and they tend to be situated at the
very base of the epidermis. They are mechanoreceptors.
Well, skin is also a very large sensory receptor organ.