In this lecture, I am going to describe skin.
It is often referred to as the integumentary
system. The integument is a system of the
body because it contains all the basic tissues
and that is why we call it a system. We will
see later on during this lecture, that skin
contains epithelium and glands derived from
epithelium, the exocrine components.
They contain connective tissue particularly the
dermis of skin that also contains muscle and
nerves. So that is why we call it a system.
Now, during this lecture, I would like you
to assemble information that will allow you
to then answer all the questions that I list
here in the learning outcomes. I am not going
to go through them all here, but I will go
through each of these in the
course of this lecture.
Skin has got many many functions and during
this lecture I am going to explain the structural
components of skin that enable these functions
to be fulfilled. First of all, let's make
sure we can understand the layers of skin.
In this section, on the right-hand side, there
is an image of a section through thick skin.
There are two main different types of skin,
thick skin and thin skin, and really the term
is a little bit confusing. Here is an image
of thick skin because the epidermis, a layer
I am going to describe in a moment is very
very thick compared to thin skin where it
is much much thinner. Here is the epidermis.
It is the surface layer. It is an epithelium.
It is a very special epithelium. It is a stratified
squamous epithelium because there are many
many cell layers making up the epithelium
as we will see in a moment. But the layer
on top, that pale purple thick layer happens
to be keratin and that is a very important
component of skin that I will describe later
on. Now this thick skin is found on the palms
of our hands or the soles of our feet where
we have a lot of wear and tear. But really
as I said before, the term is a little bit
confusing. Because really the thickest part
of our skin is on our back because of the thick
underlying layer, the dermis indicated here.
The thinnest part of our skin on our body
is the eyelid or perhaps the scrotum. In both
those areas, the eyelid and the back, the
epidermis is probably the same thickness.
So as I said the term is a little bit confusing.
During this lecture, when I refer to thick
skin, I am talking about skin on the palms
of our hands and the soles of our feet. On
other occasions, I would refer to thin skin.
But it is easy to describe the structure of
skin looking first of all at this place of
thick skin. You know sometimes we call skin
either hairy or non hairy. That is another
way in which we can classify skin and it is
probably an easier one to understand.
We certainly do not have hair on the palms of
our hand or the soles of our feet. So the two main
layers indicated here, are the epidermis, the
epithelium is derived from ectoderm and the
underlying dermis, which is derived from mesoderm.
The dermis is dense connective tissue.
When you look at it, it is very pink stained because
it is full of connective tissue fibres and
that dermis supports the epithelium, makes
our skin very strong. Underneath the skin,
underneath the dermis, you can see a very
clear layer. That is the hypodermis.
It contains adipose tissue or lots and lots of fat
cells. And if you look very very carefully in that
hypodermis, you can make out some little
round circular profiles stained bluish.
These are sections through sweat glands, one of
the skin appendages I will talk about later
on. They're all derived from the epidermis and
during development, they grow down and become
very specialized in areas of the dermis and
as you see here in areas of the hypodermis.
Well, let's concentrate first of all on the epidermis.
And as you see listed on the left-hand side
of the slide, there are five main layers and
I am going to go through these layers in some
detail now. First of all, let us look at the
stratum basale. It is often called the stratum
germativum because it is a germinating layer.
It is a single layer of cells, a single
layer epithelial cells tightly bound to
the underlying connective tissue, the underlying
dermis by very very large numbers of hemidesmosomes. These
are macular adherens special bonding structures
that bind epithelial cells to the underlying
connective tissue. Well the nuclei of the
stratum basale lie very close together, and that's
because there's not a lot of cytoplasm associated with
the cells. These cells divide constantly
because skin is continually renewed.
They divide constantly and as they divide, they
move towards the surface to form the other
layers of skin. The stratum spinosum is a
thicker layer. It consists of several layers
of cells. It is often called the prickle cell
layer or the spinous cell layer, and that's because
if you look very very carefully in this section,
better still if you had the opportunity to
look at this layer under higher magnification,
you will see between the cells, there is a
fine little clear space. And if you look carefully
into that clear space, you will see these
little tiny spike looking processes. These
represent an artifact. In actual fact, in reality,
the cells of this layer, the stratum spinosum,
are linked together by long cell processes
like my fingers here interlocking. And the fingers,
my fingers here join to each other by very
strong desmosomes. So skin, the cell layer,
the stratum spinosum, the cell processes are
joined together from one cell to another by
very strong desmosomes join the process
together, like desmosomes are holding my fingers
together. It is very hard for me to separate
my fingers apart because of those junctions.
That makes this layer of skin a very strong
layer that maintains the strength of the epidermis.
Then during processing, tissue shrinks.
So the cell shrinks apart and creates spaces between
the cells and you see these projections or
processes like my fingers are showing here.
They are the spinous processes of the cells.
They are the prickles, and that is why its
called the prickle cell layer. These cells then
move up and as they move up, they accumulate
little granules, granules of keratohyalin.
This is the very beginnings of these cells
transforming into being squames of keratin.
These keratin are hardened granules accumulate
in the cells. They are very very strongly
basophilic, dark blue staining. The stratum
lucidum is the layer that really is initiated
as being the stratum that the cells begin
to die off once they have accumulated all
these keratin. You only see it in thick skin.
And its not described that often as being a
major component of the skin epidermis. It
is often a very thin very basophilic line
that you see here. The top layer of the stratum
corneum is the layer you see at the surface.
This layer consists merely of squames or cells
that have died off and accumulated an enormous
amount of keratin. The cells or the squames
of keratin are lost from the surface of skin
as a result of wear and tear. But as they
move up from the stratum granulosum into the
stratum corneum, the desmosomes I've mentioned
earlier that hold cells together disappear.
They are broken down. And that is how the squames
or the keratinized components are released
into the exterior. Now these squames or these dead
cells full of keratin also have, during their
final development, a very thick cell membrane
layer. And on the inside of the cell layer
of this membrane is an insoluble protein, and
on the outside of the cell membrane is an
insoluble lipid layer. And these two layers,
the protein and the lipid layer and the keratin
are a waterproofing component of skin. Sometimes
lipid soluble components can move through
the epidermis and that is used often in therapeutics.
For instance, the nicotine patch that people
sometimes wear to try and get rid of their
addiction to smoking and nicotine allows the
nicotine to be lipid soluble and move through
the surface of the epidermis into the underlying
tissue and help treat that condition. So the
stratum corneum then is a very important layer.
It is a waterproofing layer in skin. And as
I mentioned earlier, it is very very thick
in thick skin. Well,
let us look at the dermis now. Let us first
of all have a look at the way in which the
epidermis attaches to the underlying dermis.
The interphase between the epidermis and the dermis
is corrugated, it is uneven and that's because
projections of the dermis called dermal papilla
push up into the overlying epidermis. And also
epidermal components bury down, invaginate
into the dermis and that creates a large surface
area of the interphase between the epidermis
and the dermis. And because of such a large
surface area, then the epidermis is tightly
bound to the underlying dermis. Now sometimes if
you see in this image, you can see the
surface of the keratin layer. It is sort of
undulating, goes up and down. These actually
reflect to some degree the random and different
arrangements of these dermal papilla and the
epidermal pegs in the thick skin. And those
attachments or those ways of increasing the
surface area of the attachment, the dermal
papilla and the epidermal pegs are reflected
on the surface as our fingerprints and our
fingerprints are very different between individuals.
It is a reflection of the different random
array of the arrangement of these dermal papilla
and epidermal pegs. The dermis has got two
layers and they are best seen when we look
at the dermis very close to the junction between
the epidermis and the underlying dermis. It
has a papillary layer. It is a very thin layer
just immediately adjacent to the epidermis,
it is loose connective tissue consisting of
only type I and type III collagen, the lesser,
stronger collagens in connective tissue.
Underlying that is the reticular layer and you can see
it's a lot denser. The collagen fibres there are
arranged very differently. They are thick
collagen fibres. They make up that very thick
reticular layer of the dermis and give strength
to the dermis and therefore strength to the
epithelium. Immediately below the dermis is
the hypodermis. it is not really termed or
classified as being part of skin, but all
skin layers have to some extent underlying
fatty tissue called the hypodermis. It is
really a layer of adipose tissue in varying
thicknessses. It helps us be insulated.
It stores energy. It stops us from losing body heat.
It is a very important layer. Sometimes
in gross anatomy we refer to it as subcutaneous
fascia, but again it is just adipose tissues,
fat cells. Here, you see it here, clear staining,
because during processing, the components,
the lipid droplets, the fatty droplets are
lost from the cell during normal histological
processing. Sometimes in the skin, there are
muscle attachments. In the platysma, which
is in the neck region and in the face, the
muscles of facial expression and also the
platysma are striated muscle. And they insert
into the dermal areas of skin, into the strong
dermis of skin you see in this image here,
rather than inserting on the bone. The arrector
pilli muscle, which is really the muscle I've listed
on this slide, is a very important muscle particularly
in animals. It attaches the root of the hair
follicle to the very strong dermis. And in
animals, when that arrector pilli muscle contracts,
it can make the hair protrude a lot taller.
It makes the animal look a lot scarier.
It is not so important in humans. But in humans
sometimes that arrector pilli muscle contracts
and creates what we experience as being goosebumps.