00:00
Well let's now look at skin appendages.
00:04
Again they are epidermal in origin. During
development, the epidermis burrows down into
the underlying dermis and becomes specialized
into various structures. Shown here in this
diagram is a picture of three-dimensional
structure of skin. And it shows you on the
right hand side diagram, the epidermis and
hair projecting from the surface.
00:39
Underneath there are various other structures that are
derived from the epidermis. Sweat glands
of two types, eccrine and apocrine are also
derived from the epidermis. They are examples
of these epidermal appendages. We will talk
about these in more detail in a moment.
01:01
Hair follicles that are areas where the hair is
produced and renewed and associated with hair
follicles are sebaceous glands. They are all
skin appendages that have very important roles
in the function of skin.
01:21
Let us look at the sweat gland in more detail.
There are two types. Eccrine sweat glands
that are widely distributed over the body
and also the apocrine sweat glands that are
restricted mainly to the locations of the
body I've listed there, particularly the axilla.
01:40
Here is an eccrine sweat gland. It consists
of a secretory portion that I have labeled
here, a paler stained portion of the gland
and a duct portion which is a darker stained
area. Much smaller profile the duct portion
has than the large, wider lumen secretory
portion. If you look very carefully at the
secretory portion, it is also surrounded by
myoepithelial cells, which are contractile
cells and they help to dispel the sweat.
02:24
On the right hand side image, you can see if
you look towards the left hand edge of this
image, the duct proceeding its way through
the dermis and out through the epidermis.
02:38
Now the best way to imagine what a sweat gland
looks like, I think you just think of
a cooked piece of spaghetti. Next time
you go home and cook spaghetti, get a piece
of this spaghetti and hold it up on your hand
and stand next to the edge of your kitchen
bench and allow that piece of spaghetti until
it comes in contact with the bench and then
keep lowering it some more. What happens is the
spaghetti will start to coil up. What you hold
in your hand is the duct portion and what
is coiled down the bottom on your kitchen
bench is the secretory portion. So when you
cut through that piece of spaghetti at various
levels, you are going to see different profiles
of the spaghetti. Such is what you see here
in this image, you can see parts of the duct
system cut transversely perhaps, sometimes
longitudinally or obliquely, the same with
the excretory portion. Now when we secrete
sweat, we are doing two things. We are secreting
water, which evaporates from the surface above
skin. So sweat has a thermoregulatory
role, that cools the body. The evaporation of
that water or sweat from the surface cools
the body. So one important function of sweat
is thermoregulation. Other components of sweat
are components of the body that we want to
get rid of such are uric acid, urea, ammonia.
And therefore sweat glands are also not just
a thermoregulatory organ, but an excretory
organ. Here is an apocrine sweat gland.
04:36
It has got a wider lumen, a large luminal space
lined by cuboidal like epithelial cells. There
are also myoepithelial cells wrapping around
these secretory components that help to eliminate
the secretion product. The ducts are usually
the same size as what you see in the sweat
gland but remember they do not modify the
secretion product. Accrine sweat glands the
ducts do modify the product. These apocrine
sweat glands become active during puberty
and they secretory proteins and pheromones.
And these are odourless, but they can be acted
upon by bacteria and create an acute odour.
It is important to understand something about
the innervation of these sweat glands. They
are both innervated by sympathetic divisions
of the autonomic nervous system. But in both
situations, the neurotransmitter substance
used is different. The eccrine sweat glands
are cholinergic. They are said to be cholinergic
whereas apocrine sweat glands are said to
be controlled by adrenergic nerve fibres. And
I will explain that in a lot more detail when
I lecture on the nervous tissues. And that
brings about different reactions because when
we secrete sweat with the role of thermoregulation,
we first of all sweat from our forehead, and then
our scalp, and then the rest of our body and
finally the palms of our hands and the soles
of our feet. But when we sweat because we are
emotionally activated, or we're stress, we start
secreting sweat from both the eccrine sweat
glands and also the apocrine sweat glands,
particularly the axilla. There we tends to
sweat first on the palms of our hands and
the soles of our feet. So the difference stimuli
for sweating whether it be by the eccrine
sweat glands or by the apocrine sweat glands
are controlled by separate types of fibres
from the sympathetic nervous system, cholinergic
and also adrenergic.
Let us briefly look at the structure of hair
and the hair follicle. It is not something
that we emphasize a lot in histology of the
skin, but shown here are three profiles through
sections of the hair at different levels as
they travel up through the dermis before the
hair finally protrudes from the surface of
skin. On the left hand side, you can see the
dermal papilla, projecting into the base of
the hair follicle. That dermal papilla is
no different to the dermal papilla projecting
up into the overlying layer of epidermis that
I've described before. The very surface of that
dermal papillae has on it epidermal cells,
typical of the epidermis, stratum basale and
also melanocytes. And they're going to contribute to
the colour of hair. And then on the outside of the
hair follicle and on the outside of the hair
as it proceeds to the surface, are a number
of root sheaths, the internal and the outer
root sheath. The outer root sheath is really
just a representation of the down growth of
epidermis. And the internal root sheath is
really a few layers of soft keratin, as it's
produce and gives rise to the more firmer
shaft of keratin in the center of the hair.
And on the remaining two slides, the one in
the middle and the one on the outside right
hand side, you see a transverse section taken
through the hair, showing different layers
of the hair. On the far right-hand side, you
see a central white unstained region, which
represents the hair shaft and two layers on
the outside that represent the hair cortical and
the hair cuticle regions of the hair.
09:36
But again as I said earlier, we do not emphasize
really a lot about the histological structure
of hair in our lecture on skin.
Associated with each of these hair follicles
are sebaceous glands that I'd like to describe
here. Sebaceous glands secrete sebum or oil.
10:02
It keeps out hair oily and it keeps the surface
of our skin oily. And again these sebaceous
glands are formed by downgrowths of epidermis,
by downgrowths of the external root sheaths
that I've mentioned earlier. And sebaceous glands
are an example of holocrine secretion.
10:30
Holocrine secretion is where the cell accumulates its
secretory product and then the cell dies.
10:38
It disintegrates by a process called programmed
cell death or apoptosis. And here in this image,
you can see the very basal parts of the sebaceous
gland, the cells being derived from basal
stem cells. They accumulate their secretory
product, the sebum and then finally towards
the luminal space associate with the hair
follicle, the cells die and the oily secretory
contents, the sebum is liberated into the
luminal space.Let us finally look briefly
at the nail. On the right-hand side, you see
a section through the tip of the finger. You
can see typical epidermis. On the top left
hand part of this image, you can see a yellow
stained component, that is the section through
the nail, which is just keratin. Again formation
of nail is just like the formation of keratin
on the surface of our skin and also like the
formation of the hair. But there's four or five
major components of this fingernail or this
section through the fingernail that I want
to point out to you. First of all, the nail
plate is a part of the skin where the epidermis
loses its layer of keratin as it comes around
to start to form where the nail is going to
exit. And then along the nail bed all you have
really is remaining components of the epidermis,
the stratum basale and the stratum spinosum
because you are not producing typical keratin
that you do on the surface of skin, you are
producing the nail. And underneath the nail
is the nail root. That is the area that germinates
the layers of keratin, germinates the cells
that are going to go through the process of
keratinization ,forming the nail root and the
nail matrix. And as more and more keratin is
formed, the fingernail is going to start to
proceed and protrude from the surface of the
finger. There are two parts of the fingernail
that are very very important. That is the
eponychium and the hyponychium. And those two
components, the eponychium and the hyponychium
are areas that seal the fingernail, the underneath
surface of the fingernail from the exterior.
13:44
The eponychium is the cuticle region of the
fingernail. It stops pathogens from getting
into underlying germinal layers of the fingernail.
And the hyponychium is where the nail protrudes
from the finger. And immediately underneath
that nail again is a region that you do not
want pathogens to be able to access, the germinating
layers of the nail bed and the nail root.
14:17
Well, I hope you have learned now a fair bit
about the structure of skin. Make sure you
understand the layers of the epidermis and
all their functional significances.
14:29
Make sure you know the difference between the thick
and thin skin and know all the functions of skin,
particularly the functions that are served
by all the cells that make up the epidermis,
and the function of all the skin appendages.
And also know how to recognize those cells
and also those skin appendages. So thank you
very much for listening to this lecture. Again
I hope you have enjoyed learning about the
histology of skin.