Accessory Structures of the Skin

Accessory Structures of the Skin


– The skin is a complex
organ that carries out an enormous number of
functions, and there are a number of
specialized structures that are found in the skin. That’s what we’re going to
talk about in this video. We’re going to talk
about sebaceous glands, we’re going to talk about
sweat glands, hair follicles, and fingernails. All of these structures
are formed from the skin. They are found in the skin
or extending from the skin. Sebaceous glands are glands
that release oil into the hair follicle, and the more technical
term for that oil is sebum. That’s why they’re
called sebaceous glands. These glands become
active during puberty. Sebaceous glands are
attached to hair follicles. They actually have a
little duct extending from the productive
region of the gland, carrying the sebum
into the hair follicle. So the sebum then travels
up the shaft of the hair and spreads out on the
surface of the skin. Sebum lubricates the skin,
keeping it moist and supple so that it doesn’t dry out,
so that you don’t get as many of those really
annoying cracks that form an open pathway for
bacteria to get deeper tissues. So it helps keep
the skin intact. It also is bactericidal. There are anti-bacterial
molecules in sebum. Here you’re looking at a
micrograph of a sebaceous gland and a hair follicle. So here is the hair
itself in its follicle. Here’s the surrounding
dense irregular connective tissue of the dermis. And here are the secretory
cells of the sebaceous gland. See how they have a kind
of frothy appearance? It’s because they’re packed
full of vesicles full of sebum. And sebaceous glands
are holocrine glands, which means that the secretory
cells just basically fill with sebum until they
actually rupture, releasing the sebum into the
duct, which takes it into the hair follicle. Sweat glands we’re probably
all somewhat familiar with. They’re more
technically referred to as sudoriferous glands. They have a long duct that
carries product directly to the surface of the skin, and
then at the bottom of the duct, there’s kind of this
knot-like structure. And if you are looking at
a cross section of skin under the microscope,
imagine taking a crazy straw and cutting it in half. You can see, when you look
at that cross-section, you can see the duct-like
knot of the sweat gland cut at all sorts
of different angles. You may have some
straight on cross-sections where you can see the lumen in
the center and secretory cells around it, but most of
what you’re going to see are going to be
oblique sections. You can still see a
little bit of the lumen and the secretory
cells around it. Sweat glands use
merocrine secretion where their product is just
secreted through exocytosis into the duct, and then
there are surrounding cells called myoepithelial cells
with a contractile function that can actually squeeze
and push sweat up and out of the duct. And the activity of
those myoepithelial cells increases as your body
temperature increases. That’s one of the triggers
for the myoepithelial cells to contract and squeeze
sweat up onto your skin. And that’s why you get
sweaty when it’s hot out, or when you’re exercising. Then that sweat can
evaporate and cool you down. Sweat itself is
basically 99% water. There’s some salt to it, so
if you’ve ever tasted sweat or had it run into
your mouth, you know, it has kind of a salty taste. There’s anti-bacterial proteins
and some metabolic waste. If you look at the
page on skin functions, they talk a little bit
about one of the functions of the skin being
excretion of urea, normally not really enough
to be noticed, though. There are actually two
types of sweat glands found on your body. The sweat glands that you’re
most familiar with are the eccrine sweat glands. And these are the ones that are
found over most of your body, basically all parts of
your body that have hair. And these are the ones
that are important for thermal regulation. You also have sweat glands
that are called apocrine sweat glands. And these guys are found
in a very limited location. They’re only found in the
armpits, the axillary region, and your genital regions. And the apocrine
sweat glands, they aren’t really important
for thermal regulation, because if you
think about it, you don’t get a lot of
evaporation at your armpits or your genital region. Apocrine sweat glands,
actually, their sweat is a little bit different. It’s still a lot
of water and salt and anti-bacterial
proteins, but it also contains more fatty
molecules and proteins that are broken down by bacteria. And when these fat and proteins
are broken down by bacteria, that releases a smell. So if you’ve ever
wondered why your arm pits and genital regions
tend to get a little smellier than the rest of
your body, that’s actually because the bacteria
are more active in that region, because they are getting
fed by specialized sweat glands in that region. The apocrine glands are
also a little larger, a little bit deeper, and they
secrete into hair follicles at this region. There’s a lot of speculation
about what the main function of the apocrine
sweat glands are, since they have such
little effect on thermal regulation. It’s thought that there
may be sexual function, in terms of that the smell
produced by this apocrine sweat may function as
human pheromones, kind of molecules that
signal others around us, molecules of attraction. But that’s a pretty
hypothetical idea, and the idea of humans actually
responding to pheromones is pretty controversial. So the eccrine sweat glands are
the most abundant sweat glands. Apocrine sweat glands are
the smellier sweat glands in your armpits and
genital regions. You actually also have
specialized sweat glands in your ear and the mammary
glands in the breast. So the glands in your
ear that secrete earwax are kind of specialized
sweat glands, with a much waxier, much
more lipid rich secretion. And the mammary
glands in the breast are modified sweat glands as
well, highly modified product that is obviously much
better for feeding an infant. So you can think of breast milk as
basically being highly, highly modified sweat. On most places in your body,
you have hair follicles. This is basically every
surface of your body except for the palms of your hands
and the soles of your feet. And the hair follicles of your
scalp are a little bit deeper. So the hair on your scalp is
much more deeply embedded. Hair follicles you
can basically think of as being infoldings,
pockets in the skin. So if we’re tracing the
epithelium, the epidermis of the skin, it
continues on like that, and then it just
kind of dives down to form the pocket
of the hair follicle with the connective
tissue still all around. This is a hair follicle
image where you can actually trace from the bottom
of the hair follicle up to the surface of the skin. Oftentimes, in looking
at cross sections of skin under the microscope,
you may just see a portion of
the hair follicle. You may not necessarily
be able to trace it all the way up to the surface. So there’s a hair follicle,
there’s a hair follicle, there’s a hair follicle. This is probably a scalp slide
because the hair follicles are very deep, extending down
into the hypodermis, which you can tell because you
can see the adipose tissue around the hair follicles. I don’t want to spend
a huge amount of time on the anatomy of
a hair follicle. This is a kind of
a cartoon version. You can see that you have
the hair bulb is the deepest part of the hair follicle. This is where the hair itself
actually grows out from– the actual actively dividing
region of the hair bulb is called the hair matrix. So this is where cells,
keratinocytes, are born, and then they’re pushed up and
become part of the hair itself. There is an arrector pili muscle
attached to each hair follicle. This is just a single
smooth muscle cell that, when it’s stimulated, can
actually pull the hair erect. In us, this just
looks like goosebumps. In your cat or dog,
when your cat or dog gets agitated and their
fur kind of stands on and making them look a little
bit bigger and fluffier, that’s because
their arrector pili muscle are getting activated. We don’t have dense
fur, so it just looks like we get little
bumps in our skin. So the arrector
pili muscle can– when it’s stimulated, it
contracts, pulls the hair upright, giving us goosebumps. You can also see
the sebaceous gland attached to the hair follicle. You can see the
hair shaft itself. The hair shaft is the
region where the hair has become completely keratinized. Deeper in the hair follicle
it’s referred to as the hair root, where the hair is in the
process of becoming keratinzed, but it’s not fully
keratinized yet. In these images, we’re zooming
in on the hair bulb itself. You can see you have the
hair papilla down here. And this is like
the dermal papillae that we talked about in the
anatomy of the skin video. This is just a little extension,
a little bump of dermis that extends up into the hair bulb. There are capillaries inside
to bring nutrients and oxygen to the hair matrix,
which is the actually dividing region of the hair. So if your hair
is growing, that’s because the matrix is dividing
and pushing cells up and out. And this is, you know,
the cartoon version. This is the actual
[histological] version. You can see
this paler region, that’s the hair papilla. The matrix is this
initial layer of cells that are actively dividing. You can also see here you
can see melanin deposits giving coloration to the hair. And they’re acquired
by the keratinocytes, and they stick with the
hair as it grows up and out. Fingernails are kind of
related to hair follicles in that they’re also
pockets of epithelium. So you have, again, you
have your epidermis, and it kind of goes in
and wraps around the base that the fingernail grows
from, with connective tissue underneath that, and then
adipose tissue underneath that. And this cutaway image
actually shows the bone, too. Again, you have the nail
root, or the nail matrix, which is the region that the
nail actually grows from. So you actually have
mitotic cells here that are pushing
the nail up and out. And both the nail and hair
are made of keratin, just like the keratin in
the stratum corneum of the rest of your
skin, but the keratin is arranged a little
bit differently in hair versus nail. In hair, keratin proteins
are helix shaped. They’re arranged in spirals. Remember, helix is just a
fancy term for a spiral. But in nails,
keratin proteins are arranged kind of like a
sheet woven back and forth. And so that’s why nails
take on a different shape than hair
does, and also tend to be a lot stronger than hair. A little bit of
terminology– you have the eponychium
of a nail as well. And you may be more familiar
with it as your nail cuticle. It’s a layer of skin
that actually extends up over the nail body, providing
protection from bacteria. It keeps bacteria from
entering into the nail root and damaging the spot
that the nail grows from. You also have the
hyponychium here, which is underneath the nail. One important part of
the nail is the lunula, and that’s this
kind of half moon shape at the base of the nail
where the nail is attached firmly to the skin. Basically, it’s the
part of the nail root that you can actually see. The lunula is
clinically important because it is a good place to
look for changes in skin tone. So if a person is developing
jaundice or cyanosis, the lunula may be
the first place that you can actually
see it before you see it in the rest of the skin that
may be more heavily pigmented. So if you see a yellowish tint
or bluish tint in your lunulas, you might want to have it
checked out by a doctor. Nails in general can
be revealing of health, not necessarily just
the lunula itself, but the way the nail is growing. If the nail is white
or yellow or thickened, or if it increases in size
at the end of the fingers– that’s a condition
called clubbing– it can indicate liver disease. Yellow nails may be
associated with diabetes or a fungal infection. Inflammatory diseases, like
psoriasis or arthritis, can often cause
kind of a ripping or pitting of the nails. Thyroid disease can lead
to cracked and split nails. And if you see a dark line
in the skin under the nail, that can be a
melanoma, actually. All right we’ve just
done a quick introduction to some of the important
accessory structures of the skin. After studying this
video, you should be able to identify these
accessory structures and talk about what they do. So what do sebaceous glands do? What do sweat glands do? Talk about the two different
types of sweat glands, eccrine and apocrine,
as well as describe the basic structures of a hair
follicle and a fingernail.

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