Skin. Another thing on a long list of
things that amoebas don’t have – but greatly admire – because skin, which
is your largest organ, is remarkable. Being the largest organ, it’s very fitting that
it’s part of a system with a very long word: integumentary system. Skin
- and structures associated with it – make up the integumentary system. Why do we admire skin so much? Well, skin
is critical for homeostasis because it helps maintain internal body temperature
and fluid balance. As a physical barrier, it protects your internal structures and organs
from being damaged. It protects from invasion of pathogens like bacteria or fungi for example.
Skin is where Vitamin D is produced. Skin also has sensory functions – for example, if a ladybug
lands on your arm, you are usually aware of it. The integumentary system, which
includes skin, has some depth to it: this system has layers. Layers of
different tissues and cell types. And you’ll find major layers can be divided
into smaller layers as well. We’re going to take a little exploration of this – and a
reminder- this is a general exploration. We’re going to start with the epidermis
–first, I want to mention cells that can be found in different locations in the epidermis
called keratinocytes. These are cells that make keratin. Keratin is actually a protein – a
protein that helps cells be water resistant and tough. Keratinocytes are produced at
the bottom layer of the epidermis – which we’ll get to on our tour – and they get pushed
upward to the top, superficial layer of the epidermis where they are considered cornified.
Cornified cells are hardened , flattened, and tough – they’re also dead – they’ve lost
their organelles and are just full of keratin. Now before we start exploring the layers of
the epidermis, I think it’s really helpful to have a mnemonic to remember the different
layers of the epidermis from the outer layer to the inner layer. Here’s an epidermis
mnemonic: Can lemurs get some bamboo? I may or may not have made that mnemonic up and
there are probably better ones out there but many lemurs do like bamboo… something to be said for
factual mnemonics. The first letter in each word stands for each epidermal layer- which will
be called a stratum - which we’ll get to now. Outer top layer: we’ve got the stratum corneum.
This layer consists of cornified cells – dead cells. They are continuously shed off with
replacement cells coming from layers below. Now the next layer down, the stratum
lucidum, is not in all skin areas. It’s generally just for thick skin
areas like the bottom of your feet or on your palms. This layer also consists
of cornified cells. They contain a type of protein that gives them a transparent kind
of appearance, hence the name of this layer. Next, stratum granulosum. In this particular
layer, keratinocytes that have been pushed from the lower layer and they will develop a flatter
structure in this layer. They contain granules – like the name of the layer suggests- that
have a variety of functions. The keratinocytes here will eventually lose their organelles and
become the cornified cells of the layers above. Next, stratum spinosum. There are
many layers of keratinocytes here, but there’s also a type of cell that will
consume worn out cells or bacteria and by doing so – it acts like a macrophage. By the
way, the name of this layer is related to how the layer looks under the microscope when it’s
stained – the layer looks spiny when stained. Now, the deepest layer of the epidermis: the
stratum basale. The stratum basale consists of one layer of cells called basal cells. These cells
are the ones constantly doing mitosis and actually give rise to the keratinocytes in the layers
above. Interesting fact: do you have especially thick skin on your feet? Or maybe on a finger?
This could be a callus. Calluses generally form when the stratum basale layer is trying to respond
to frequent abrasion by making more cells, which mature and result in a thicker stratum corneum.
The stratum basale also has other types of cells like melanocytes – this is the cell that makes a
protein called melanin. Melanin is a pigment that can result in an array of different skin colors,
and melanin protects skin from being damaged by ultraviolet (UV) rays. The melanin actually gets
transported to the keratinocytes in organelles called melanosomes. Merkel cells are another
cell type and while their exact function is a bit uncertain – check out the further reading links
on that – they are thought to have involvement with the nervous system as they work with nerves
in helping us have the sense of touch. The stratum basale layer of the epidermis will be bonded to
the dermis, which we’ll move into right now!
The dermis. Unlike the epidermis, you will find
blood vessels in the dermis. The dermis is a type of connective tissue, which is a non-epidermal
type of tissue that connects things together in the body. You’ll also find sweat glands,
hair follicles, and nerves in the dermis. The dermis has fibers of two types of proteins:
collagen– providing support - and elastin – which gives it elasticity. These proteins are made
by specialized cells in this layer called fibroblasts. The dermis has two general layers:
a papillary layer – which has connective tissue that is more loose – and a deeper reticular layer
– where connective tissue is more tightly packed. Now before we move to one more layer – this is
a good time to mention scars. Many cuts that are isolated to the epidermis won’t scar but if a cut
makes it to the dermis, it can often scar. Scars tend to look different from the rest of the skin,
because they aren’t put together exactly like the original skin was. What I mean by that is that
when the fibroblasts are generating collagen to fix the damage, they tend to not arrange
it in the pattern that had originally been there before and accessory structures that
we’ll get to later (like sweat glands or hair) won’t be re-created there. Scar tissue
also tends to have less elasticity – meaning really large scars from large wounds can even
affect range of movement. And occasionally, the collagen production keeps on going even
after everything is healed, and you result in a raised scar that can be referred to as a keloid.
That’s from this excessive producing of collagen. Ok – moving on to the last layer we’ll discuss.
The hypodermis. It’s under the dermis and it connects the skin above to bone and muscle
tissue. The hypodermis contains adipose tissue, which is stored body fat, and this has
important functions such as insulation. Now after looking at the layers, there are
accessory structures that are part of the integumentary system that we haven’t mentioned.
Sweat glands, for example. We talked about sweat in our homeostasis video and how important
for being used to cool the body. We should point out that sweat glands aren’t the only
ways that your skin helps with temperature regulation. Blood vessels in your dermis will
dilate (meaning widen) so that heat can escape through the skin. And if you’re really
cold? Those same blood vessels will now constrict and be kept away from the surface
of the skin so that heat can be conserved. Sebaceous glands are another accessory structure.
They produce oil and help keep the skin – and hair –waterproof. They also help lubricate all those
dead keratinocytes we’ve been talking about. Hair. Remember which layer we said
has hair follicles? That’s right, the dermis. There are cells that are found in
this hair bulb that are doing mitosis rapidly, and when they do, the cells get
pushed outward as the hair root grows. The hair shaft you see itself
is made of keratin and non-living. Nails. The base of your nail is part of the
epidermis. While the nail body – that portion that protects the ends of fingers and toes- is made
of dead keratinocytes, the nail root has cells that are doing mitosis frequently and as those
cells get pushed outward, they help the nail grow. So we always like to mention the “why” – why learn
about the integumentary system? Well first of all, we do want to mention that skin cancer – which
according to the American Academy of Dermatology is the most common cancer in the United States
– can occur when some of the integumentary cells we’ve been talking about start to not function
correctly and divide out of control. For example, basal cells can also give rise to the
most common type of skin cancer in humans, the basal cell carcinoma. Melanocytes can
give rise to a type of skin cancer called melanoma. Merkel cells can give rise to an
aggressive rare type of cancer called Merkel cell carcinoma. It is by understanding
and studying the integumentary system that we can better understand how to
treat these types of skin cancers. In continuing to emphasize how important the
functions of the integumentary system are – let’s consider a condition where the functions can be
compromised such as a burn. The degree of a burn tends to be placed in a type of category depending
on which levels are affected. I do want to point out though that we’ve noticed classification
of burn degrees are not consistent across all sources; check out our further reading to learn
more. If going on a system that recognizes four different classification categories of a burn:
a first degree burn generally is isolated to the epidermis, a second degree burn affects the
epidermis and part of the dermis, a third degree burn affects the epidermis and all of the dermis,
and a fourth degree burn goes beyond these layers to even affect bones and muscles. In third and
fourth degree burns, the nerves are often damaged, so it can actually be less painful despite being
more severe of a burn. Significant burns can be dangerous because those functions we talked
about with the skin like maintaining fluids and protecting all the internal structures – that
function is compromised with a significant burn. In addition, the skin is now very vulnerable to
getting infected so that needs to be addressed. There are medical personnel trained to work
specifically with burns, and they have to have extensive knowledge of the integumentary
system in order to make a treatment plan and restore the integumentary system so that it can
do all those amazing functions we’ve mentioned. Well, that’s it for The Amoeba Sisters,
and we remind you to stay curious.