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.