Back To CourseCLEP Natural Sciences: Study Guide & Test Prep
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Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.
Did you know that your skin alone makes up about 16% of your total body weight? And that your skin is considered to be an organ? In fact, it's the largest organ of your body. Without your skin, you would have no protective barrier between you and the environment outside your body. In fact, I bet your skin would be considered one of the most important organs of your body! Without it, you would be vulnerable to all sorts of infections and injuries.
The skin helps your body defend against microorganisms - you know, like bacteria and parasites, all those nasty things that we don't want living on or in our body. It excretes salt, water and organic wastes, preventing fluid buildup in your body. Without excretion, you might look a little bloated. Your skin protects you from the UV radiation of the sun - well, to a certain extent. You still need to wear sunscreen, or you might end up looking a little bit like a lobster!
It helps regulate your body temperature through releasing or keeping in heat. And your skin also acts as a water repellent. Without this ability, you would take in too much water when you went swimming or took a bath and might end up looking like a human Jabba the Hutt.
And your skin even synthesizes the hormone vitamin D. This helps make you happy. That's why people tend to be a little more gloomy during the wintertime. Your skin also contains numerous nerve endings, allowing it to relay the sense of touch, pressure, pain and temperature to the brain.
The integumentary system has two main parts: the skin and its accessory structures. They work together to carry out all these functions. The skin, like everything else in science, has an official scientific name. It is officially termed the cutaneous membrane and is made up of multiple layers.
The topmost layer, up here, is the outer epidermis. Underneath that is the inner dermis, and all the way down here at the bottom is the hypodermis. You may remember from other science lessons that the term epi refers to something that is above, over or on top of something else. That 'something else' in this case is the dermis. So the epidermis is on top of the dermis.
In this lesson, we will be focusing on the epidermis, or outer skin layers. The epidermis is made up of multiple layers, or strata, of stratified squamous cells. These are flattened cells that look kind of like sheets or scales, but without all the spikes or roughness that a lizard scale might have.
In some cases, these cells are filled with the protein keratin and are called keratinized cells. This is the same protein found in your hair and your nails. In the skin, though, it helps make the skin water-resistant. This is important because otherwise every time you are exposed to large amounts of water, like a pool, water would enter your body quickly, causing it to bloat, bringing us back to our human Jabba the Hutt! The keratinized cells of the epithelium help prevent this from happening.
The stratified nature of your epidermis also functions to help keep microorganisms outside your body, and it is avascular. This means it doesn't contain any local blood vessels.
I know what you may be thinking: 'But if it doesn't contain blood vessels, then why do you bleed when you're cut?' Well, that's because the layer underneath the epidermis (the dermal layer) does contain blood vessels. And remember, the epidermis is only the top layer of your skin, and compared to the other layers, it isn't really that thick. That's why really shallow cuts bleed very little but deeper cuts - those that reach the dermis - bleed more.
The lack of blood vessels also means the epidermis has to get its nutrients and oxygen from elsewhere. They diffuse out of the vessels in the dermis and into the epidermis. Therefore, epidermal cells that require the most nutrients are located closest to the dermis, and those that require the least are located at the surface of your skin. Let's start with the innermost strata, the one closest to the dermis.
Did you know the epidermis constantly regenerates all throughout one's life? In fact, you get new cells in your epidermis about every 40 to 60 days. New cells are made down here at the stratum basale. This is the innermost layer of your epidermis, located directly above the dermis. But the rate of new cell generation decreases as you age. That's why seniors have a harder time healing from a cut than a child does.
As each new cell is made, the cells above it are pushed upwards, replacing the dead skin cells lost at the surface of your skin through washing and other daily activities. As the cells move further away from the stratum basale, their shape changes, and they get flatter as their need for nutrients decreases.
For example, when you get a sunburn, you can affect different layers of your skin. In many cases the burnt cells will peel off; these are then replaced by newer cells underneath. This process of division and new cell creation is carried out by the stem cells of your skin, called basal cells.
The stratum basale also contains melanocytes that produce the pigment melanin. This is what gives your skin and freckles their color. They also help protect it from UV radiation. But they have their weakness; when exposed to too much sun, they can mutate and cause skin cancer.
A mutated melanocyte, or skin cell, has had its DNA damaged by the sun, creating a cancerous cell that can spread throughout your body. However, sunlight is also good for you. Your skin needs sunlight to produce the vitamin D, a hormone important for many things, including bone and cell growth, your immune system and neuromuscular functions. Lack of vitamin D can lead to abnormal bone growth, a condition known as rickets disease.
Directly above the stratum basale is the stratum spinosum. This layer was named for the spiny look of the cells within it. Cells in this strata come from the stratum basale. Remember how basal cells divide into two new cells? Well, as each basal cell divides, one is pushed upward, into the spinosum, while the other remains in the basal layer so it can continue dividing. The spinosum is made up of eight to ten layers of keratinocytes, the cells that produce the protein keratin.
Above the stratum spinosum is the stratum granulosum. This strata has three to five layers of cells that have moved up from the spinosum. What makes this layer different is that by the time cells reach the granulosum layer, they have stopped dividing and have started making keratin.
Now, in some areas of the body you need an extra layer of protection. Any ideas? What areas do you think contain more skin than others? Not sure yet? Well, these areas contain an extra layer called the stratum lucidum, which is found only in the thick skin of the palms of your hands and the soles of your feet to add a little extra protection.
Above the stratum lucidum or the stratum granulosum (depending on where we look) is our last and topmost layer, the stratum corneum. Remember how we said that cells furthest from the dermis don't require as many nutrients as those closest to the dermis?
Well, what kind of cells don't require nutrients? The answer is simpler than you might think. You see, the topmost layers of your epidermis are made up of dead cells, and a dead cell doesn't require any nutrients!
The stratum corneum is made up of about 15 to 30 layers of keratinized cells. As mentioned before, keratinization is the formation of protective layers of cells filled with the protein keratin. This process occurs on all of your exposed skin surfaces except one.
Any ideas? Okay, I'll give you a hint: What part of your face is NOT covered in skin? Got it yet? If you said your eyes, then you are correct! Your eyeballs are exposed to the outside environment, and they are the only exposed surface that does not contain keratinized cells. Why? Because they're protected by your eyelids!
These dead keratinized cells are more durable than underlying layers. They're also water-resistant. This makes their surface a dry and unsuitable habitat for most organisms. That's why cuts can get infected so easily - because they open up a pathway to moisture and more nutrient-rich areas of your skin.
Now that you know a bit more about each part of the epidermis, let's run through a quick review. The epidermis consists of five to six strata. All cells in the epidermis come from the innermost layer, the stratum basale. This is where the basal cells, which divide to produce new skin cells, are located. The basal layer also contains melanocytes, the cells responsible for the color of your skin.
Next is the stratum spinosum where the keratinocyte cells are located. These cells begin making keratin, a water-repelling protein, as they move up towards the next strata, the stratum granulosum. By this time, the cells here have stopped dividing and they've started producing keratin.
In areas of the body where you have thicker skin, you have an extra layer, the stratum lucidum, which adds protection to the hands and feet to help reduce friction.
If there is no stratum lucidum, then directly above the granulosum layer is the topmost layer, the stratum corneum. This strata contains 15 to 30 layers of keratinized cells that aid in protection against microorganisms and act as a barrier to water.
All of the layers of your skin - the epidermis, dermis and hypodermis - work together to:
After this lesson is done, you might be able to:
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Back To CourseCLEP Natural Sciences: Study Guide & Test Prep
26 chapters | 302 lessons | 25 flashcard sets