Heather has taught reproductive biology and has researched neuro, repro and endocrinology. She has a PhD in Zoology/Biology.
The Dermal Layer
Did you know that a large amount of the dust in your home is actually dead skin cells? Pretty gross, right? All those dead skin cells fall off the surface of your skin as new cells are made in the top layer of your skin called the epidermis. Underneath that layer is a deeper layer called the dermis.
The dermis plus the epidermis make up the integument, or skin. The epidermis contains layers of cells that contain the protein keratin. These layers help protect the dermis. The dermis lies underneath the epidermis and contains most of the blood vessels and nerves that your skin needs to be healthy.
Both the epidermis and dermis are made up of layers of cells and tissue, as you can see here. But the dermis is a lot thicker than the epidermis, even though the dermis only has two layers and the epidermis has about five to six layers. That's because the cells in the epidermis are smaller and flatter, while those in the dermis are larger.
The Papillary Layer
The dermis actually contains different types of tissue, including loose connective tissue, elastic and collagen fibers, nerve cells, hair cells and sweat glands.
Loose connective tissue is pretty unorganized-looking - almost like someone took of bunch of cells and fibers, threw them up in the air and just let them drop, sort of like a game of pick-up-sticks or jacks, if you're old enough to remember the pre-video game days. It's characterized by a loose connectivity between cells and fibers, with lots of spaces in between. This tissue is found all over your body, mostly around your internal organs and blood vessels.
But let's get back to the skin. Here, it's found in the papillary layer of the dermis. This is the layer of the dermis closest to the epidermis. You know when your skin gets itchy and inflamed - a condition called dermatitis? Well, just as the name suggests, dermatitis is an inflammation of the dermis, specifically the papillary layer.
Now, look at the boundary between the epidermis and the papillary layer. See all those projections going upwards, like somebody poked the papillary layer? Those are called dermal papillae, and they help secure the dermis to the epidermis, kind of like how Velcro works, with all those spikes that interlock with each other. And the cool thing is the ripples from these projections can be seen on the skin of your hands and feet. That's what gives each person his or her unique fingerprints.
Okay, so the papillary layer's job is to connect the dermis to the epidermis. But that's not all it does. It also contains many tiny little blood capillaries (that's just the fancy term for really, really small blood vessels). These capillaries come up from deeper layers and supply the blood and nutrients for your skin. And, like the rest of your body, without blood, the skin would die.
Your papillary layer also contains another important set of structures for a properly functioning integumentary system: sensory nerve endings. These are receptors for the nervous system that relay the sensing of pain, temperature, touch and pressure information to the brain. In fact, without these structures, or with dysfunctional ones, you might have a condition known as CIPA, where you're unable to feel extreme hot or cold temperatures on your skin and even unable to feel pain! Or, if your nerve endings in your skin were overly active, you might have a condition known as fibromyalgia, which can make even the lightest touch painful.
While the sensors for light touch and temperature are located in the papillary layer, the pain sensors are actually located in the epidermis, while the pressure sensors, the ones that sense strong touch, are located further down, in the reticular layer.
The Reticular Layer
This layer lies underneath the papillary layer. So what makes the reticular layer different from the papillary layer? Well, the reticular layer is made up of a different type of tissue called dense irregular connective tissue. And, as the name suggests, it's denser than the loose tissue found in the papillary layer. That means it doesn't have as much space in between cells. It's a weave of collagen and elastic fibers that provide structural support for the skin. Both loose and dense irregular tissue are called irregular because when you look at them there is no apparent sense of organization.
These fibers help make the tissue impact-resistant, each with a distinct and opposite role. You see, the collagen fibers are thicker, tougher and resistant to stretching, giving skin its strength, while their opposing fibers, elastin fibers, are, as the name suggests, elastic. They recoil, returning to their original shape after being stretched, kind of like when you stretch a rubber band and it returns to its original shape when you let go. So, while elastic fibers allow the skin to stretch out, like when a mother is pregnant, the collagen fibers prevent the skin from stretching too much!
Both fibers extend from the reticular layer into the papillary layer, making the boundary separating the layers hard to see. And, just like our papillary layer, we have blood vessels, nerves and other accessory structures, like hair cells and sweat glands, in the reticular layer.
Now, last but not least, we have one more layer to get through. I know; I told you the dermis only had two layers. Well, this next layer isn't part of the dermis; it's a separate layer underneath the dermis called the hypodermis, 'hypo' meaning 'under' or 'below.'
The hypodermis is connected to the reticular layer by both collagen and elastin fibers and is made up of loose connective tissue and fat cells. It separates the integument (or skin) from the tissue surrounding the organs and muscles. This separation is important so that your skin is not directly attached to your muscles. That way, they can each move independently of each other.
You know how babies have a lot of what we call 'baby fat,' making them cute and chubby? Well, that's because they have extra amount of fat cells, or adipose tissue (as it is called in the science world), in their hypodermis. This helps them stay warm and provides them with the extra energy they need when they're growing. But, like all things, this changes as we grow older. As you age, men will deposit more fat in the neck, arms, lower back and buttocks areas, while women tend to accumulate more fat in the breasts, buttocks, hips and thighs.
And that's it, the different layers that make up the largest organ in your body, the skin! It starts up top with the protective epidermis. Underneath that is the dermis, where all the blood vessels, nerves and accessory structures that a healthy and normally functioning skin needs are located.
And remember, the dermis is divided into two layers: the papillary layer, which is made up of loose irregular connective tissue, and the reticular layer, made up of dense irregular connective tissue.
And let's not forget the last layer, the hypodermis, which separates the muscles from the skin. That's where all the fat cells that lie underneath your skin are located. They help provide insulation and energy reserves, especially in children.
When this lesson is done, you should be able to:
- Recognize the dermal layer of the integumentary system
- Understand how the dermis and epidermis works
- Identify the use of the papillary layer
- Remember what separates the skin from the muscle
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