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Structure and layers of the skin.
Hello everyone! This is Joao from Kenhub, and welcome to this tutorial where we’re going to be discussing the integumentary system. So, right now, you're probably wondering to yourself, what is the integumentary system, anyway. Well, it is basically an organ system that surrounds you and is the biggest organ that covers the external surface of the human body which is then the skin. The system is also comprised of the appendages of the skin like hair, nails, sweat glands, and sebaceous glands. In addition, the integumentary system is also comprised of subcutaneous tissue, deep fascia, mucocutaneous junctions, and the breasts.
Well, in this tutorial, we will look at the different components that make up the system citing examples of each as well as their functions. So we will get started.
I have a question before we start all of this. I want to ask you, what do you see when you look in the mirror or look at your body?
Well, you see, the outermost layer of your skin which is called then the epidermis. The skin also known as the integument is made up of two closely related layers – the epidermis seen here highlighted in green and the dermis which we will discuss later. The skin is the first line of defense for your body. It functions to then protect the body from ultraviolet radiation, variations in temperature, pathogens, and toxins.
The epidermis which is the outermost layer of the skin is devoid of blood vessels. It is densely packed with cells and is made up of stratified squamous keratinizing epithelium – a tissue that is colonized with keratinocytes which are cells that produce a protective protein called keratin. The epidermis also has pigment-producing cells known as melanocytes and it is the cells that determine our complexion or skin color based on the amount of melanin that they produce. There are other cells also present in the epidermis which are the Merkel cells. These are cells that let you feel light touch. And the other cells you find here are the Langerhans cells – these are part of then the immune system.
Now those were characteristics of the epidermis. Now, let's look at the specific layers of the epidermis.
The epidermis is composed of five layers – the stratum corneum, the stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. It should be noted that depending on which area of the skin we are talking about, the epidermis can have either four or five layers. Basically, thin skin has only four layers while thick skin has five layers. But we will discuss this in a bit more detail later in this tutorial.
First, let's look at the outermost layer of the epidermis. The outermost layer of the epidermis – the stratum corneum – is made up of mature keratinocytes. This layer of the epidermis is comprised of up to twenty cell layers thick of keratinized squamous epithelium. The dead cells of this layer help make the skin such a great barrier mainly functioning by fighting infections as well as protecting the body against chemical damage, daily wear and tear, and dehydration.
Now looking down under the layer underneath the stratum corneum, we will find this one here which is the stratum lucidum. The second layer of the epidermis can only be found in areas of your body where the skin is thicker such as your palms. So, I'm talking about the palms of your hands or also the soles of your feet. This is where four versus five layers of epidermis comes in. So as I mentioned earlier, thin skin is comprised of four layers of epidermis which means the stratum lucidum would not be present in thin skin.
By looking at these two micrographs, you can see the visible difference between the number of layers present in thick skin and thin skin. The stratum lucidum is characteristically clearer on histological slides. It gets its name from the Latin word "lucid" which means clear.
The next layer of the epidermis is the granular layer of the epidermis and it is appropriately known as the stratum granulosum. This comprised of mature keratinocytes that contain granules in their cytoplasm. You might have noticed that we haven't seen any nuclei yet. That's because in this layer, the keratinocytes lose their organelles including the nucleus and then they die.
Now moving on, we're going to look at the layer underneath the stratum granulosum which is the stratum spinosum. Interestingly enough, this layer is called stratum spinosum because the cells in this layer shrink as a result of histological fixation during slide preparation. The points where they connect to their neighboring cells are called desmosomes and they stay intact and this makes them look like they are spiny. The keratinocytes found in the stratum spinosum layer are less developed.
Antigen-presenting dendritic cells of the immune system known as Langerhans cells are found in this layer. Please keep in mind that Langerhans cells can also be found in other layers of the epidermis but they are most abundant in the stratum spinosum layer.
So the final and innermost layer of the epidermis is the stratum basale seen here. The cells in this layer of the epidermis are attached to the basement membrane below the hemidesmosomes. Cells both divide and grow in this layer and keratinocytes originate from this layer of the epidermis and it also contains melanocytes. Remember that melanocytes produce then melanin which is the pigment responsible for giving skin its color. Melanin is also responsible for the color of your hair and eyes.
Okay, so we have discussed the epidermis of the skin and looked at the five sublayers that make up the epidermis. The next layer of skin just below that is the dermis. You will notice from this micrograph that the dermis, unlike the epidermis which we just saw, does not have densely packed cells. Instead, the cells of the dermis are loosely packed or spaced out with fibers in between them. The dermis is located deep to the epidermis just beneath the basement membrane of the epidermis. Here you see the basement membrane highlighted and the dermis is located just beneath it.
Unlike the epidermis, the dermis contains blood vessels. So nutrients and oxygen diffuse from the dermis up into the epidermis. And for that reason, part of the dermis' function is to support and sustain the epidermis.
The dermis also contains fibroblasts and collagen. It can be distinguished into two sublayers – the papillary dermis and the reticular dermis. So let's look at these two sublayers of the dermis. We're going to start with the papillary dermis or stratum papillare if you're feeling a bit more fancy which is the most superficial sublayer of the dermis. The papillae of this layer of the dermis serve to increase the surface area for the transfer of oxygen and nutrients between the dermis and the epidermis. This layer is made up of loose connective tissue – areolar connective tissue to be more exact.
Next, we're going to move in a bit deeper into the sublayer of the dermis which is known as the stratum reticulare or the reticular dermis. This sublayer of the dermis is composed of thicker connective tissue which surrounds the hair follicles of the skin, sweat glands, sebaceous glands, nerves, and the plexus of blood vessels. The stratum reticulare also contains fibroblasts, collagen which provide strength and resistance, and elastic fibers.
Now that we have discussed the layers of the dermis, let's look at some of the other structures that can be found within the dermis. These structures are known as the appendages and they serve various functions. We will look at the hair follicle, the arrector pili muscle, several kinds of glands, and tactile corpuscles like the one pictured here for then sensation.
The first structure we will look at is the hair follicle seen here. As you can see, we have magnified it so you can see it a bit more clearly. The hair follicle is a sac-like downgrowth of the dermis and epidermis out of which hair grows. Hair is found on the skin all over the body except the palms of your hands, soles of your feet and also your lips. The hair follicles go through a cyclic activity of hair growth and loss. At the terminal end of the active hair follicle is an expanded area known as the hair bulb. It consists of a germinal matrix comprised of stem cells and then the upper bulb which arises from the stem cells.
Now let's look at two other structures that we associate with the hair follicle. First one, the sebaceous gland which is closely associated with the hair follicle and also found in the dermis. These small sacular structures are made up of cluster of cells called secretory acinus. The name comes from the Latin word for "berry" because these structures resemble a lobed berry like a raspberry.
Sebaceous glands are exocrine glands which are glands that have ducts and open to the outside of the body or to a cavity in the body. These ducts open into the dermal papillary canal of the hair follicle where the gland secretes a fatty oily substance called sebum that is crucial in creating the skin's epidermal barrier. As you can see here, the sebaceous glands are closely associated to the hair follicle.
Another structure associated with the hair follicle is the arrector pili muscle. This small bundle of smooth muscle pulls the hair shaft erecting it when it contracts. It is this small muscle that is responsible for making your hair stand on ends when you have goosebumps or when you are cold.
Here we see a sweat gland. Like the sebaceous glands we just looked at, these glands are exocrine glands and are located in the dermis. They are distributed in the skin all around the body and they produce sweat which is important for thermal regulation. On a really hot day or when you're working out, the sweat glands produce sweat which is secreted onto the surface of the skin through small openings on the surface of the skin called sweat pores. Here you can see a sweat pore that opens onto the surface.
Incidentally, there are two types of sweat glands – the apocrine sweat glands which can typically be found in the areas of the axillary region, the perianal region, the nipples, the periumbilical region, the prepuce as well as in the scrotum, the penis, the mons pubis, the labia minora, and the clitoris. The other type of sweat glands are the eccrine sweat glands. This type of sweat gland is found all over the body although looking at our micrographs, it may appear to you that these two types of glands look very familiar. It should be noted that apocrine sweat glands are closely associated to the hair follicles which is why they are predominantly found in the regions we just mentioned. Eccrine sweat glands, on the other hand, are found all over the body but most especially on the palms of the hands and the soles of the feet.
Next structure we're going to be seeing is this one, the tactile corpuscle, which is one of the ways that the integumentary system helps you gather sensory input about your environment. Tactile corpuscles such as the Meissner's corpuscles seen here facilitate the sensorial capabilities of your skin such as light touch, low frequency vibrations, and other sensations. There are other kinds of tactile corpuscles in the skin as well, some of which we'll see soon when we get into the subcutis.
And we are there now. Next we are going to look at the layer situated underneath the dermis called the subcutis seen here. It is also known as the hypodermis or subcutaneous layer. This layer is comprised of loose connective tissue particularly adipose tissue, blood vessels, and other structures such as nerve tissue. The adipose tissue of subcutis functions as energy storage, shock absorption and helps keep in heat while blood vessels are important for thermal regulation.
Here we see the subcutaneous vascular plexus which is a series of blood vessels that supply and drain the subcutis. This subdermal vessel plexus which is also a network of blood vessels is found at the level of the dermis as you can see here. Located superficial to the subdermal plexus is then the dermal vessel plexus. This vascular plexus plays an important role in thermal regulation. It is important to note that these blood vessels which are important for thermal regulation are superficial to the layer of fat in the subcutis otherwise that fat which is so helpful in insulation would prevent those blood vessels from releasing heat.
Along with blood vessels, cutaneous nerves can also be found in the subcutis. These nerves provide sensory innervation to the skin and have a number of sensory receptors. One example is the lamellar corpuscles. Lamellar corpuscles which are also known as Pacinian corpuscles detect pressure and high-frequency vibrations. These corpuscles are larger than Meissner's corpuscles that we just saw earlier in this tutorial.
And just beneath this subcutis is a layer of fascia seen here highlighted in green that separates the subcutis from the underlying muscle. Fascia is just a fancy word for a thin layer of fibrous connective tissue. It provides protection to the muscles below and helps reduce friction. The muscle layer is closely associated to the fascia layer. The muscle layer is also quite well vascularized. It contains intramuscular arteries and intramuscular veins which are essentially these blood vessels that you find within the muscle layer.
Now that we have a good understanding of the composition of the skin, we're going to be looking at some conditions and clinical notes that can affect the skin.
As we have seen, the skin acts as a protective barrier against chemical damage, UV radiation, and pathogens. Well, when this doesn't work properly, it can lead to certain skin conditions like skin cancer. There are many types of skin cancer which differed depending on the types of cells involved and the nature of the cancer. For example, basal carcinoma is a type of cancer that arises from the stratum basale while squamous cell carcinoma involves the squamous cells of the stratum spinosum. Melanoma is a type of malignant skin cancer that involves melanocytes.
Other skin conditions like naevi or moles as you might know them can either be congenital or acquired and they are caused by a benign overgrowth of melanocytes. Psoriasis which is an autoimmune condition causes pink scaly plaques all over the body but it can be treated using phototherapy, immunosuppressants, topical agents, and alternative therapies.
Eczema which is basically a dry inflammation of the skin, which is also known as atopic dermatitis, and this skin condition has been linked to other hypersensitive disorders such as asthma and hay fever and it can relatively easily be treated with the use of steroid creams and moisturizers. Of course, there are a number of other skin conditions and the ones you see here are just the tip of the iceberg.
This, everyone, brings us to the end of this tutorial and before we finish off, let's quickly go over what we have looked at so far. So we have seen that the integumentary system is comprised of the skin and its appendages. We've seen that the skin is made up of an outer layer known as the epidermis and an inner layer known as the dermis. We also discussed the five sublayers of the epidermis – the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. And we looked at what distinguished these sublayers in terms of the types of cells present in them.
The two sublayers of dermis were also discussed – the stratum papillare and the stratum reticulare. We also discussed the difference between the epidermis and dermis and as we continued with the tutorial, we had the chance to describe and understand the characteristics of the subcutis and the appendages of the skin such as the hair follicles, glands, and muscle fibers.
Finally, we discussed irrigation, innervation, and how to look at the clinical importance of this topic by describing a few conditions related to the skin such as a basal carcinoma, squamous cell carcinoma, melanoma, naevi or moles, psoriasis, and eczema.
And that's it for this tutorial. I hope you guys enjoyed it and I will see you on the next one.