Epidermal Layers

The Epidermis

The epidermisThe outermost layer of the skin, made of stratified squamous epithelium. is the most superficialNear the surface of the body. layer of skinThe body’s largest organ, providing protection and regulation.. As I’ve mentioned before, I like to call it the hard candy shell. Use the terms superficial and deepAway from the surface of the body.. Do not say outside or inside or on top or down below. Remember to try to use the correct regional and anatomical term when referring to organs.

This red line in this picture separates the epidermis from the dermisThe thick inner layer of the skin that contains blood vessels, nerves, and connective tissue.. Most students incorrectly think that this is the border of the epidermis and dermis it is not. Lightly purple stained cellsThe basic structural and functional units of life. here are dead whereas these darkly purple stain cells here are alive. However both swaths of purple belonged to the epidermis. The dermis is composed of this pinkish tissue below which is rich in collagenA structural protein in the dermis that provides strength and elasticity.. Cells of the epidermis are squamous shaped which is a shape that is like a fried egg.

When viewed superficially these cells look like your body is covered in little tiny eggs. We make a cut into the body as shown in the picture. These squamous, or egg-shaped, cells look more like the shape of an eye or an almond. Think about looking at an egg from the top down. Then imagine cutting it and looking at it from the side. Whenever we talk about epithelial tissues, we must name the cells based on their shape. This also applies to tissues that line in open space. Here, the shape is squamous. We also must designate whether the cells are one layer or many layers.

Here these squamous cells are many many layers.  These layers allow the cells to endure abrasion from the external environment. They do not compromise the integrity of the cutaneous membrane.  Because these layers are the most superficial they contain a protein called keratinA strong, fibrous protein that forms the structure of skin, hair, and nails.. Because these stratified squamous cells make the protein keratin they can also be referred to as keratinocytesThe most abundant cells in the epidermis, responsible for producing keratin.. Keratin is a waterproofing protein. In a second or third degree burn, you lose the epidermis. This loss removes the cutaneous membrane’s ability to stop waterThe universal solvent essential for life. loss from the dermis. 

Layers of the Epidermis

Let’s just take a look at the five layers that make up the epidermis.  I have taught from a book that called these layers . The most recent book that I’ve taught from calls these strata. Strata is a word that just means layers. The word stratum is the singular version, meaning layer.

The deepest layer is called the stratum basaleThe deepest layer of the epidermis, where new skin cells are formed..  It is a line of cells, just one line of cells, that all borders on the dermis. Various kinds of cells are here. Stem cells are the most abundant and they do mitosis very rapidly. They are creating new keratinocytes deep in the cutaneous membrane. Keratinocytes will move superficially and die and get sloughed off as you can see up at the top here. The epidermis is like a conveyor belt. It generates new cells, moves them, and then sloughs them off.

The stratum spinosumA layer of the epidermis with cells connected by desmosomes, giving a spiny appearance. is better seen in the picture on the left than on the right. It is two to three cells thick. Another book I read described it as 5 cells thick, but it is certainly less than 10 cells thick. Here in the stratus spinosum, the keratinocytes are bound to each other. If we enhance the stratum spinosum, we see these prickly or thorny little attachments between the cells. The word spinosum means spiny or thorny.

The stratum granulosumThe layer of the epidermis where keratinization begins. is visible. It is superficial to the stratum spinosum. You can sense a little bit of a lighter purple color here in the granulosum. The cells are also becoming flattened in their shape. These cells are still alive. However, they are beginning to be cut off from the glucoseA simple sugar that is the main source of energy for cells. and oxygen in the blood vessels in the dermis. Their appearance is called granular. This is because the melanosomes are little vesicles containing melaninA brown-black pigment made by melanocytes in the stratum basal and given to keratinocytes as melanos. The keratin in the cytoplasmThe gel-like substance within a cell that contains organelles and cytosol. of the cells also contributes to their grainy or sandy type appearance.

The stratum lucidumA thin, clear layer of the epidermis found only in thick skin. is only in thick skinSkin found on the palms and soles, containing an extra layer (stratum lucidum).. It is the fifth layer of keratinocytes that the epidermis can have. This layer is only present on your palms and the soles of your feet. The stratum lucidum is really only one layer of cells thick it’s also very very darkly colored. The cells here are kind of bursting apart and releasing that purple stain making a pretty dark line of purple.

The stratum corneumThe outermost layer of the epidermis, consisting of dead, keratinized cells. is superficial to the stratum lucidum. It is 30 to 50 cells thick. These cells are all dead. They’re flattened. They have no nucleusThe control center of the cell that contains DNA and directs cellular activities.. Their organellestructures within a cell that perform specialized functions. are gone. They don’t take up a lot of stain because of that. These cells are starting to slough off.  I disagree with this picture of your book because it shows the cells sloughing off individually, and that’s not true they actually slough off in sheets

The Epidermis has Variable Thickness

As you can see from this picture if the epidermis has all five layers we call that thick skin. Thick skin is present on your palms and on your feet. These are the two areas that receive the most abrasion as you navigate the external environment. Thin skin covers every other part of your cutaneous membrane. It only contains four of the five layers that the epidermis can have.  The thickness of thin skinSkin found on most of the body, lacking a stratum lucidum. and thick skin differs significantly. The difference is similar to comparing the thickness of a plastic bag to that of a paper towel.

This difference in thickness can be sensed by mechanoreceptorsReceptors that respond to mechanical stimuli such as touch or pressure. in your fingers. It is such a small difference in thickness. People always find it very difficult to differentiate between the relative thicknesses. I feel the thickness differences are similar to toilet paper quality. One is like the good toilet paper you buy for yourself at home. The other is what you find in public bathrooms. The toilet paper in public bathrooms does not even have enough tensile strength. You can’t move the big cheese wheel of toilet paper. All that happens is you end up with individual squares.

The Avascular Epidermis

These hills are called epidermal ridgesThe ridges on the skin surface that form fingerprints..  They have two very important functions.  These ridges create more surface area for exchange. They do this instead of forming a straight line as the border between the epidermis and the dermis.  This is really important because the epidermis is avascularTo be devoid of blood capillaries. Epithelial tissue is avascular, kind of like a cap of dead cells, meaning that it has no blood vessels.  Therefore, nutrients such as glucose and oxygen need to move from deep to superficial areas. This movementA fundamental property of life involving motion of the body or its parts. is done via diffusionPassive movement of molecules from areas of high to low concentration. so that the keratinocytes of the epidermis can receive them.  This diffusion takes the nutrients only so far into the epidermis. The keratinocytes of the strata lucidum and corneum are cut off from the nutrients.  This is why the cells of the corneum are flat and have less purple stain.  They lack the organelles and the nucleus to take up the stain.  Even though the epidermis is avascular, it relies on the dermis for oxygen and nutrients. 

The other important function is attachment.  The epidermis and the dermis are like wearing two body stockings.  You want them to move together so that they are not rubbing past each other.  This situation occurs when the dermis attaches to your hypodermisThe layer beneath the dermis, composed of connective and adipose tissue, providing insulation and cu. When you buy new sneakers for the gym, it’s really natural to tie new sneakers too tightly.  Meanwhile, the epidermis becomes more attracted to your shoe.  The epidermis starts to detach from the dermis and a fluid filled bubble starts to form.  As the layers move past each other, they create friction. When there’s friction in the body, lubricating fluid is present.  This bubble of serous fluid is a blisterA fluid-filled sac between layers of skin caused by friction, burns, or infections..  Put some tape on the back of your heel when you wear new sneakers.  Again, the surface area expanding epidermal ridges provide more surface against which the two layers can attach.