For some readers the material might be too elementary ( or too much detail). I would challenge you to think deeply about the simplicity of the descriptions and also I will include links for you too (on the side with greater detail).
With this in mind, lets explore the "function of the skin." To understand the function each macroscopic layer plays, the easiest way is to start from the outside and work our way down - inside. I use the term "macroscopic" to emphasize that the cellular picture is too specific for the discussion and will be referred to when needed. The outermost layer of the skin is referred to as the "Epidermis." If we continue down inside the organ (skin), the next two layers would be: Dermis, Hypodermis. Since the purpose of the "Anti-Aging Skin Series" is to break down a long post into edible bites, I think in the current post, introducing the function of the Epidermis will be more than enough for one reading. Enjoy!
What is the function of the Epidermis?
A diagram of the structure of human skin taken from a book authored by Dr. Zoe Draelos and Dr. Peter Pugliese titled "Physiology of the Skin" is shown below:
In the first chapter of the book, the content is captured appropriately by the title of the chapter -- "Behavior of Normal Healthy Skin." Of the three types of skin mentioned in the introductory post about skin (access here), the two types of skin that are relevant to the cosmetic industry are the outer two layers: epidermis and dermis.
I am a big believer in not "reinventing the wheel" -- meaning, I like to cite other authors work and use their explanations when better understood than mine. From the first chapter of the book, here is a description of the epidermis that captures the function in a simple but complex manner:
The epidermis is the outer skin layer that forms a outer barrier to the outer world. This barrier keeps out water, sunlight, insects, germs, heat and cold, dirt and gases. It keeps in fluids such as water and blood, and holds safe minerals, vitamins, hormones, proteins and heat. An incredible self-renewing system, the epidermis provides replacement of the outer cells lost to the environment. It provides a water proof outer layer, yet permits internal water to carry nutrients to the outermost living cells. It provides a tough outer layer to resist friction, abrasion and pressure, yet is sensitive to the lightest touch or softest breeze. Less glamorous, but equally as important, the skin serves as a vast waste disposal system, ridding the body of many toxic substances.
I had to read that excerpt a few times before comprehending the entire range of the skin. Remember when your older sister or brother use to grab onto your arm skin and twist it -- to cause a sudden escalation of pain along with the outcry...."stop that you bully..." Well, I had my fair share with dispensing a few to my younger brother. Yet, at the time, the last idea running through my head was the dynamic range of the skin.
To be able to withstand abrasion, yet feel the softest breeze. The body is amazing. From the diagram above, a person would never have been able to ascertain the many functions that the outer layer of skin provided. At least, I would not have been able to (I cannot speak for anyone else).
Here is a short -- 2 minute video worth watching to give you a basic idea about the epidermis that was taken from the website "Chemist's Corner" titled "What Is Skin For?" shown below. I highly recommend viewing the short video before reading further into the blog post:
The producers did a great job of listing the relevant steps in the process of producing the various layers of the epidermis. If I had just started with the weird names such as "stratum corneum" you might have been asleep already, right?
The Epidermis can be split up into five layers which are of importance. Especially, when formulating a product. The outermost layer of the epidermis is the "stratum corneum" and for functional purposes is the hardest. Keeping foreign invaders out is just one purpose along with the many others listed in the excerpt above. The four layers underneath the stratum corneum serve to different steps in production of the outermost layer -- as you will see shortly. The video above illustrates the point just made that the bottom few layers manufacture the final outer layer of skin. Lets take a look at the stratum corneum below.
Function of the Stratum Corneum
As I just mentioned, the stratum corneum is fueled by the layers underneath. Below is a brief breakdown of the layers along with an excerpt to clarify a few key terms which will help us understand anti-aging products claims and effects in later blogs. Further, at times, the Stratum Corneum will be referred to in abbreviated form -- "SC". Here is a diagram of the Stratum Corneum taken from the book above -- "Physiology of the Skin" shown below:
Working our way to the top from the bottom, a few points about each layer should be understood by a cosmetic chemist or a consumer. There are many claims that originate out of the "marketing" or "Public Relations" departments in product lines that have no or very little scientific basis. After reading about the various layers, as a consumer, you might have a different perspective toward your next purchase.
Without further ado, starting with the Stratum Basale:
Cells known as keratinocytes start forming in the stratum basale layer. Additionally, another type of cell of importance is made here -- melanocytes which produce the chemical pigment melanin. Melanin is devoted to skin color but also contributes as a defense mechanism again harmful UV rays.
An important note about the various layers of the epidermis is that there is an upward movement of the layers. Think of a reptile like a snake which over time will shed it's skin or the teeth of a shark which produce a new set every weeks. Similarly, the keratinocytes and melanocytes that are made in the basale layer will eventually make their way up to the outer stratum corneum layer.
Note: But first, on their way, transformations will occur. These transformations will be a change of polarity in the proteins itself, death (cells will flatten out and dry out/die), and production of natural moisturizer factor. That is to say, cells that arise in the basale layer will eventually die when they reach the stratum corneum.
Next, the Stratum Spinosum:
In the stratum spinosum, the keratin cells start becoming "spiny" -- which is to say, their shape starts to change. Lipid production has already occurred in the previous layer, therefore, the lipids are very similar in structure. This layer is referred to as "early differentiation."
The "Zone of Transition" -- Stratum Granulosum:
In the "zone of transition," many changes are starting to occur. There is the keratin proteins have started to fill the cells completely. Earlier, in the stratum spinosum, lipids were 'static' (i.e. not changing). That is not the case in the stratum granulosum. Along the way, the production of various macromolecules have started to take place. In this phase, sterol production is increased along with glycolipids, and cholesterol sulfate. The SC is now close to formation.
Finally, the Stratum Corneum:
In the last layer -- the layer closest to the environment (the outside world), the keratinocytes have lost enzyme function and are doomed to flatten out into their final shape. The cell has lost most of the function -- except that the new function is to be flat and hard -- to serve as a barrier toward to outside world. As the cells (keratin protein filled) harden the matrix forms of "flattened cells" -- called corneocytes.
The matrix of corneocytes have intercorneocyte layers. Intermixed in the layers are lamellar bodies. These bodies serve an important purpose. The secretion of vital chemicals (such as free sterols, sphingolipids, and other compounds like glycoproteins). Combined, this chemical matrix permits both hydrophilic and hydrophobic materials to pass through the intercorneocyte space. The matrix serves as a "guard" agains foreign invaders.
Overall, the layers of the underlying skin have produced the critical components for the formation of the stratum corneum. Here is another excerpt that might help clarify the complex stratum corneum from the book:
Keratin is the protein that makes up the bulk of the SC. Keratin is a helical, or coil-shaped fibrous protein made up of a series of building blocks, known as polypeptides. These polypeptides are, in turn, made up of the most basic substances, known as amino acids. Amino acids are arranged in a variety of orders to form chains of polypeptides, which are then twisted around each other to form proteins. These polypeptides vary in different parts of the body so that the skin protein is not homogeneous, but rather heterogeneous. The protein is resistant to water and many chemicals. It is this complex structure that provides part of the protection from the outside. Manufacturing proteins is one of the major functions of the skin.
These keratin proteins are formed and arranged into cells known as corneocytes that are held together with fats, known as lipids. The SC can be thought of as a brick wall, with the protein-rich corneocytes forming the bricks and the lipids functioning as the glue. It is this brick wall that provides the barrier necessary for the beauty and health of the body.
The lipids that keep the proteins glued in place in the SC are water-insoluble, oily substances. They can be classified by their electrical charge and by their structure. The two major groups of lipids are polar lipids and nonpolar lipids. Polar lipids have an electrical charge. Examples of this type of lipid are phospholipids, glycolipids and cholesterol. Non-polar lipids have no electrical charge. Tryglycerides, squalene and waxes are examples of this group.
The six major structural groups of lipids are: triglycerides, the most abundant lipid in the body, which function as energy strorage compounds and make up between 12-25% of the lipids in the SC; fatty acides, which give the oily feel and make up between 12-20% of the lipids in the SC; waxes, which make up 6% of the lipids in the SC; and cholesterol, shingolipids and ceramides, which make up between 14-25% of lipids in the SC.
Does the Stratum Corneum seem complicated? This layer is the smallest layer and most crucial layer toward the outside world. The total thickness is around 150 micrometers. To give you an idea of the relative size of this layer, a human hair is around 70 micrometers. The stratum corneum is roughly two hair strands in diameter and of the upmost importance with the multifunctions that it provides in relation to protecting the body from outside forces.
And last but not least regarding the stratum corneum -- it is the only layer that is visible on the skin. Therefore, the stratum corneum defines the image of the skin on the body. Scary to imagine that a layer of the skin that is roughly two strands of hair thick -- gives rise to the image of healthy or diseased skin -- Wow! Additionally, the process of producing a new layer is accomplished over a ten week cycle (every 2 months a new SC)!
The structure of the outer layer of the skin -- the Epidermis -- is more complicated than you would have initially thought. Although, I would like to highlight that you do not have to be a professional medical doctor or a dermatologist (specialist) to understand the physiology of the skin -- the basics. Additionally, I would like to drive home the point that you need not be a professional to tease out the "false" or "absurd" claims that cosmetic manufacturers use to get the consumer (you ) to buy their product. Each product has different benefits and disadvantages. Some more than others. Ultimately, you will decide which you will purchase and use. I just provide some insight in the process.
A few final questions that you (the reader) should entertain after reading the post (and the next blog post) would be:
1) How do cosmetics (skin, creams, lotions, etc.) affect the function of my skin?
2) How is my skin different than other people's skin? - i.e., dry skin, itchy skin, psoriasis, etc.
3) What does healthy skin look like for my skin?
Each person is different. Therefore, each of us have different skin needs. There exist general skin functions as highlighted above. Although, when a person has a unique skin condition or need, the function might be slightly different. This is not a bad observation. In fact, this observation might be a saving grace for you -- in terms of money and health. Until next time, have a great day!