Skin is like a glove that wraps around the human body to prevent it from both external aggressions and dehydration. This wonderful organ is a thing of beauty, a tool of pleasure, but above and before all, it is an incredibly well-designed protective envelope. Its beauty lies as much in the way it looks as in the way is operates.
Skin is our first line of defense and the guardian of our vital functions. You may have a perfectly functioning heart, a great liver, clean lungs and a pristine brain, but if your skin is burnt beyond a certain percentage of total body area (and depending on your age, according to the Baux score system used by medical professionals to predict the chance of mortality of patients suffering burns), you may die—severely damaged skin impedes the ability to breathe, retain or expel water, and defend against harmful bacteria, oxidants and toxins.
The superficial layers of the skin, although thought of as “dead,” are its most important part from the standpoint of its vital functions. The outermost layer is called the stratum corneum. It is about 10–15 microns thick and constituted by about 10 layers of cells, and it is those 10 layers that we call “dead.” And, technically, they are, but there’s much more to that story.
The stratum corneum is one of the most important and most functional parts of our bodies. So important and so functional that we wanted to tell you the story and the latest on what we have learned about the skin barrier.
The Barrier: A Primer
The skin barrier regulates water loss by regulating evaporation rate. In addition, it prevents the penetration of toxic substances and pathogens.
Historically, the barrier has always been described as a fairly passive system. That is the old “bricks-and-mortar” model: dead cells, surrounded by lipids, preventing water loss and the entry of toxic molecules or pathogens.
Born within the epidermis, keratinocytes progressively move toward the skin surface and undergo a series of transformations (including the loss of their nucleus) that turn them into corneocytes, a breed of very important and very functional “dead” cells, constituting the skin barrier.
The differentiation is the process by which a keratinocyte will transform into a corneocyte. That process occurs during the move in the epidermis from basal layer to the stratum corneum and results in the loss of the nucleus and all other organelles of the keratinocytes, as well as the initial production of lipid and the keratinization of the cell.
A very essential notion is that of the movement upward to the stratum corneum. It is important to understand that although the term “migration” is often used, the movement is not a migration. Cells migrate when they move from one place to another under the influence of chemotactic factors, for instance, using their own means of motility. Cell motility involves deformation, extension and so on. In the case of the keratinocytes’ move to the surface and differentiation into corneocytes, the movement depends on a push created by the formation of new keratinocytes. Like a crowd moves under its own force because people in the back push other people that are in the front, so do keratinocytes push the corneocytes above them.
And the speed at which the cells move impacts the time they have to go through the differentiation process. So, the more keratinocytes are formed, through proliferation, the quicker the movement will be and the less time will be left for the differentiation process, which is dependent on the time it takes for the corneocytes to reach the surface. (More on the implications of this follows in A Story of Time and Synchronization.)
Lipids: A Key in Control Transepidermal Water Loss (TEWL)
The lipids that constitute the lipid mantle, coating the corneocytes, are produced and extruded by the keratinocytes in the basal layer, and are secreted throughout the whole differentiation process.
The lipids, organized in a lamellar fashion (in layers), are cerebrosides, ceramides, fatty acids and cholesterol. Present in the barrier, they constitute a hydrolipidic cement.
These layers of lipids, produced and secreted during the maturation of the keratinocytes, coat each cell in the stratum corneum in such a way that water can still circulate between the lipidic lamellae through a passive diffusion process. Water moves between corneocytes in a very slow and controlled fashion, resulting in a very controlled evaporation rate.
The proteins in the cornified envelope are linked to the lipids that coat the cells. Some ceramides are bound covalently to the proteins, especially involucrin—a structural protein. And transglutaminases controlled by calcium play an essential role in the liaison between involucrin and lipids.
An Active Proteinic Nature
The false notion that the stratum corneum is a dead structure comes from the fact that the corneocytes, born as keratinocytes in the basal layer, are highly proteinic entities that become cornified as they move up in the granular layer and lose their nucleus in the process. However, they are organized in a very complex way, and are the host of a frantic enzymatic activity—forming a very efficient, very active and very “lively” skin barrier.
It is important here, before examining structural proteins, to note the “active” proteins that these enzymes constitute.
Enzymes intervene in a number of metabolisms, allowing structural proteins to link up (transglutaminases I) to form a stronger barrier as well as break these bonds (proteolytic enzymes). They also allow desquamation. But there are also antioxidant enzymes (SOD) protecting the proteins from oxidation, or histidase, allowing for the production of urocanic acid, the skin’s own sun protectant.
Structural Proteins: Building a Strong Yet Flexible Barrier
Structural proteins are the main component of the horny layers, the outermost layer of the skin. Keratin and its various forms are the most common.
During the differentiation process, the cornified envelope replaces the plasma membrane of differentiating keratinocytes. It consists of keratins that are enclosed within an insoluble amalgam of proteins, which are crosslinked by transglutaminases and surrounded by a lipid envelope.
But there are other proteins forming the structure of the stratum corneum—such as filagrin and involucrin. These are very important players in the structure of the skin barrier function, and their role is to link the lipids with the key elements of the corneocytes structure, the cornified envelope, through covalent liaisons. The term “covalent” means that those liaisons are strong and hard to break, allowing for the lipids to stick to the cells and for the skin to maintain the essential lipidic mantle produced during the differentiation process.
If these proteins are dysfunctional or absent, the lipids can’t stay anchored onto the cells and the barrier function is impaired—resulting in many different disorders, starting with irritable, dry and sensitive skin and, possibly, severe dryness, skin lesions, etc.
In some pathologies where the barrier is deeply impaired, such as atopic dermatitis or eczema, there may be a genetic deficiency in filagrin.
New research on the skin barrier reveals very important and very interesting structures: Also called “kissing points,” the tight junctions are proteinic structures between the cells that allow cell adhesion and barrier function control. Their crucial role in the barrier function has only been recently demonstrated and researchers are still in the process of understanding it fully.
Tight junctions restrict ions circulation but allow for the controlled diffusion of water. They control the evaporation of water. They also function as landmarks by limiting the circulation of signaling molecules.
Claudin is one of the anchoring molecules in the tight junctions. When the synthesis of claudin is inhibited, the water loss becomes much more important. This tells us that lipids are necessary but not enough to control TEWL.
At the granular level, tight junctions regulate the penetration of certain high molecular weight components such as serin protease, which results in a modulation of the effect of enzymatic peels on the skin. Tight junctions buffer the enzymatic peels, preventing the enzymes from going where they could do some damage, beyond the upper strata of the stratum corneum. The enzymes’ activity is then restricted to the surface of the skin.
The Actin Connection
Tight junctions also fulfill a very important structural role: anchoring two cells together. Cadherin and adheren are among the main components of tight junctions, acting as this anchor. They are also anchored to a very important intracellular structure: the actin.
Actin filaments do many things in a cell, but they are interesting in this discussion due to two functions in particular:
- Actin allows the cells to move (cell motility).
- Actin filaments are part of the cytoskeleton, which gives shape and structure to a cell.
The fact that actin is bound to the components of the tight junctions adds to the whole stratum corneum structure—both in terms of rigidity and flexibility. It creates a continuum between cells and intercellular liaisons. It creates coherence.
The subtle interactions that take place there are what allows skin to be strong yet flexible, along with the very important keratin structure.
Flexible Stratum Corneum to Prevent Wrinkles
Everything in the stratum corneum is linked in a flexible manner; this is what gives it its strength, both structural and functional. The flexibility of the stratum corneum is indeed our first line of defense against wrinkles—as well-hydrated, supple skin is flexible and will flex without breaking or marking the fold. Additionally, keratin needs to hold enough water so as to keep the hardened corneocytes flexible enough as well.
The Importance of Bound Water
When discussing the barrier, one also needs to discuss water—noting the importance of the hydrated state of keratins. When bringing water to the skin, you should understand what the state of water is in the skin. Only bound water (water molecules organized in clusters, linked by hydrogen liaisons) is functional in skin.
To visualize bound water, think of grapes.These “water grapes” have very low entropy and also very low mobility—and because of this, their ability to link to adjacent structures is very high as they linger near the sites that they need to link to.
The high number of molecules comprising hydroxyl groups is what allows the natural moisturization factor to be an efficient means of hydration, as these groups will link to the water clusters easily. The more hydroxyl groups there are, the higher the probability for linking between water and keratin and for the corneocytes to remain flexible.
If free water is applied to the skin, it will not link to these structures, but rather will wash away the amino acids that are there.
A Story of Time and Synchronization
The keratinization process, which allows for keratinocytes to turn into corneocytes is a complicated one, dependent on many factors.
During the movement upward of the epidermal cells to the skin surface, as they enter the granular layer, the keratinization process is in its fullest. The cells become harder because their envelopes’ keratin level increases tremendously.
So, very simply, the differentiation process is inversely proportional to the proliferation process. When the proliferation increases, the differentiation processes decrease, and so does the keratinization. Conversely, when proliferation decreases, the differentiation processes increase.
It sounds perfectly logical that if you force the skin into proliferating more, it produces more new cells that, thusly, need to move up faster and push the old ones out faster. However, there is less time to move up and cover the same distance—resulting in less time for differentiation, keratinization, lipid mantle secretion, etc.
We can then conclude that stimulating proliferation too much can only affect the barrier function in the short- or even long-term, as long as the treatment lasts.
Because time and maturation speed cannot be considered as an isolated notion, we also have to consider relative speed. If the speed is too great, it becomes a problem unto itself, but it also becomes a relative problem: In perfectly healthy skin, variations in the migration speed exist and are compensated by cell synchronization. Cells talk to one another and make sure that they, more or less, all function on the same rhythms.
Biological rhythms are regulated by a host of systems—a central clock (regulated mainly by light), peripheral clocks (clock genes, individual cellular genes activated by light), light itself and other cues, and the cells themselves, communicating among each other with signaling molecules. And when moving upward to the skin surface, cells move in columns. If two groups of columns move at different speeds, and that difference is too great or if the synchronization systems between cells falter (impaired cell communication is a common characteristic of aging skin), cells will come to the surface in different states and create different zones on the skin. In this case, there is no longer a harmonized synchronization between differentiation and proliferation. And that is the case and cause of blotchy skin. Patches of skin with an immature barrier, and thus an impaired barrier function, are created by the conditions noted.
Largely caused by the loss of synchronization between the cells, the loss of homogeneity in the look of skin is one of the marked signs of aging. Most of the time, areas lacking homogeneity are the result of inflammatory reaction, which could be represented as micro volcanoes: The cells go up very fast just like the lava in the cone of the volcano, pushing the cells outward too fast. The cells in those zones tend to hyper-proliferate and move toward the surface very fast.
But not only will the inflammation create a hyperpigmentation, it also creates an impaired barrier; thus exposing the skin to more inflammation [Editor’s note: Inflammation is further discussed in Inflammation and Aging].
Often, attempts are made to treat hyperpigmentation with exfoliation; this is a losing tactic for multiple reasons—notably when inflammation occurs, it activates a number of immune skin cells, as well as very specific enzymes (metalloproteases) that destroy specific components of the extracellular matrix in the dermis and key elements of the basement membrane/basal layer. Under these conditions, melanocytes, which usually stay above the basal layer, literally fall into the dermis, under the basal layer, where they stay permanently and give rise to an age spot.
But because this is fairly recent research, few people know that and still try to exfoliate the melanocytes away. And exfoliation, which tends to be pretty drastic, is another activator of inflammation.
In the case of hyperpigmentation, exfoliation provides temporary relief by taking away the melanin that has migrated in the superficial layers, but is bound to worsen the phenomenon by reactivating the melanogenesis induced by the inflammation it triggers all the way down to the basal layer, where the affected melanocytes live.
In addition, a number of dermatological treatments, unfortunately, still put the emphasis on accelerating cell division and increasing proliferation speed. The consequence is that the cells migrate too fast and bring about a very immature and dysfunctional stratum corneum. The cells that are exposed to the environment are just not ready for it.
One strategy used in skin care product development has been to simply add lipids to the formula to compensate for the loss of lipids in skin. For the past 30 years, the de facto motto has been: lipids, lipids and more lipids. It started by adding occlusive lipids such as mineral oil and then progressed to oils that were particularly rich in essential fatty acids, because these were functional.
Finally, the practice has moved on to cerebrosides and ceramides.
About 10 years ago, product developers started using equivalent concentrations of the three lipids: cholesterol, fatty acids and ceramides. When applied to the skin, that formula temporarily restores the barrier effect, but it is purely a palliative effect—not a cure.
Differentiation and Proliferation
Approaches to creating skin care products have also involved vitamin D3, which is essential in the differentiation process, and it seems to be a good idea. Although vitamin D3 is not allowed for use in cosmetics in Europe, its precursor, 7-dehydrocholesterol, is allowed.
As with every biological process, it is important to be careful and use the right concentration. Remember, too much differentiation implies a slow down in the proliferation. If the differentiation processes is excessively increased, skin becomes excessively thick as the migration speed slows down too much and cells accumulate instead of being properly exfoliated in a timely fashion, and cells take four to six weeks in a normal cycle to reach the stratum corneum.
A major direction of research on new skin barrier technologies involves looking at how to nurture and activate the tight junctions.
Some teams are working on activating the claudins, a family of proteins that are crucial structural and functional components of the tight junctions, in order to increase the tight junctions formation. But new strategies must also focus on restoring or improving the role of the proteins, filagrin and evolucrin in strengthening bonds between the lipids and the surface coat proteins to provide optimal barrier effect.
New research on the different proteins of the skin and their metabolism could also lead to technologies helping skin produce its own sunscreen, which it already does to some extent. By activating histidase, an enzyme in the stratum corneum that breaks down into urocanic acid, a mild UV protectant results. Activating urocanic acid production in the skin would not replace the use of sunscreen during sun exposure, but it could provide an additional protection for everyday by enhancing the skin’s own ability to produce greater UV protection. In addition, it could allow for a very uniform protection, which, unless applied flawlessly, current sunscreen does not.
What the Research Reveals
The skin barrier may be composed of cells that appear dead but it is far from being a dead structure. It is an extraordinarily important, active, rich and complex structure. Apoptosis, or programmed cell death, is part of life and, more often than not, means the survival of a system by its overall improvement.
The stratum corneum represents a very specific and very interesting example. The challenge is not so much to improve the barrier but to move away from old practices that are deeply ingrained in our product design culture and that damage the barrier.
We need to let the skin protect itself, by nurturing a healthy barrier rather than to try and activate cell proliferation and, in the long run, risk hurting the very barrier that allows for these cells to be healthy and functional.
This is definitely the most important lesson that nature teaches us as we learn more about the way it works: Skin is beautifully made, and what makes it dysfunction is our lifestyle more than anything. In the light of what we know, we have to adapt our strategies to restore the skin barrier and, more importantly, not impair it while trying to rejuvenate, whiten or firm the skin. Our job is not to try and force it into doing better, but protect it from getting bad and trust that the healthier skin becomes, the more beautiful it is.
Marie Alice Dibon, PharmD, is the principal at Alice Communications, Inc., helping companies in the life science sector to develop innovative technologies and communications strategies.