First and foremost, skin is a permeability barrier. It keeps water inside the body, which is critical for survival. It also keeps potentially detrimental influences, such as chemicals, microbes and antigens, outside of the body. Our skin is also an immunological barrier. Epidermal keratinocytes are strongly immunologically active and are part of our body’s innate immune system. The epidermis also contains dendritic Langerhans cells, which are an integral part of our body’s adaptive immune system.

Skin moreover protects our body against UV radiation. Melanin is produced in the epidermis, and melanin-containing melanocytes form a “parasol” over the nuclei of the viable keratinocytes to protect against DNA mutations induced by UV radiation. In relation to this, keratinocytes have the ability to repair their DNA when needed. This is important for them in maintaining their ability to optimally provide our body with skin’s permeability barrier function.

Skin cells have the ability of quenching free radicals in different ways. Anti-oxidative enzymes are produced in the epidermis, but several constituents of the cornified envelope of the corneocytes in the outer layers of our skin, the stratum corneum, have the ability to act as an antioxidant. As such, skin also possesses a much-needed antioxidative barrier function. An important additional barrier function of skin includes its active production of antimicrobial peptides. This helps skin in the defense against pathogenic microbes.

Other barrier functions of the skin include it ability to resist physical impact. The dermis is rather thick and contains polymers like collagens, elastin and hyaluronic acid and water, which help the dermis to act like a “cushion.” The skin is also a thermal barrier for our body, helping to insulate our body. “Antifreeze” and heat-shock proteins are produced in the skin to help maintain cellular integrity and functionality in the keratinocytes to ensure that the skin barrier function is retained.

Our skin’s barrier functions are indeed its reason for being, its purpose. Interestingly, skin’s look and feel, which are extremely important parameters for a person’s wellbeing and quality of life, are strongly intertwined with the quality of skin’s barrier function. Especially its role as a permeability barrier is of great importance in this respect. Loss of skin permeability barrier function is strongly linked to loss of moisture in the stratum corneum – i.e., to dry skin. Dry skin feels rough and looks unattractive.

Skin barrier, as a commercial or marketing concept, has enjoyed strongly increased attention in the personal care industry. The modern consumer is reported to be clearly more health-oriented than in the past. Together with the above-described implications of the loss of the skin’s permeability barrier function, this explains the increased interest in skin barrier as a concept. At this point in time, skin barrier is mainly seen in the light of the scientific concept of skin as a permeability barrier. This merits a closer look.

The permeability barrier properties of the skin originate from the stratum corneum, the outermost layer of our skin and, in essence, of our body. This was first recognized in the 1940s. At that time, however, it was not yet clear how this barrier function was established biologically. In the 1950s and 1960s it was shown that solvent extraction of the skin leads to dramatically increased water permeability. This implied that lipids, which were removed by the solvent extraction, played an important role in the skin’s permeability barrier function. Only in the 1970s, though, a more accurate picture of the stratum corneum lipid composition was established. It was found that 45–50% of the lipid content of stratum corneum was ceramides, 25% cholesterol, 10–15% free fatty acids and up to 5% other lipids.

In the early 1980s, in an effort to describe the structure of the stratum corneum, the “brick-and-mortar model” was introduced, in which the stratum corneum is composed of flat dead cells (bricks) surrounded by a lipid matrix (mortar). The flat dead cells – the corneocytes – provide structural support for the stratum corneum and act as hydrating reservoirs to enable enzymatic processes needed in the stratum corneum. The surrounding lipids represent the main permeability barrier, as described above.

Current scientific knowledge shows that the skin’s barrier function comprises much more than “bricks and mortar.” Junctional protein structures which ensure intercellular anchoring, such as tight junctions, desmosomes and corneodesmosomes, play an integral role in the skin’s permeability barrier function, for instance. Another important topic in this context is the cornified envelope, which comprises a physical and biochemically active “cell wall” of the dead skin cells, the corneocytes, in the stratum corneum. These are two examples of structure in the skin which have now also been recognized as vital for the skin’s permeability barrier function, but there are many more.

Biologically, all entities which comprise the stratum corneum’s permeability barrier function originate from the epidermal differentiation process. The stratum corneum is the top layer of our epidermis and, apart from the keratinocytes’ innate immune role, the keratinocytes in the epidermis act as a “stratum corneum factory.” The epidermis is a stratifying epithelium with multiple layers of keratinocytes, where the keratinocytes gradually move upward over time. During this upward movement they differentiate, losing their nucleus and, thus, their viability as a living cell. They remain extremely biochemically active though. This allows them essentially to go through a metamorphosis from a viable living cell to the stratified corneocyte we can observe in the stratum corneum.

The process of differentiation is extremely complex and, in healthy skin, well-regulated. Virtually all above-described aspects of the skin’s barrier functions originate from the epidermal differentiation process. As already described in this paper, this also includes important “cosmetic aspects” of the skin, such as look and feel. The epidermal differentiation process is at its core the most important feature for both the skin’s reason for being (its barrier function) and the fulfilment of the skincare consumer’s basic needs.

Interestingly, a new barrier function related to skin has emerged in the last few years. Both scientifically and conceptually, the skin microbiome is now recognized as our “first line of defense” for skin and our body. We at CLR have written multiple papers on the skin microbiome and we refer to these papers for more details, but it is worth mentioning that the quality of our skin microbiome as a “first line of defense” depends fully on the epidermal differentiation process.

Skincare products can play a positive role for the quality of the skin microbiome, skin’s barrier function, and thus its most important “cosmetic features,” its feel and look. Different approaches can be taken, depending on the type of skin for which the skincare product is intended. Sensitive skin has an inherent problem with the epidermal differentiation process and its barrier function, which leads to the initiation of inflammatory processes and, therefore, irritation. An anti-inflammatory approach (AnnonaSense CLR™) is important, but an ingredient which supports the skin in solving this inherent problem (ProRenew Complex CLR™) is essential too. Additional ingredients which lead to immediate hydrating effects (MultiMoist CLR™, Vitamin F forte) have important added value, providing direct effects and resulting in customer satisfaction and compliance, where the consumer is motivated to continue using the product daily, to make sure that ingredients which work deeper in the skin (and, inherently, take longer to work) are able to elicit their beneficial effects.

Aged skin can be another starting point for the use of a skincare product. Sensitivity is not necessarily a problem, and aged consumers are on average more realistic in their expectations of a cosmetic product. For the mature consumer it is beneficial to provide immediate relief with hydration (MultiMoist CLR™, Vitamin F forte), but this is not a must. An approach where the epidermal differentiation process is actively supported (ProRenew Complex CLR™) is absolutely necessary, though.

There are, obviously, many other skincare concepts where epidermal differentiation and skin’s permeability barrier function stand at the core, but it would go too far to go into all the details in this paper. Microbiome skincare is a topic which merits mentioning, however. As stated above, we have already written different articles about the skin microbiome, but in the relationship between our skin and the microbes which live on and in it, the skin is “leading.” This means that the environment which is provided to the microbes and which originates from the epidermal differentiation process is decisive for the composition of the skin microbiome. The composition of the microbiome, in turn, is decisive for its ability to be beneficial for us, its role as our “first line of defense.” Here, ProRenew Complex CLR™, which was proven to improve the epidermal differentiation process, was also shown to provide support for the healthy skin microbiome.