External challenges for skin
Healthy-looking skin is closely related to the actual main purpose of our skin: to function as a physical barrier between our body and the harmful outside world. Our skin’s top layer, the epidermis is responsible for maintaining this function.
Our skin is constantly challenged by fluctuating environmental humidity and temperatures, UV, microbes, daily hygiene (water, surfactants), foreign molecules, etc. UV radiation is the most dangerous stress factor among permanent environmental impacts on human skin. Consequences of UV exposure are aberrant tissue architecture and alterations in skin cells, including functional changes. Keratinocytes are subject to significant cellular damage after continuous UV exposure.
Air pollution – a growing concern
Air pollution is a growing concern for the consumer, especially for those who live in big cities. In many big cities the level of air pollution exceeds the maximum values determined by the World Health Organization. Epidemiological studies have clearly shown that people who live in a polluted environment have worse skin barrier function, lower moisture level in the stratum corneum and skin which is more prone to become irritated. Production of filaggrin and KLK7 has been reported to be significantly lower in skin which is exposed to pollution. Moreover, strong indications have been found that skin in a polluted environment shows a lower level of immunocompetence. Interestingly, corneocyte cohesion and the overall desquamation process were also reported to be disturbed in skin in a polluted environment.
Skin renewal is key in maintaining skin’s protective barrier function
In order to maintain its vital function as physical barrier, the epidermis is constantly renewing itself, a process which makes it the most dynamic part of our body. Every day an average of one sheet of dead skin cells is shed from the outer surface of our body. During a lifetime, up to 30 kg of dead skin cells are “produced” by our epidermis, and the epidermis renews itself in its entirety approximately 1,000 times. If the epidermal renewal process takes place successfully, skin shows optimum barrier function, hydration level, and esthetic properties.
The renewal process of the epidermis is the product of a careful and sensitive balance between proliferation of the living keratinocytes in the basal layer and the differentiation process which takes place above the basal layer, one in which the differentiating keratinocytes move slowly upwards. In healthily functioning skin, approximately 14% of the basal keratinocytes are in the process of cell division at any given time. They go through 3-5 cell divisions before they detach from the so-called basement membrane, after which they enter the extremely well-organized and delicate process of differentiation and move upward. The total turnover time of epidermis is reported to be between 3-5 weeks. During half of this time, the cells are part of the stratum corneum, where in the process of moving upwards they continue to differentiate until they are fully transformed into corneocytes, reach the skin surface, and are shed.
The role of keratinocytes
As mentioned above, the reason why skin is renewed so dynamically is because our stratum corneum is constantly challenged and cannot maintain its barrier function without constant renewal. To safeguard the skin’s ability to uphold its barrier function, the epidermis can and must act as a “biosensor”. The differentiating keratinocytes are able to instantly react and compensate for the negative influence of an outside challenge on the skin’s barrier function. Although the keratinocytes which are in the process of differentiation are no longer formally alive, they are still biochemically and immunologically active. Where the physical barrier properties of our skin can be considered to be the first line of defense, the keratinocytes’ ability to react to insults immunologically would be the second line of defense against negative impacts. The immunocompetence is therefore of eminent importance.
The protective function of the skin microbiome
Another important health-relevant aspect is to maintain the physiological microflora both internally in the intestinal tract and externally on the surface of the skin. The drive to cleanse ourselves of our microscopic friends and to create a germ-free environment may be disrupting this physiological microflora and causing health problems. Introducing microorganisms into the human body to replenish depleted bacterial levels is becoming a highly researched field which is yielding positive results. New findings in medical research are increasingly strengthening the case for the medicinal administration of live microorganisms, otherwise known as probiotics.
The microbes living on our skin are different from the probiotic bacteria living inside our body. They play the exact same role, though, just in different parts of our body. The skin microflora makes sure that the skin cannot be invaded by pathogenic bacteria and it positively influences the quality of our skin. This includes the skin’s own ability to manage its microbiome and keep it healthy. A perfect example of a positive feedback loop.
The epithelial cells inside our body, on which the probiotic bacteria reside, are strongly related to the keratinocytes in the top layer of our skin and if you know the science behind the interaction between the bacteria and the cells they live on top of, no matter inside our body or on our skin, only one conclusion can be drawn: probiotic lysates potentiate the skin’s ability to manage its microbiome.