What constitutes the skin barrier?

The skin is divided into three layers: the epidermis (outermost layer), the dermis (middle layer) and the hypodermis (deepest layer). The stratum corneum is the scientific term for the skin barrier; it constitutes the most superficial layer of the skin, serving as an essential protective shield.

The skin’s protective barrier is often compared to bricks and mortar, highlighting its highly organised nature. The barrier’s homeostatic preservation is best characterised by its protective functions and its capacity to retain moisture:

• Bricks (Corneocytes): The first layer of the epidermis comprises flattened, non-viable cells whose composition is predominantly keratin and natural moisturising factors (NMFs), which regulate water retention. Their compact, layered organisation renders the corneocyte layer akin to bricks in a wall, providing stability and durability to the skin barrier against external influences.

• Mortar (Intercellular Lipids): The channels between corneocytes are filled with intercellular lipids comprised of ceramides, cholesterol and free fatty acids. This entire intercellular lipid matrix binds corneocytes into a network, forming a waterproof seal that minimises transepidermal water loss and the penetration of external substances such as pollutants and pathogens.

• Hydrolipidic Film: The skin’s surface is covered by a thin layer comprising emulsified water and lipids, also known as the hydrolipidic film. Perspiration combines with sebum and water to form an additional protective layer, which helps maintain pH balance and provides an extra barrier against environmental aggressors.

Without an intact skin barrier, the body would be susceptible to external damage and prone to excessive moisture loss, resulting in dryness and irritation. The skin barrier fulfils several functions that are beneficial to the human body and to life in general.

Defence against external aggressors.

The skin barrier protects us from outdoor stressors such as wind, temperature extremes and air pollution. It also shields us from harmful microorganisms, viruses, bacteria and fungi. Corneocytes form a brick-like barrier that prevents pathogens from penetrating the deeper layers, much like neatly arranged bricks. Without adequate protection, all of these stressors could readily damage and irritate the skin.

Regulation of moisture content.

This barrier also aids moisture retention by serving as an occlusive layer to reduce trans-epidermal water loss (TEWL). It further supports skin flexibility and smoothness, imparting an overall radiant appearance.

Protection against UV radiation.

Deeper layers of the skin, such as the dermis, are chiefly responsible for repairing and mitigating long-term UV damage. On the external surface, densely packed corneocytes provide an initial reflection and scattering of UV rays, reducing their impact on the skin. When combined with a healthy diet, the addition of antioxidants to the hydrolipid film provides an extra line of defence by neutralising UV-induced free radicals on the skin.

Maintaining pH balance.

The hydrolipidic film maintains the natural pH of the skin, which plays an important role in creating a slightly acidic environment. This environment will ultimately promote the development of intercellular lipids and form an intact, functional barrier. Damage or imbalance in pH disrupts the skin’s harmony, compromising its defence and making our body susceptible to bacterial and fungal infections.

The skin barrier is dynamic and can repair itself when compromised. The rate and success of restoration depend on age, health and other external factors. When this barrier is damaged, processes that upregulate lipid biosynthesis and restore corneocyte architecture are initiated, enabling the barrier to resume its protective functions.

This complex structure in the architecture of the skin is essential for protection, hydration and overall health. Barrier functions underline the crucial importance of maintaining skin integrity. In addition to dermatology, scientific research will further advocate improved methods of skin care that support and protect this remarkable system.

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