The skin and its barrier function

The skin is the body’s largest organ, representing approximately 15% of adult body weight. The skin performs many vital functions, including protection against external physical, chemical, and biological assailants, prevention of excess water loss from the body and a role in thermoregulation (1).

The layers of the skin:

1. Epidermis (upper, outermost layer) – consists of cells known as keratinocytes, which function to synthesise keratin, a threadlike protein with a protective role (1).
2. Dermis (middle layer) – consists of a fibrillar structural protein known as collagen (1).
3. Subcutaneous tissue

The epidermis represents the physical, the chemical/biochemical (antimicrobial, innate immunity) and the adaptive immunological barrier (2).

The layers of the epidermis (2, 3):

1. Stratum corneum (outermost layer)
2. Stratum lucidum (only fingertips, palms and soles of feet)
3. Stratum granulosum
4. Stratum spinosum
5. Stratum basale (the innermost layer that contains epidermal stem cells)

The stratum corneum of the epidermis is the principal barrier against the penetration of chemicals and microbes and can withstand mechanical forces. It is further involved in regulating water release from the organism and into the atmosphere, known as trans-epidermal water loss (TEWL) (2). The stratum corneum is thick and consists of dead cells (corneocytes) (3).

The epidermis does not have a blood supply but is nourished by the blood vessels below (4).

The dermis is divided according to the thickness of its collagen content into an upper stratum papillare and lower stratum reticulare, containing thin and thick collagen fibres, respectively (3). The dermis is a thicker layer (up to 3–4 mm depending on the body site) with a relatively low cell volume compared with the epidermis. The dermis predominantly consists of the extracellular matrix, such as collagen, which is made of fibroblasts. In addition to the extracellular matrix, the dermis contains structures such as blood vessels, lymphatics, nerves, sweat glands and pilosebaceous units (4).

The deepest layer of the skin, the hypodermis or subcutaneous tissue, is a layer of connective tissue and elastin that provides insulation from cold temperatures, shock adsorbent capability, and a reservoir of nutrient and energy storage. The hypodermis is thickest in the buttocks, palms of the hands, and soles of the feet (3, 4). As we age, the hypodermis begins to atrophy, contributing to the thin, wrinkled appearance of the aged skin (3).

The barrier function of the skin

The skin barrier protects against extensive water loss in one direction (inside-outside barrier) and the invasion of harmful substances from the environment (outside-inside barrier) (2).

The barrier to water permeation is not absolute, and the normal movement of water through the stratum corneum into the atmosphere is known as trans-epidermal water loss (TEWL). Any changes in the cellular composition and arrangement result in altered barrier function, leading to conditions such as eczema and psoriasis, as well as symptoms such as dry skin, sensitive or inflamed skin, pigmentation, wrinkles and infection (5).

Ageing and skin structure

Skin ageing is a complex, multi-factorial process characterised by decreased collagen levels, loss of fibrillin-positive biostructures and broken elastin (3).

As we age, our skin structure changes and the epidermal layer is thinner due to keratinocyte atrophy. This leads to increased trans-epidermal water loss in elderly individuals, resulting in increased skin dryness. The extracellular matrix components collagen and elastin, which provide tensile strength and elasticity, change significantly with age. The total amount of collagen is reduced with age. Elastin is an inert protein formed during early development and is not replenished; therefore, any changes to elastin that occur over a lifetime tend to be permanent, resulting in reduced skin elasticity and wrinkling, the classical sign of skin ageing (4).

Dermal fibroblasts contribute to age-associated dermal thinning as they are reduced in size. In addition, dermal fibroblasts from elderly individuals make less pro-collagen which can contribute to increased collagen fragmentation. Other skin changes observed with age are reduced sweat and sebum production (4).

Finally, a thinning of the adipose tissue is observed with age due to a reduction in white adipose tissue, and subsequent antimicrobial protection (by the dermal fat) in response to infection is significantly decreased.

Changes in skin structure with age depend on lifestyle choices and environmental challenges, including UVB exposure and the use of sunscreen, smoking and environmental pollution. Collectively these changes render older people more susceptible to mechanical injury, alter the skin microbiome and have important implications for skin barrier immunity (4).

How to protect the skin barrier

• Avoid excessive sun exposure

UV rays can cause cellular damage, which is why it is recommended to avoid excessive sun exposure and use sun protection (i.e., broad-spectrum sunscreen of minimum SPF30) (5).

There are two options:

1. Mineral sunscreens – sit on the skin and block UV rays
2. Chemical sunscreens – absorb and deactivate UV rays, causing heat and irritation, especially in patients with certain skin conditions such as rosacea (5).

• Healthy diet

The skin’s TEWL and hydration can be improved through a healthy diet:

1. Foods containing omega-3 fatty acids (e.g., oily fish and flaxseed)
2. Other foods beneficial for the skin include broccoli (which contains sulforaphane, which can protect against sun damage by neutralising free radicals) (5)

• Lifestyle factors

Certain lifestyle factors, such as long hot baths, aggressive skin rubbing when drying, and picking spots and scabs, can irritate the skin. Stress and lack of sleep can also negatively impact the skin barrier, leading to inflammatory skin disorders and reduced skin barrier function (5).

• Skincare routines for improvement of skin barrier function

Moisturisers

Urea, propylene glycol, glycerin, hydroxy acids (e.g., lactic acid) and humectants are used in many moisturising skincare products, but they also promote skin exfoliation (5).

Ceramides

Skincare products containing ceramides are very beneficial in strengthening the skin barrier. They can improve skin dryness, itchiness and skin scaling (5).

Lipids

Topical application of lipids (e.g., triglycerides, phospholipids, free fatty acids, and antioxidants) can improve skin barrier physiological function, promote antioxidative and anti-inflammatory activities in the skin, promote anti-microbial properties, wound healing and anticarcinogenic properties (i.e., coconut oil, jojoba oil and almond oil) (5).

Niacinamide

Niacinamide is a form of Vitamin B, which can reduce inflammatory activities, decrease TEWL and increase the thickness of the stratum corneum of the epidermis (5).

pH

The skin surface is acidic, so reduced pH emollients and cleansers should be used to avoid compromising skin barrier function and prevent conditions such as acute dermatitis (5).

REFERENCES

1. Kolarsick, P.A.J., Kolarsick, M.A. & Goodwin, C. Anatomy and physiology of the skin. (2012). Journal of Dermatology Nurses’ Association; 3(4): 203-213.
2. Proksch, E., Brandner, J.M. & Jensen, J-M. The skin: an indispensable barrier. (2008). Experimental Dermatology; 17:1063–1072.
3. Bragazzi, N.L., Sellami, M., Salem, I., Conic, R., Kimak, M., Daniele, P., Pigatto, M. & Damiani, G. Fasting and Its Impact on Skin Anatomy, Physiology, and Physiopathology: A Comprehensive Review of the Literature. Nutrients 2019, 11, 249
4. Vukmanovic-Stejic, M. & Chambers, E.S. Skin barrier immunity and ageing. (2019). Immunology; 160, 116–125.
5. Francis, T. Understanding the skin barrier. (2022). Aesthetics; 9(12): 48-49.