What is a skin barrier or is it just permeable?

Your skin is your largest organ and your first line of defense against the outside world. But is the skin barrier really impenetrable? Or can substances still enter? In this blog post we discuss the functioning of the skin barrier and how substances can penetrate your skin. We also answer the question why you don’t swell when you’re in the pool.

The skin barrier consists of several layers:

The epidermis: a shield of cells

Our skin, the largest organ of our body, is a complex tissue that protects us from the outside world. The outer layer, the epidermis, is the first line of defense. This layer is made up of dead skin cells that cover our body like scale armor. These dead cells are rich in keratin, a sturdy protein that protects against moisture loss and harmful external influences. The epidermis is continuously able to renew itself: dead cells are shed and replaced by new cells that migrate upwards from the deeper layers. This process, called desquamation, takes an average of 28 days.

The dermis: firmness and elasticity

Beneath the epidermis is the dermis, a vibrant layer that ensures the skin’s firmness and flexibility. This layer is rich in collagen and elastin, two important proteins that provide structure and resilience. The dermis also contains blood vessels, nerves, hair follicles and sebaceous glands. The blood vessels provide the supply of nutrients and oxygen, while the nerves provide sensitivity. Hair follicles and sebaceous glands produce hair and sebum, which moisturize and protect the skin.

Subcutaneous fat: an insulating layer

The deep layer of the skin is the hypodermis, which consists of fat cells. These fat cells provide insulation and protect the body organs against cold and shock. It is also an important store of energy.

The three layers of the skin work together as a team to protect, hydrate and regulate our body. The epidermis forms a barrier against the outside world, the dermis provides strength and flexibility, and the subcutaneous fat layer insulates and protects.

Several factors strengthen the barrier function:

• Tight junctions: The cells in the stratum corneum are connected by tight junctions, tight connections that do not allow water molecules to pass through.
• Natural hydration factor (NMF): The stratum corneum contains NMF, a mixture of water-soluble substances that attract and retain water. This component works with your sebum and helps keep the skin hydrated and strengthen its barrier function.
• Skin acidity (pH): The normal pH of the skin is slightly acidic (around pH 5.5). This pH value helps keep the lipid barrier intact and protects the skin against microorganisms. More about skin acidity and why it is important

Factors that can affect the barrier function:

• Prolonged bathing: Prolonged contact with water can damage the lipid barrier and dry out the skin.
• Chlorine and salts: Chlorine and salts such as magnesium chloride and sodium chloride in swimming pool water can irritate the skin and weaken the barrier function.
• Soaps and cleansers: Harsh soaps and cleansers can strip the skin’s natural oils and compromise its barrier function.
• Skin aging: As we age, the skin becomes thinner and less elastic, which reduces its barrier function.

Consequences of a weakened barrier function:

• Dry and irritated skin: A weakened barrier function can lead to dry, irritated and itchy skin.
• Eczema and other skin conditions: A weakened barrier function can increase the risk of eczema and other skin conditions.
• Infections: A weakened barrier function makes the skin more susceptible to infections by bacteria and viruses.

The journey of substances through the skin barrier: three main routes

Imagine: you apply a cream to your skin. The active ingredients in that cream want to do their job, but how do they get into the skin? There are three main routes that substances can take: transcellular (intracellular), intercellular and follicular penetration.

1. Transcellular (intracellular) penetration: Right through the cells

During transcellular or intracellular penetration, substances penetrate through the individual skin cells, straight through the cell membrane and the cytoplasm. The cell membrane is a type of liquid barrier that surrounds the cell.

How substances cross the cell membrane:

• Lipophilic substances: Substances that are ‘fat-loving’ can cross the cell membrane by diffusion. They dissolve in the ‘fat-loving’ inner layer of the membrane and then diffuse through the membrane to the other side.
• Small Molecules: Small molecules can cross the cell membrane.
• Active transport mechanisms: Certain substances, such as ions and sugars, are transported through the cell membrane via active transport mechanisms, using energy from the cell.

Transcellular Penetration:

1. Retinol
2. Vitamin C
3. Niacinamide
4. Peptides
5. Hyaluronic Acid

2. Intercellular penetration: Between the cells

Unlike transcellular penetration, substances that undergo intercellular penetration take the shorter route between skin cells. These spaces, filled with intercellular lipids, provide a less formidable barrier.

Which substances use this route:

• Hydrophilic substances: Substances of a ‘water-loving’ nature, which cannot cross the lipophilic cell membrane, take this route.
• Larger molecules: Molecules that are too large to fit through the pores in the cell membrane can diffuse intercellularly.

Intercellular Penetration:

1. Glycolic acid
2. Ceramides
3. Antioxidants
4. Peptides
5. Essential oils (e.g. neem oil, eucalyptus oil, thyme oil, lavender oil, oregano oil)

3. Follicular penetration: Through the hair follicles

The hair follicles provide a direct route to the deeper layers of the skin. Substances that take this route penetrate into the pores and follicle ducts. Despite being one of the easiest routes of all, hair follicles and sweat glands cover only 0.1% of the total skin surface, making the follicular route less likely to be used for topical application.

Which substances use this route:

• Small, lipophilic substances: These substances can dissolve the sebum in the follicle ducts and thus penetrate the follicle.
• Substances with antibacterial or anti-inflammatory action: These substances are effective in the treatment of acne and other follicular diseases.

Folliculair penetration:

1. Salicylic acid (for example in shampoos and conditioners)
2. Tea tree oil
3. Retinol
4. Antibiotics (scalp treatment)
5. Hormone lotion, cream or ointment (scalp treatment)

What factors determine skin penetration?

• The size of the molecules: Small molecules penetrate faster than large molecules.
• The oiliness of the substance: Fatty substances penetrate faster than watery substances.
• The condition of the skin: Damaged skin (skin infection, eczema, wound) is more permeable than healthy intact skin.
• Duration of skin contact (stay on or rinse off product)
• Location of skin contact: eyelid versus palm or sole of foot
• Occlusion: Is it under a plaster or under a piece of clothing
• Spread area: Apply body cream or a small area such as an eye cream

Why don’t we swell in the pool?

The key to the answer lies in the skin barrier. This barrier, formed by the outer layer of the epidermis (the epidermis), is waterproof. This means that water molecules are too large to penetrate the cells of the epidermis. This allows us to bathe in the water for hours without our skin swelling.

While you are here

As a skin specialist, it is crucial to guide individuals in maintaining healthy skin and an intact protective layer. Our skin, the largest organ of our body, performs several vital functions, such as warding off disease and regulating body temperature. But what exactly are the mechanisms behind the skin and how do the skin barrier, sebum and the micro-organisms on our skin contribute to these processes? If you want to know more, click here.